Zoology Grade-XII

Name: Dinesh Ghimire. Zoology Grade-XII.

Excretion and Osmoregulation. The process of removing metabolic wastes from the body is called excretion . The main waste products are excess of water, carbon dioxide, ammonia, urea, uric acid, salts etc. The excretory system not only eliminates waste materials but also plays a strong role in maintaining homeostasis by regulating the balance of water and other substances in the body. In mammals, kidneys are the major excretory organs. Besides kidneys; lungs, skin, liver & intestine also serve as accessory excretory organs. Types of kidney On the basis of their origin, Kidneys are of following types: Holonephric kidney The nephrons are formed by the somites from anterior to posterior region of the mesomere. Simplest type of kidney. Found in larva of Myxine, Adult apodans (Caecilians) Pronephric kidney The nephrons are formed by the somites from anterior region of the mesomere. Present in the embryo and larva of all the vertebrates. Functional pronephrous kidneys present in adult are called head kidneys. Found in tadpole larva of frog, Adult Myxine. Mesonephric kidney The nephrons are formed by the somites present in the middle region of the mesomere. Eg. Most of the fishes and amphibians. 4. Metanephric kidney : The nephrons are formed by the somites from posterior region of the mesomere. These are the most developed type of kidneys. Found in Reptiles, Birds and mammals..

Excretory system in man. The main excretory organs in man include a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. kidneys Kidneys are the chief excretory organs in human beings. There are a pair of kidneys which are small, bean-shaped, reddish brown organs located in the abdominal cavity. They lie one on either side of the vertebral column just beneath the diaphragm. They remain attached to the dorsal abdominal wall by a fold of peritoneum called Mesorchium. These are Retroperitoneal organs ; placed outside the coelom, behind the peritoneum. Each kidney is about 12cm long, 6cm wide, 2.5cm thick and about 150gm in weight. The right kidney is slightly lower than the left (due to liver lobe)..

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The outer surface of kidney is convex while the inner surface is concave. Depression found in the middle of concave region is known as the Hilus . Renal artery, renal vein and nerve fibre pass in or out of the kidney through the Hilus. Kidney consists of two parts: outer cortex, inner medulla. Cortex invaginates medulla forming Renal Column of Bertin , which divide medullary region into 15-16 medullary pyramids. Each medullary pyramid contains a large number of collecting ducts known as Ducts of Bellini . All ducts of Bellini open into minor calyces (6-8), these minor calyx unite to form major calyx (2-3). All the major calyces unite to form pelvis which leads to the ureter..

Nephrons. A nephron is a functional and structural unit of kidney. There are about 1 million nephrons in each kidney. Each one works as an independent unit that produces a miniscule amount of urine. Each nephron or uriniferous tubule is made up of two parts- Malpighian body and Renal tubules. A. Malpighian body – It consists of Bowman’s capsule and a network of blood capillaries, Glomerulus. The proximal end of each nephron forms a large double-walled cup, the Bowman’s capsule in the cortex of kidney. The cup contains a network of blood capillaries called glomerulus . The Bowman’s capsule and glomerulus together constitute the malpighian capsule. Each glomerulus in Bowman’s capsule contains about 50 capillaries network. It receives blood from renal artery through the afferent arterioles which breaks up into capillaries network. The blood leaves the glomerulus through efferent arteriole..

Visceral layer of Bowman’s capsule is made up of squamous epithelium called podocytes . Podocytes have finger like outgrowths bearing fine pores called pedicels. The filtrate released from capillaries enter into the podocytes. The contents fail to enter into podocytes are removed by the phagocytic cells called Mesangial cells..

B. Renal tubule – The remaining part of nephron behind the malphigian capsule is called secretory tubules which may be differentiated into three parts. Proximal convoluted tubules : The M alpighian capsule is followed by a slender tube like portion called the neck. The neck leads into a coiled tube called proximal convoluted tubule . It lies in the cortex region. This tubule is about 0.05mm in diameter and 15mm in length. It is lined with tall cuboidal cells. The main function of the proximal convoluted tubule is to reabsorb water, amino acids, glucose etc. from the glomerular filtrate. The loop of Henle: The proximal convoluted tubule continues into a ‘U’-shaped region known as the loop of Henle. It has two limbs descending limb & ascending limb . The wall of the descending limb of this loop is much thinner and permeable to water. The wall of the ascending limb is thicker and impermeable to both water & sodium chloride..

Distal convoluted tubule: The ascending limb gives rise to a coiled structure; the second convoluted tubule called distal convoluted tubule . This tubule open into a straight tube called the collecting tube or duct which further units to form larger ducts called ducts of Bellini. Later it opens into renal pelvis. Ureter - The pelvis of each kidney remains continued into a thick walled narrow cylindrical tube called ureter. It is about 30cm long. It runs downwards along the dorsal abdominal body wall and Open posteriorly into a common urinary bladder and bring the urine..

Mechanism of Urine formation. Urine formation is very complicated excretory function of the kidney. The various types of metabolic wastes produced in the body are eliminated through urine. The process of removal of nitrogenous wastes especially in the form of urea is called ureotelism and the organisms following this process are ureotelic organisms. Mammals including man are ureotelic. Nephrons, the functional unit of kidney performs the filtration of blood plasma and forms urine. The entire process of urine formation involves three stages: 1. Glomerular filtration (Ultra filtration of blood) 2. Selective reabsorption 3. Tubular secretion or Active secretion ..

