Samacheer Class 11 Bio Zoology - Excretion

The three filtration barriers that solutes must cross as they move from the plasma in the glomerulus to the lumen of Bowman's capsule are the glomerular capillary endothelium, the basal lamina or basement membrane, and the epithelium of Bowman's capsule. The glomerular capillary endothelium is the innermost layer consisting of endothelial cells with fenestrations or pores that allow small molecules to pass through. The basal lamina is a non-cellular layer of proteins and carbohydrates that acts as a selective filter. The epithelium of Bowman's capsule forms the outermost layer with specialized cells called podocytes that have filtration slits. These three layers work together to form a selective barrier that allows small solutes such as glucose, ions, urea, and water to pass through while excluding larger molecules. Blood corpuscles including red blood cells, white blood cells, and platelets are too large to pass through the filtration barrier and are retained in the blood. Similarly, plasma proteins such as albumin, globulins, and fibrinogen are also excluded due to their large size and negative charge, which prevents them from crossing the basement membrane.

• Glomerulus hydrostatic pressure
• Glomerulus pressure
• Opposing pressure: Colloidal osmotic pressure, Capsular hydrostatic pressure
• Net filtration pressure = Glomerular hydrostatic pressure – (Colloidal osmotic pressure + capsular hydrostatic pressure.

1. Juxtaglomerular apparatus:
Juxtaglomerular apparatus is a specialized tissue in the afferent arteriole of the nephron that consists of macula densa and granular cells. The macula densa cells sense distal tubular flow and affect afferent arteriole diameter. The granular cells secrete an enzyme called renin. It plays an important role in reabsorption of water, Na + and excretion of K +.
2. Podocytes:
The visceral layer of the Bowman’s capsule is made up of epithelial cells called podocytes. The podocytes end in foot processes which cling to the basement membrane of the glomerulus. The openings between the foot processes are called filtration slits. It is important for glomerular filtration.
3. Sphincters in the bladders:
Sphincter muscles in the bladder controls the flow of urine from the bladder. When urinary bladder is filled with urine, it stretches and stimulates the central nervous system through the sensory neurons of the parasympathetic nervous system and brings about contraction of the bladder.
Simultaneously, somatic motor neurons induce the sphincters to close. Smooth muscles contracts resulting in the opening of the internal sphincters passively and relaxing the external sphincter. When the stimulatory and inhibitory controls exceed the threshold, the sphincter opens and the urine is expelled out.
4. Renal cortex:
The outer portion of the kidney is the renal cortex. It contains renal corpuscles and the proximal and distal tubules. It is thin and fibrous.

When a molecule or ion is reabsorbed from the lumen of the nephron, it is transported back into the bloodstream. The reabsorbed substances move across the epithelial cells lining the nephron tubule and enter the peritubular capillaries, which are part of the network of blood vessels surrounding the nephron. These capillaries eventually merge to form the efferent arteriole, which carries blood enriched with reabsorbed substances away from the glomerulus. The reabsorbed molecules and ions are thus returned to the general circulation. In contrast, if a solute is filtered from the blood into the nephron but is not reabsorbed from the tubule, it remains in the filtrate and is excreted along with urine through the collecting duct, ureter, and urethra.

The distal tubule and collecting duct are the segments of the renal tubule that are the primary sites of secretion and regulated reabsorption of ions, as well as pH homeostasis. In these segments, hydrogen ions (H+) and ammonium ions (NH4+) are actively secreted from the blood into the tubular lumen and are subsequently excreted in the urine, thereby maintaining acid-base balance in the body. For each hydrogen ion secreted from the filtrate, one sodium ion (Na+) is reabsorbed in the tubules through an exchange mechanism, allowing the body to conserve essential sodium while eliminating excess acid. The secreted bicarbonate ions (HCO3-), phosphate ions (PO4-), and ammonia (NH3) combine with hydrogen ions to form carbonic acid and phosphoric acid, which serve as buffers in the urine. This buffering system prevents excessive acidification of the urine and helps maintain the pH of body fluids within the narrow range required for normal physiological function. Through these mechanisms of selective secretion and reabsorption, the distal tubule and collecting duct play a crucial role in maintaining both electrolyte balance and acid-base homeostasis in the body.

• Blood enters the glomerulus faster with greater force through the afferent arteriole and leaves the glomerulus through the efferent arterioles much slower.
• Because afferent arteriole is wider than efferent arteriole.
• The constriction of this does not affect the rate of filtration.

• A liquid which gathers in the bladder – Urine
• Produced when blood is filtered in a Bowman’s capsule – Glomerular filtrate
• The temporary storage of urine – Urinary bladder
• A ball of intertwined capillaries – Glomerulus
• Removal of unwanted substances from the body – Excretion
• Each contains a glomerulus – Bowman’s Capsule
• Carry urine from the kidneys to the bladder – Ureter
• The scientific term for urination – Micturition
• Regulation of water and dissolved substances in the blood and tissue fluid – Osmoregulation
• Consists of the kidneys ureters and bladder Excretory system
• Removal of useful substances from glomerular filtrate reabsorption
• What solute does blood contain that is not present in the glomerular filtrate? – Plasma Protein

The type of nitrogenous end product excreted by an animal depends primarily on its habitat and the availability of water. Ammonia is highly toxic and requires large amounts of water for dilution, making it suitable only for aquatic animals. Animals that excrete most of their nitrogen in the form of ammonia are called ammonotelic, and examples include most fishes and aquatic amphibians. These animals can afford to produce dilute urine because they live in or near water. Urea is less toxic than ammonia and requires moderate amounts of water for dilution. Mammals and terrestrial amphibians excrete urea and are called ureotelic. Urea is an ideal nitrogenous waste for terrestrial animals as it requires less water than ammonia but is still readily soluble. Uric acid is the least toxic and can be eliminated with minimal loss of water, making it highly suitable for animals in arid environments. Animals that excrete uric acid are called uricotelic, and examples include reptiles, birds, insects, and some terrestrial arthropods. This adaptation allows these animals to conserve water while still effectively removing nitrogenous wastes from their bodies.

Protonephridia and metanephridia are two types of excretory organs found in different organisms. Protonephridia, found in organisms like platyhelminthes (flatworms), are networks of dead-end tubules that lack internal openings. They contain flame cells as their excretory structures, which are specialized cells with beating flagella that help move excretory products through the tubules. The excretory products are filtered and selectively reabsorbed within the system before being excreted through nephridiopores. Protonephridia primarily function as osmoregulators. Metanephridia, found in organisms like earthworms and other annelids, have tubular structures with internal openings called nephrostomes that allow direct communication with the body cavity. The excretory products are collected from the coelom through the nephrostomes and are excreted through nephridiopores. Metanephridia function both as osmoregulators and as organs of excretion, playing a more comprehensive role in maintaining water balance and removing metabolic wastes from the organism.

Urine formation involves three main processes namely Glomerular Alteration, tubular reabsorption and tubular secretion.
I. Glomerular filtration:
* Blood enters the kidney from the renal artery into the glomerulus.
* Blood is composed of water colloidal proteins, sugars and nitrogenous end product.
* The filteration is started in the glomerulus. The fluid that leaves the glomerular capillaries and enters the Bowman’s capsule is called the glomerular filtrate. It formed 170-180 litre within 24 hours.
* Glomerular membrane has large surface area. Blood enters the glomerulus faster with greater force through the afferent arteriole and leaves the glomerulus through the efferent arterioles much slower.
* This is because afferent arteriole is wider than efferent arteriole and the glomerular hydrostatic pressure is around 55 mm Hg.
* Molecules larger than 5mm are barred from entering the tubule.
The two opposing forces are contributed by the plasma proteins in the capillaries.
* Colloidal osmotic pressure – 30 mm Hg
* Hydrostatic pressure -15 mm Hg
Both pressures combine. 30 mm Hg + 15mm Hg = 45mm Hg
* The net filtration pressure of 10mm Hg is responsible for the renal filtration.
* Net filtration pressure – Glomerular
* Hydrostatic Pressure – Colloidal Osmotic pressure + Capsular hydrostatic pressure.
Net filteration pressure = 55 mm Hg – (30mm Hg +15mm Hg) = 10mm Hg
Substance
Concentration in blood Plasm / g dm -3
Concentration in glomerular filtrate / g dm -3
Water
900
900
Proteins
80.
0.05
Aminoacids
0.5
0.5
Glucose
1.0
1.0
Substance
Concentration in blood PIasm /gdm -3
Concentration glomerular filtrate/gdm -3
Urea
0.3
0.3
Uric Acid
0.04
0.04
Creatinine
0.01
0.01
Inorganic ions (mainly (Na +, K + and Cl – )
7.2
7.2
II. Tubular reabsorption:
* The volume of filtrate formed per day is around 170 – 180l.
* Urine released is around 1.51 per day. 99% of the glomerular filtrate is reabsorbed by the renal tubules.
Reabsorption in proximal convoluted tubule:
* Glucose lactate amino acids Na in the filtrate are reabsorbed in the PCT.
* Sodium is reabsorbed by active transport through a sodium-potassium pump in the PCT.
* Small amounts of urea and uric acid are reabsorbed.
Reabsorption in Henle Loop:
* Descending limb of Henle’s loop is permeable to water due to the presence of aqua porlns but not permeable to salts.
* Water is lost in the descending limb. Hence Na and Cl get concentrated in the filtrate.
* Ascending limb of Henle’s loop is impermeable to water but permeable to Na +, Cl – andK +.
Distal Convoluted Tubule:
* It recovers water and secretes potassium into tubule.
* Na +, Cl -, and water remain in the DCT.
* Reabsorption of HCO 3 takes place to regulate the blood pH.
* Collecting the tubule is permeable to water potassium ions are actively transported into the tubule and Na + to produce concentrated urine.
Tubular Secretion:
* Once it enters the collecting duct water is absorbed and concentrated hypertonic urine is formed.
* For every H secreted into the tubular filtrate, a Na + is absorbed by the tubular cell.
* The H + secreted combines with HCO + 3, HPO – 3, and NH + and gets fixed as H 2 CO 3, H 2 PO 3, and NH 4 +.
* Since H + gets fixed in the fluid reabsorption of H + is prevented.

Cortical nephrons and medullary nephrons differ in their anatomical location and the extent of their loop of Henle. Cortical nephrons have their renal corpuscle and convoluted tubules located in the cortex of the kidney, and their loop of Henle is short, extending only minimally into the medulla. These nephrons lack a well-developed vasa recta, or the vasa recta is very short or absent. Cortical nephrons are more numerous and are primarily involved in the filtration and reabsorption of small amounts of solutes and water. In contrast, medullary nephrons, also called juxtamedullary nephrons, have their renal corpuscle located near the corticomedullary junction, and they possess a long loop of Henle that extends deep into the medulla, sometimes reaching the papilla. Medullary nephrons have an extensive vasa recta that runs parallel to the loop of Henle, forming a countercurrent multiplier system. This anatomical arrangement allows medullary nephrons to generate and maintain the osmotic gradient in the medullary interstitium, which is essential for the concentration of urine and water conservation. Although fewer in number, medullary nephrons play a crucial role in producing concentrated urine when the body needs to conserve water.

Tubular secretion is the active transport process by which specific substances are moved from the blood in the peritubular capillaries into the tubular fluid or filtrate within the renal tubules. Unlike glomerular filtration, which is a passive process, tubular secretion is an active process that requires energy in the form of ATP and can move substances against their concentration gradient. The substances secreted through the renal tubules include hydrogen ions (H+), potassium ions (K+), ammonium ions (NH4+), creatinine, and organic acids such as uric acid and certain drugs. These substances are secreted primarily in the proximal convoluted tubule and the distal convoluted tubule. Tubular secretion serves important functions including the regulation of blood pH by secreting excess hydrogen ions, the regulation of potassium levels in the blood, the elimination of metabolic wastes like creatinine and uric acid, and the excretion of foreign substances and drugs. This process ensures that potentially harmful substances are removed from the blood and added to the urine for elimination from the body.