1. Glomerular filtration (Ultra filtration of blood).

2. Selective re-absorption. This is the second step in the formation of urine. The glomerular filtrate also contains some useful substances and those substances are selectively reabsorbed from the filtrate into the blood. The tubule of the nephron plays an important role in re-absorption. Of the total fluid filtrated (about 180 ltr ), 99% of the filtrate is reabsorbed from renal tubules. PCT re-absorbed most of amino acids, vitamins, about 70% Na + , 75% K + , large amount of Ca ++ & CL - . The water & most of salts re-absorbed in PCT so filtrate in PCT becomes isotonic to blood plasma. The loop of Henle is divided into two parts- Descending limb and ascending limb. The descending limb is thin and permeable to water but ions are impermeable which increase the concentration of filtrate i.e. hypertonic. The ascending limb is impermeable to water but sodium passes out of the limb so filtrate become hypotonic. As the filtrate moves into the DCT and collecting duct, more water is reabsorbed, hence hypertonic urine is excreted out of the body. The reabsorption of water and sodium is controlled by the antidiuretic hormone and Aldosterone. 3. Tubular Secretion This is the third step in urine formation The blood flowing through the efferent arteriole also contains unwanted substances not cleared by the process of filtration. These substances are secreted into the tubule (DCT and CT) by the process called tubular secretion. These substances are H+, K+, ammonia, Creatinine, hormones, drugs etc. The Substances thus formed in the tubule after selective re-absorption and tubular secretion is called the urine. The urine now passes into the collecting duct, ducts of Bellini and then into the pelvis from where it passes on to the urinary bladder through the ureters..

2/11/2022. 13. DISTAL CONVOLUTED TUBULE Parathyroid Hormone GLOMERULUS efferen t afferent Atrial Natriuretic Peptide (ANP) Angiotensin II CORTEX OUTER MEDULU PROXIMAL CONVOLUTED TUBULE 2 4 cr Renin sugars amino acids Na4 3 Na LOOP Of HENLE (LON) K ca2+ Mg DESCENDING LIMB (permeable to water) Diuretic site of action: 1 Acetazolamide Osmotic diuretics (e.g. mannitol) 2 3 Loop diuretics (e.g. furosemide) Thiazides (e.g. HCTZ) 4 5 Potassium-sparing (e.g. spironolactone) ASCENDING LIMB of LOH (permeable to salts) . PTH • Vitamin D COLLECTING DUCT dosterone 5 Na H O (potentially) Antidiuretic Hormone (ADH) Cassandra Jy.

Composition of urine Urine is a transparent, pale yellow & slightly acidic fluid. The colour of the urine is due to the presence of pigment urochrome. The main constituents of normal urine is WATER=96% UREA= 2% SALTS=2% (sodium, potassium, chloride, phosphate, sulphates, calcium, magnesium, etc) Other substances as - uric acid, Ammonia, cretinine , Hippuric acid, Hormones, enzymes, etc. The characteristic smell of urine is due to the presence of chemical Urinoid (C 6 H 8 O). Osmoregulation It is the homeostatic process by which water content and concentration of salts in the body is maintained. Kidney plays an important role in maintaining water and osmotic concentration in the body. This phenomenon is called osmoregulation. Osmoregulation by kidney Kidney plays a key role in the osmoregulation in man. About 80% of the water of glomerular filtrate is re-absorbed by the proximal convoluted tubule. The salts of glomerular filtrate are also re-absorbed by the wall of this tubule. The fluid in this tubule becomes isotonic to blood. The loop of Henle acts as counter current multiplier systems. The flow of glomerular filtrate is in opposite directions in the ascending and descending limbs of a loop. The osmotic pressure in the descending limb rises (hypertonic) due to diffusion of water out from limb into surrounding tissue and sodium in from tissue into limb. The osmotic concentration in the ascending limb decreases (hypotonic) due to loss of sodium from limb to surrounding tissue but water cannot diffuse. When filtrate enter the distal convoluted tubule the water & salts both are diffused out from tubule into surrounding tissue so filtrate becomes isotonic to the blood..

As the isotonic filtrate enter in collecting duct, the water diffuses out into the surrounding tissue thus final urine becomes hypertonic to the blood. Thus, kidney prevents the water loss from the body and helps in Osmoregulation. Role of ADH in Osmoregulation The antidiuretic hormone (ADH) or vasopressin is released by posterior lobe of pituitary gland. This hormone affects (increase) the permeability of distal convoluted tubule (DCT) and collecting duct (CD). Balance of water when a person takes small amount of water When a person takes small amount of water and low water content in blood then increases the osmotic pressure in blood. It is detected by osmo- recepor in hypothalamus and simulate the Pituitary gland to increase the secretion of ADH. It increases the permeability of DCT & CD & increase in re-absorption of water from tubules into blood stream. Thus urine is more concentrated i.e. hypertonic . Balance of water when a person takes large amount of water When a person takes large amount of water and blood contains high amount of water then decrease the osmotic pressure in blood. It is detected by osmoreceptor in hypothalamus and stimulate to Pituitary gland to decrease the secretion of ADH. It decreases the permeability of DCT & CD & decrease in reabsorption of water from tubules into blood stream. Thus urine is less concentrated i.e. hypotonic. Any variation in the osmotic pressure of blood due to amount of water intake is quickly detected by osmoreceptor in hypothalamus & a homeostatic feedback mechanism operates..