• When the volume of blood decreases the flow pressure, decreases.
• This can be sensed by the hypothalamus and osmoreceptors are stimulated and the antidiuretic hormone is secreted from the neurohypophysis.
• The aquaporins in the proximal convoluted tubules and collecting tubule reabsorb water. Hence the blood volume increases and the blood pressure increases.

• ADH which is also called vasopressin or Antidiuretic hormone is secreted from the neurohypophysis.
• Fluid loss or if blood pressure increases the osmoreceptors of the hypothalamus is stimulated and hence neurohypophysis is stimulated and secretes ADH.
• When the fluid level and pressure are maintained due to the negative feedback mechanism ADH secretion stops.

• Due to the stimulation of Angiotensin II the adrenal cortex secretes aldosterone. That causes reabsorption of Na +, K + excretion, and absorption of water from distal convoluted tubule and collecting tubule.
• This mechanism is known as Renin – Angiotensin Aldosterone System.

The heart plays an important role in regulating renal function through the secretion of atrial natriuretic peptide (ANP), also called atrial natriuretic factor (ANF). When blood volume increases, excessive stretching of the cardiac atrial cells triggers the release of ANP into the bloodstream. This hormone travels to the kidneys where it acts on the afferent and efferent arterioles of the glomerulus. ANP functions as a vasodilator on the afferent arteriole, increasing blood flow into the glomerulus, and as a vasoconstrictor on the efferent arteriole, further increasing glomerular filtration pressure. Additionally, ANP increases sodium ion excretion in the kidneys and promotes increased blood flow to the glomerulus, thereby enhancing the filtration rate. This mechanism helps reduce blood volume and blood pressure when they are elevated, making ANP an important natriuretic hormone that works in conjunction with other regulatory mechanisms to maintain homeostasis.

Various animals produce different nitrogenous waste products depending on their habitat and metabolic requirements. In addition to ammonia, urea, and uric acid, marine teleosts produce trimethylamine oxide (TMO), which helps them maintain osmotic balance in saltwater environments. Spiders excrete guanine as their primary nitrogenous waste. Mammals, reptiles, and other vertebrates produce hippuric acid as a nitrogenous waste product. Other nitrogenous wastes found in different animals include allantonin and allantoic acid, which are produced during purine metabolism. Additional nitrogenous compounds excreted by various organisms include ornithiuric acid, creatinine, creatine, purines, pyrimidines, and pterines. The type of nitrogenous waste produced by an animal reflects its evolutionary adaptation to its specific environment and its metabolic pathways.

Net filtration pressure is the effective pressure that drives the filtration of water and small solutes from the blood in the glomerulus into Bowman's capsule. It is calculated by considering three opposing forces: the glomerular hydrostatic pressure, which favors filtration, and two forces that oppose filtration, namely the colloidal osmotic pressure of blood and the capsular hydrostatic pressure. The formula for net filtration pressure is: Glomerular Hydrostatic Pressure minus Colloidal Osmotic Pressure minus Capsular Hydrostatic Pressure. Using the standard values, the calculation is 55 mm Hg minus 30 mm Hg minus 15 mm Hg, which equals 10 mm Hg. This positive net filtration pressure of 10 mm Hg is responsible for forcing water, glucose, amino acids, urea, and other small molecules out of the glomerular capillaries into the capsular space, initiating the process of urine formation.

A hypertonic solution is one in which the solute concentration is higher than that of another solution separated by a semi-permeable membrane. When two solutions A and B are separated by a semi-permeable membrane, if water moves from solution A to solution B across the membrane, then solution B is hypertonic and solution A is hypotonic. In a hypertonic solution, the higher concentration of solutes creates a lower water potential, causing water to move into this solution by osmosis. Cells placed in a hypertonic solution lose water and undergo crenation or plasmolysis. The hypertonic solution has a greater osmotic potential, drawing water molecules from the hypotonic solution across the semi-permeable membrane until equilibrium is reached or the cell becomes severely dehydrated.

• When we drink or eat salty products the Na+ enters into the body fluids.
• The sodium ions helps in the reabsorption of water
• But when we drink only water as there is no sodium ions the tubules cannot reabsorb water.
• Hence there is an increase in the urine output.

Malfunction of the kidneys can lead to accumulation of urea and other toxic substances, leading to kidney failure. In such patients, toxic urea can be removed from the blood by a process called hemodialysis. A dialyzing machine or an artificial kidney is connected to the patient’s body. A dialyzing machine consists of a long cellulose tube surrounded by the dialyzing fluid in a water bath.
The patient’s blood is drawn from a convenient artery and pumped into the dialyzing unit after adding an anticoagulant like heparin. The tiny pores in the dialysis tube allow small molecules such as glucose, salts, and urea to enter the water bath, whereas blood cells and protein molecules do not enter these pores.
This stage is similar to the filtration process in the glomerulus. The dialyzing liquid in the water bath consists of a solution of salt and sugar in the correct proportion in order to prevent loss of glucose and essential salts from the blood. The cleared blood is then pumped back to the body through a vein.

• The blood vessels supplies to skin constricts and thus there is a decrease in the secretion of sweat prevents loss of water.
• There is a reduction in the glomerular blood pressure and the rate of filtration decreases.
• The reaborption of water in the proximal distal convoluted tubules increase.
• There is absorption of water from the small intestine and large intestine and thus increases the water content in the blood.

Aquaporins are water-permeable channel proteins present in cell membranes that facilitate the rapid transport of water molecules across epithelial cells. These channels are integral membrane proteins that allow water to move passively in response to osmotic gradients without requiring metabolic energy. The primary function of aquaporins is to enable water movement from the lumen of the nephron to the interstitial fluid surrounding the peritubular capillaries, thereby facilitating water reabsorption. In the kidney, aquaporins are particularly abundant in the collecting duct and proximal convoluted tubule, where they play a crucial role in concentrating urine and maintaining water balance in the body. The movement of water through aquaporins is driven by the osmotic difference created by the active reabsorption of solutes such as sodium ions and glucose, allowing the kidney to regulate body fluid osmolarity and conserve water when necessary.

• The main function of Henle’s loop is to reabsorb water from filtrate.
• If the length of the loop is longer then there is more reabsorption of water and if the lengh of Henle’s loop is shorter then the reabsorption of water is less.

The capsular membrane of Bowman's capsule acts as a selective barrier during ultrafiltration. Blood cells such as red blood cells and white blood cells, along with most blood proteins including albumin and other large plasma proteins, are too large to cross the capsular membrane and therefore remain in the blood within the glomerular capillaries. However, the membrane possesses numerous slits and pores that allow smaller molecules to pass through into the capsular space. These smaller molecules include water, mineral salts such as sodium and potassium ions, glucose, amino acids, polypeptides, and various waste products including urea, ammonia, and creatinine. This selective permeability ensures that useful substances and cells are retained in the blood while waste products are filtered out to form the glomerular filtrate.

Blood enters the glomerulus through the afferent arteriole, which branches from the renal artery. The afferent arteriole is wider in diameter than the efferent arteriole that exits the glomerulus, creating a pressure gradient that forces blood into the knot of capillaries forming the glomerulus. The blood enters at high hydrostatic pressure, which is sufficient to force water and other small solutes such as glucose, amino acids, urea, and ions out of the capillaries into the capsular space of Bowman's capsule. This process is called ultrafiltration and is the first step in urine formation. The large proteins and blood cells remain in the capillaries because they are too large to pass through the filtration barrier.

The proximal convoluted tubule is the region of the nephron located immediately after Bowman's capsule. This tubule is lined with cuboidal epithelial cells that possess numerous mitochondria and microvilli, providing a large surface area for selective reabsorption. In this region, much of the water filtered from the blood is reabsorbed back into the surrounding peritubular capillaries through osmosis. Additionally, useful substances such as glucose, amino acids, mineral salts including sodium and potassium ions, and vitamins are selectively reabsorbed from the filtrate into the blood through both active transport and facilitated diffusion. This selective reabsorption ensures that essential nutrients and water are conserved and returned to the bloodstream, while waste products remain in the filtrate to be excreted.

The peritubular capillaries, also called the vasa recta, form a network of blood vessels that surrounds the renal tubules. These capillaries play a crucial role in reabsorption, recovering up to 99 percent of the water that was initially filtered into the tubule. The peritubular capillaries reabsorb various useful substances from the filtrate including glucose, amino acids, and mineral salts through both passive and active transport mechanisms. Active transport pumps in the capillary endothelium actively transport sodium ions from the blood into the tubular fluid while simultaneously reabsorbing other ions and water back into the blood. This bidirectional movement of substances helps maintain the osmotic gradient necessary for water reabsorption and ensures efficient recovery of essential substances while concentrating the remaining waste products in the urine.

• As the loop of the Henle dipsin to the renal medcula more water moves from the tubule into the blood as well as small amounts of salts and some urea and creatinine.
• Some acids and amines may move into the tubule in which ammonia cango in both the direction.

• Distal tubule is far from capsule. This region may see water go in or out of the tubule depending on the concentration of water already in the tubule/ while hydrogen and potassium ions move to regulate both blood and urine pH.
• Acids amines and ammonia compounds may also transported into the tubule.

• Fine adjustment of urine composition continues into the collecting duct system.
• About 5 percent of all the water and sodium being reabsorbed into the blood is recovered here.

The efficiency of the kidney can be estimated by calculating the renal clearance, which is the volume of blood completely cleared of a particular substance per unit time. If the renal clearance of a substance is equal to the glomerular filtration rate, it indicates that the substance is filtered at the glomerulus but not reabsorbed or secreted in the renal tubules, suggesting efficient filtration with little or no reabsorption and secretion. If the renal clearance is less than the glomerular filtration rate, it indicates that some of the filtered substance has been reabsorbed in the tubules. If the renal clearance is greater than the glomerular filtration rate, it indicates that the substance has been secreted into the tubules in addition to being filtered. By comparing the renal clearance values of different substances, we can assess the kidney's ability to selectively filter, reabsorb, and secrete various substances, thereby evaluating its overall efficiency in maintaining the composition of blood and producing urine.

Different organisms possess specialized excretory structures adapted to their body organization and habitat. In invertebrates, the primary excretory structures are protonephridia and metanephridia. Platyhelminthes such as planarians use flame cells, which are specialized cells with a tuft of cilia that help move waste through tubules. Amphioxus possesses solenocytes, which are similar to flame cells but found in this chordate. Nematodes like Caenorhabditis elegans have renette cells that function in osmoregulation and excretion. Annelids such as earthworms and polychaetes use metanephridia, which are more complex tubular structures with funnels that collect coelomic fluid. Insects including grasshoppers and beetles have Malpighian tubules, which are blind-ended tubules that extend into the hemocoel and actively transport wastes into the tubule lumen. Crustaceans such as prawns and crabs possess green glands, also called antennal glands, which are located near the base of the antennae and function in excretion and osmoregulation. These diverse excretory structures reflect the evolutionary adaptations of different organisms to their specific environments and physiological needs.

• In the reptiles the glomerulus is reduced or there may be no glomerulus and 1-lenle’s loop and hence produces dilute urine (chypotonic).
• In the mammals the long Henle’s loop produces concentrated urine (hypertonic)
• A glomerular kidneys of marine fishes produce little urine that is isoosmotic to the body fluid.
• Amphibians and freshwater fish lack Henle’s loop hence produce dilute urine.

a. Each kidney weighs an average of 120 – 170 gms.
The outer layer of the kidney is covered by three layers of supportive tissue namely renal fascia perirenal fat capsule fibrous capsule.
b. Draw the LS of kidney and name the parts.
c. Internal Structure of kidney
* The longitudinal section of kidney shows an outer cortex inner medulla and pelvis.
* The inner concave surface of the kidney is renal hilum through which ureter blood vessels and nerves enter.
* Inner to the hium is a funnel shaped renal pelvis with projection called calyces.
* The calyces collect the urine and empties in to the ureter.
* The medulla consists of conical tissues called medullary pyramids or renal pyramids.

a) Hie structural and functional unit of kidney is nephron. It is composed of Malpighian body or renal corpuscle and Urine ferous tubule.
b) Structure of nephron
c. Malpighian body/Renal Corpuscle.
* The Bowman’s capsule and the glomerulus together constitutes Malpighian corpuscle.
* Bowman’s capsule is made up of two layers. It contains blood vessels called glomerules.
* The endothelial of glomerulus has many pores. The viscral layers of glomerulus is made of epithelial cells called podocytes.
* Tire podocytes end in foot processes which cling to the basement membrance of the glomerulus.
* The openings between the foot processes are called filtration slits.

a)
1. proximal convoluted tubule.
2. Henle’s loop
a. Thindescending limb of Henle’s loop.
b. Thick ascending limb.
2. Distal convoluted tubules
The distal convoluced tubules opens in to acollecting duct.
Several collecting ducts fuse to form papillary duct that delivers urine in to the calvces which opens into the renal pelvis.
b) The PCT and DCT are situated in the cortical region of the kidney.
The loop of Henle is in the medulla region.
c) The loop of Henle is too short and extends only very little into the medulla and are called cortical nephron.
Some nephrons have very long loops of Henle that run deep into the medulla and are called Juxta medullary nephrons.

The capillaries of the nephron consist of two main types: the glomerular capillary bed and the peritubular capillaries. The glomerular capillary bed is formed by the afferent arteriole, which branches into a network of capillaries within Bowman's capsule, and the efferent arteriole, which collects blood from the glomerulus. The afferent arteriole is broader in diameter than the efferent arteriole, creating a pressure gradient that facilitates ultrafiltration. The glomerular capillaries have a specialized structure with fenestrations that allow water and small solutes to pass through while retaining large proteins and blood cells. The efferent arteriole that exits the glomerulus forms a fine capillary network called peritubular capillaries that surrounds the renal tubule, facilitating the reabsorption of useful substances back into the blood. In juxtamedullary nephrons, the efferent arteriole serving the glomerulus forms bundles of long, straight vessels called vasa recta that run parallel to the loop of Henle. The vasa recta are specialized for maintaining the osmotic gradient in the medulla by allowing selective reabsorption and secretion of ions and water, which is essential for the concentration of urine. These capillary networks work together to ensure efficient filtration, reabsorption, and secretion in the process of urine formation.

The glomerular filtrate resembles blood plasma because during ultrafiltration, small molecular weight substances are filtered from the blood into the Bowman's capsule, while large molecules like plasma proteins are retained in the blood due to their size. The filtrate contains all the substances present in blood plasma except the plasma proteins, which cannot pass through the filtration barrier. This is why the composition of glomerular filtrate is similar to blood plasma in terms of water, glucose, amino acids, urea, uric acid, creatinine, and inorganic ions. The following table shows the concentration of various substances in blood plasma and glomerular filtrate: Substance, Concentration in blood plasma (g dm⁻³), Concentration in glomerular filtrate (g dm⁻³); Water, 900, 900; Proteins, 80.0, 0.05; Amino acids, 0.5, 0.5; Glucose, 1.0, 1.0; Urea, 0.3, 0.3; Uric acid, 0.04, 0.04; Creatinine, 0.01, 0.01; Inorganic ions (mainly Na⁺, K⁺ and Cl⁻), 7.2, 7.2. The presence of proteins in the filtrate is negligible (0.05 g dm⁻³) compared to blood plasma (80.0 g dm⁻³), which is the main difference between the two fluids.

Tubular Secretion:
* The collecting tubule of nephron secrete H + NH 4, Creatinine and Organic acid and liberated into the tubules and excreted through urine.
* Most of the water is absorbed in the proximal convoluted tubule.
* Na – is exchanged for water in the loop of Henle.
* The hypotonic fluid enters the distal convoluted tubule.
* Substances such as urea and salts pass from peritubular blood into the cells as distal convoluted tubule and then to collecting duct.
* Water is absorbed and concentrated hypertonic urine is form ed.
* For every H + secreted into the tubular filtrate a Na + is absorbed by the tubular cell.
* The H secreted combines with HCO 3, HPO 3 and NH 3 and gets fixed as carbonic acid CH 2 CO 3 and Phosphoric acid CH 2 PO 4
* Since H + gets fixed in the fluid reabsorption of H + is prevented.

a) The major function of Henle’s loop is to concentrate Na + and Cl –.
* There is low osmolarity near the cortex and high osmolarity towards the medulla.
* This osmolarity in the medulla is due to the presence of the solutes transporters and is maintained by
the arrangement of the loop of Henle collecting duct and vasa recta.
* The osmolarity of interstitial fluid is 300 m Osm.
* The Henle’s loop create a countercurrent multiplier.
* As the fluid enters the descending limb water moves from the lumen into the inter stitial fluid the osmolarity reduces.
* To counteract this dilution the region of the ascending limb actively pumps solutes from the lumen into the interstitial fluid and the osmolarity increases to about 1200 m OSM in medula.
b) The vasa recta maintains the medullary osmotic gradient via counter current exchanger.
* The counter current exchanger of vasa recta preserves the medullary gradient while removing reabsorbed water and solutes.
* The vasa recta leaves the kidney at the junction between the cortex and medulla.
* When the blood leaves the efferent arteriole and enters vasa recta the osmolarity in the medulla increases (1200 rnOsm) and result in passive up take of solutes and loss of water.
* As the blood enters the cortex the osmolarity in the blood decreases and the blood loses solutes and gain water to form concentrated urine.

Vasa recta maintains the medullary osmotic gradient via counter current exchanger.
* Vasa recta preserves the medullary gradient while removing reabsorbed water and solutes through counter current exchanges.
* The vasa recta leave the kidney at the junction between the cortex and medulla.
* The interstitial fluid at this point is iso – osmotic to blood.
* When the blood leaves the efferent arteriole and enters the vasa recta the osmalarity in the medulla increases (1200 mOsm) and results in passive up take of solutes and loss of water.
* As the blood enters the cortex the osmolarity in the blood decreases (300mOsm) and the blood loses solutes and gains water to form concentrated urine.
* Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.

The kidney functioning is regulated by several important structures. The hypothalamus acts as the primary regulatory center by monitoring blood osmotic pressure through osmoreceptors. The juxtaglomerular apparatus, located in the afferent arteriole of the nephron, monitors blood pressure and regulates the renin-angiotensin-aldosterone system. The heart also plays a regulatory role through the secretion of atrial natriuretic peptide. Antidiuretic hormone (ADH), also called vasopressin, is secreted by the neurohypophysis in response to stimulation by osmoreceptors in the hypothalamus. When there is excessive loss of fluid from the body or when there is an increase in blood osmotic pressure, the osmoreceptors of the hypothalamus are stimulated. These osmoreceptors then stimulate the neurohypophysis to secrete ADH. ADH facilitates the reabsorption of water by increasing the number of aquaporins on the cell surface of the distal convoluted tubule and collecting duct, thereby preventing excessive loss of water from the body. This hormone makes the collecting duct more permeable to water, allowing more water to be reabsorbed back into the blood and reducing the volume of dilute urine produced. Diabetes insipidus is a condition caused by deficiency or absence of ADH secretion. The symptoms of diabetes insipidus include excessive thirst (polydipsia), excretion of large quantities of dilute urine (polyuria), and a fall in blood pressure due to excessive water loss from the body.

a) Renin is secreted by granular cells.
b) Granular cells are present in the afferent arteriole.
c) The role of renin
* A fall in glomerular blood flow blood pressure and filtration rate can activate granular cells of juxtaglomerular cells to release renin.
* Renin converts the plasma protein angiotensinogen into angiotensis I and angiotensin II.
* Angiotensis II stimulates Na + reabsorption in the proximal convoluted tubule by vasoconstriction of the blood vessels and increases the glomerular blood pressure.
* Angiotensis II stimulates adrenal cortex to secrete aldosterone that causes reabsorption of Na +,K + excretion and absorption of water.
* This increases the glomerular blood pressure and glomerular filtration rate.
* Hence renin regulates osmoregulation.

Atrial natriuretic peptide (ANP) is liberated from the atrium of the heart, specifically from the cardiac muscle cells of the atrial wall. This hormone is released in response to increased blood pressure and increased blood volume. Atrial natriuretic peptide has several important physiological functions in regulating kidney function and blood pressure. It increases sodium excretion by the kidneys and increases the blood flow to the glomerulus, thereby increasing the glomerular filtration rate. ANP acts on the afferent glomerular arteriole as a vasodilator, increasing blood flow into the glomerulus, while simultaneously acting on the efferent arteriole as a vasoconstrictor, reducing blood flow out of the glomerulus. This combination of effects increases the pressure gradient across the filtration membrane, promoting increased filtration. Additionally, ANP reduces the secretion of aldosterone from the adrenal cortex and inhibits renin secretion from the juxtaglomerular apparatus. By reducing renin secretion, ANP decreases the formation of angiotensin II, which is a potent vasoconstrictor and promoter of sodium reabsorption. Thus, atrial natriuretic peptide acts antagonistically to the renin-angiotensin-aldosterone system and vasopressin (ADH), helping to lower blood pressure and increase sodium and water excretion when blood pressure is elevated.

Micturition is the process of release or expulsion of urine from the urinary bladder to the outside of the body. It is also called urination. The urine formed by the nephrons is transported through the collecting ducts and ureters to the urinary bladder where it is stored until the bladder receives appropriate signals from the central nervous system to expel the urine. The regulation of urination by the central nervous system involves a reflex arc mechanism. When the urinary bladder becomes filled with urine, the stretch receptors present in the bladder wall are stimulated by the distension of the bladder. These stretch receptors send sensory signals to the sacral spinal cord through sensory neurons. The sensory information is processed in the spinal cord, and motor signals are sent back to the bladder through parasympathetic neurons. At the same time, the internal sphincter, which is made of smooth muscle, opens involuntarily in response to these signals. The external sphincter, which is made of skeletal muscle and is under voluntary control, relaxes when the individual consciously decides to urinate. Once both sphincters open, the detrusor muscle of the bladder contracts, and the urine is expelled out through the urethra. In infants, micturition is purely a reflex action, but in adults, it can be voluntarily controlled through higher brain centers.

a) in the patients of kidney failure toxic urea can be removed from the blood by a process called haemodialysis.
b) The dialyzing machine is a artificial kidney.
c) Hemodialysis
* A dialyzing machine consists of a long cellulose tube surrounded by the dialysing fluid in a water bath.
* A patient’s blood is drawn from a convenient artery and pumped into the dialysing unit after adding an anticoagulant like heparin.
* The tiny pores in the dialysis tube allow small molecules such as glucose salts and urea to enter into the water bath.
* Whereas blood cells and protein molecules do not enter these pores the cleared blood is then pumped back to the body through a vein.

• This involves transfer of healthy kidney from one person (donor) to another person who is with kidney failure.
• The donated kidney may be taken from a healthy person who is declared brain dead or from sibling or close relatives to minimize the chances of rejection by the immune system of the host.
• Immuno suppressive drugs are usually administered to the patient to avoid tissue rejection.

The major nitrogenous wastes produced during protein metabolism are ammonia, urea, and uric acid. These are the primary end products of amino acid deamination and nucleic acid metabolism. Besides these major nitrogenous wastes, several other nitrogenous compounds are also formed during protein and nucleic acid metabolism. Trimethylamine oxide (TMO) is produced in some aquatic animals and helps in osmoregulation. Guanine is a purine base produced from the breakdown of nucleic acids. Allantoin is formed from the oxidation of uric acid and is excreted by some animals. Creatinine is produced from the metabolism of creatine phosphate in muscles and is a useful indicator of kidney function. Creatine is an amino acid derivative found mainly in muscle tissue. Purines are nitrogenous bases derived from nucleic acid metabolism. The relative amounts of these different nitrogenous wastes vary depending on the diet, metabolic state, and the type of organism, with different animals showing preferences for excreting different forms of nitrogenous waste.

Animals can be classified based on the form of nitrogenous waste they excrete. Ammonoteles are animals that excrete most of their nitrogen in the form of ammonia. Examples include aquatic animals such as bony fishes, aquatic amphibians, and aquatic insects. In bony fishes, ammonia is highly soluble in water and diffuses out across the body surface and through the gills into the surrounding water. Ammonia is toxic and requires large amounts of water for dilution, so this strategy is only feasible for aquatic animals. Uricoteles are animals that excrete uric acid in the form of crystals or paste with minimal loss of water. Examples include reptiles, birds, land snails, and insects. Uric acid is relatively insoluble in water and requires very little water for its excretion, making it an ideal nitrogenous waste for terrestrial animals living in dry environments. The conversion of ammonia to uric acid requires more energy but conserves water, which is advantageous for animals living on land. Ureoteles are animals that excrete urea as their primary nitrogenous waste. Examples include mammals and terrestrial amphibians. Urea is less toxic than ammonia and requires moderate amounts of water for excretion, making it a suitable compromise between water conservation and energy expenditure. The type of nitrogenous waste excreted by an animal reflects its evolutionary adaptation to its particular habitat and lifestyle.

• Reptiles have reduced glomerulus or lack glomerulus and Henle’s loop and produce hypotonic urine (dilute)
• Mammalian kidneys produce concentrated urine due to the presence of long Henle’s loop.
• Aglomerular kidneys of marine fishes produce little urine that is iso osmotic to the body fluid.
• Amphibians and fresh water fishlack Henle’s loop hence produce dilute urine.

The nephron contains two important capillary beds that play crucial roles in urine formation and selective reabsorption. The first capillary bed is the glomerulus, which is involved in ultrafiltration. Blood enters the glomerulus through the afferent arteriole, which has a larger diameter than the efferent arteriole. This size difference creates high hydrostatic pressure within the glomerulus, forcing small molecular weight substances through the filtration barrier into the Bowman's capsule. The filtered blood is then drained from the glomerulus through the efferent arteriole. The second capillary bed consists of the peritubular capillaries, which surround the renal tubule. The efferent arteriole, after leaving the glomerulus, branches to form a fine capillary network around the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. These peritubular capillaries are involved in the reabsorption of useful substances from the filtrate back into the blood through active and passive transport. In the case of juxtamedullary nephrons, which have their loops of Henle extending deep into the medulla, the efferent arteriole forms bundles of long, straight vessels called vasa recta. These vasa recta run parallel to the loop of Henle and are involved in the countercurrent multiplier mechanism that helps concentrate the urine. Vasa recta are absent in cortical nephrons, which have short loops of Henle that do not extend far into the medulla.

• In the proximal convoluted tubule glucose lacticacid aminoacid sodium ions are reabsorbed.
• Sodium is reabsorbed – potassium pump in the proximal convoluted tubule.
• Small amounts of urea and uric acid are also reabsorbed.

The filtrate undergoes significant changes as it passes through the loop of Henle, which is a U-shaped structure consisting of a descending limb and an ascending limb. These two limbs have different permeability characteristics that create a countercurrent multiplier system. In the descending limb of the loop of Henle, the epithelial cells contain aquaporins, which are water channel proteins that make the membrane highly permeable to water but impermeable to salts such as sodium ions (Na⁺) and chloride ions (Cl⁻). As the filtrate moves down the descending limb, water is reabsorbed into the surrounding peritubular capillaries and vasa recta by osmosis, while sodium and chloride ions remain in the filtrate. This causes the concentration of Na⁺ and Cl⁻ to increase progressively as the filtrate moves down the descending limb. The ascending limb of the loop of Henle has a different permeability profile. It is impermeable to water, preventing water reabsorption, but is permeable to solutes such as sodium ions (Na⁺), chloride ions (Cl⁻), and potassium ions (K⁺). In the thick portion of the ascending limb, active transport mechanisms actively pump out these ions into the interstitial fluid, further reducing the osmotic concentration of the filtrate. This countercurrent multiplier mechanism in the loop of Henle is essential for the production of concentrated urine and the conservation of water in the body.

Tubular reabsorption is the process by which useful substances are selectively reabsorbed from the filtrate back into the blood as it passes through the renal tubules. The volume of glomerular filtrate formed per day is approximately 170 to 180 liters, while the volume of urine released per day is only about 1.5 liters. This means that nearly 99 percent of the glomerular filtrate is reabsorbed by the renal tubules, a process called selective reabsorption. Selective reabsorption occurs because only useful substances such as glucose, amino acids, vitamins, and ions are reabsorbed, while harmful substances and excess water remain in the filtrate to be excreted as urine. Reabsorption takes place through the tubular epithelial cells in different segments of the nephron, including the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The mechanisms of reabsorption include active transport, which requires energy in the form of ATP and is used for reabsorbing glucose, amino acids, and ions against their concentration gradients; passive transport and diffusion, which allow substances to move along their concentration gradients without requiring energy; and osmosis, which allows water to move across the tubular epithelium in response to osmotic gradients. Different segments of the renal tubule have different permeability characteristics and reabsorb different substances, allowing for fine-tuned regulation of the composition and volume of urine produced.

• Depending on the body’s need the reabsorption taking place here and is regulated by hormones.
• Reabsorption of bicarbonate HCO 3 – takes place to regulate the blood pH.
• Homestasis of K + and Na + in the blood is also regulated in this region.

Diabetes insipidus is a metabolic disorder characterized by the deficiency or complete absence of antidiuretic hormone (ADH), also called vasopressin, which is secreted by the neurohypophysis. This hormone deficiency results in the inability of the kidneys to concentrate urine and conserve water. The condition can also occur due to the inability of the kidneys to respond to ADH, a condition known as nephrogenic diabetes insipidus. The symptoms of diabetes insipidus are quite distinctive and include excessive thirst, medically termed polydipsia, which occurs because the body attempts to compensate for water loss by increasing fluid intake. Patients also experience the excretion of large quantities of dilute urine, a condition called polyuria, sometimes producing up to 20 liters of urine per day compared to the normal 1.5 liters. This excessive water loss through urine leads to dehydration and a fall in blood pressure, which can cause dizziness, weakness, and fatigue. If left untreated, diabetes insipidus can lead to severe dehydration and electrolyte imbalances. The condition is managed by administering synthetic ADH or by treating the underlying cause if it is nephrogenic diabetes insipidus.

• Specialized tissue in the afferent arteriole of ncphron is the juxta glomerular apparatus.
• It consists of macula densa and granular cells.
• The macula densa cells sense distal tubular flow and affect afferent alteriole diameter.
• The granular cells secrete renin.

• Female’s urethra is very short and its external opening is close to the analopening.
• Hence improper toilet habits can easily carry faecal bacteria into the urethra.
• The urethral mucusa is continuous with the urianary tract and the inflammation of the urethra is called urethriti’s.

Cystitis is a urinary tract infection that leads to inflammation of the bladder wall. It is one of the most common urinary tract infections and can be caused by bacterial infection, most commonly by Escherichia coli (E. coli), which normally inhabit the intestinal tract but can ascend through the urethra to infect the bladder. The infection causes irritation and inflammation of the bladder mucosa. The symptoms of cystitis include painful urination, medically termed dysuria, which is often accompanied by a burning sensation in the urethra. Patients experience urinary urgency, which is a frequent and persistent desire to urinate even when the bladder contains only small amounts of urine. The urine may appear cloudy or blood-tinged due to the presence of white blood cells, bacteria, and sometimes red blood cells in the urine. Back pain and headache often occur as associated symptoms. Other symptoms may include suprapubic pain, which is pain above the pubic bone, and in some cases, fever and chills if the infection spreads to the upper urinary tract. Cystitis is more common in females than males due to the shorter length of the female urethra, which allows bacteria easier access to the bladder. The condition is typically treated with antibiotics to eliminate the bacterial infection and with increased fluid intake to flush out the urinary tract.

Renal failure is a condition in which the kidneys lose their ability to filter metabolic wastes and excess water from the blood effectively. When the kidneys fail to excrete wastes, there is accumulation of urea, uric acid, and creatinine in the blood with a marked reduction in urine output. This leads to serious complications as toxic substances build up in the body. Renal failure is classified into two main types: Acute renal failure occurs suddenly due to severe injury, shock, or acute glomerulonephritis, and kidney function may be restored with proper treatment. Chronic renal failure develops gradually over months or years due to conditions like diabetes, hypertension, or chronic glomerulonephritis, and kidney damage is usually irreversible, eventually requiring dialysis or kidney transplantation for survival.

• Though the kidney stops its function abruptly there are chances for recovery of kidney function in acute renal failure.
• But in chronic failure there is a progressive loss of function of the nephron which gradually decreases the function of kidney.

Glomerulonephritis, also known as Bright's disease, is a serious kidney disease characterized by inflammation of the glomeruli of both kidneys. This condition is often caused by streptococcal infection in children, particularly following a streptococcal throat infection or skin infection. The immune system produces antibodies against the streptococcal bacteria, and these antibodies form immune complexes that deposit in the glomerular basement membrane, triggering an inflammatory response. This inflammation damages the filtration barrier and impairs the kidney's ability to filter waste products and regulate fluid and electrolyte balance. The symptoms of glomerulonephritis are varied and reflect the damage to the glomeruli. Hematuria, which is the presence of blood in the urine, occurs because the damaged glomerular membrane becomes permeable to red blood cells. Proteinuria, which is the presence of protein in the urine, results from the increased permeability of the filtration barrier to plasma proteins. Salt and water retention occurs due to impaired kidney function, leading to oliguria, which is a reduced urine output. The retention of salt and water causes an increase in blood volume and blood pressure, leading to hypertension. In severe cases, pulmonary edema may develop, which is the accumulation of fluid in the lungs due to increased hydrostatic pressure in the pulmonary capillaries. Other symptoms may include facial puffiness, particularly around the eyes, due to fluid retention, and general malaise. The condition may resolve spontaneously in children, but in adults, it can progress to chronic kidney disease and renal failure if left untreated.

• Transfer of healthy kidney from one person (donor) to another person with kidney failure is called kidney transplantation.
• The donated kidney may be taken from a healthy person who is declared brain death or from sibling or close relatives.
• Immuno supressive drugs are administered to the patient to avoid tissue rejection.

• The visceral layer of glomerulus is made of epithelial cells called podocytes and ends in foot processes which cling to the basement membrane of the glomerulus.
• The openings between the foot processes are called filtration slits.

• Blood enters the glomerulus faster with greater force through afferent arteriole.
• Because the afferent arteriole is broader than efferent arteriole that is why the pressure reduces when it goes through the efferent arteriole.

• The osmo receptors in the hypothalamus is stimulated.
• The neurohypophysis is stimulated and anti diuretic hormone is liberated.
• The aquaporins in the tubuler are increased and water is reabsorbed and enters into the interstitial cell and the water loss is rectified.

Acute renal failure
* Chronic renal failure
1. Acute renal failure
* In acute renal failure the kidney stops its function abruptly.
* There are chances for recovery of kidney function.
2. Chronic renal failure
In chronic renal failure there is a progressive loss of function of the nephron which gradually decreases the function of kidneys.

• When we drink much fruit juice the osmo receptors in hypothalamus is not stimulated and hence the secretion of vaso pressin from neuro hypophysis is reduced.
• The aquaporin escapes from collecting duct to cytoplasm and hence water reabsorption is prevented and formed dilute urine.

The level of urea in the blood is 17 -30 mg /100ml.
* In chronic kidney failure there is 10 times increase in urea level.
Five marks
IV. Detailed Answers –