EXCRETORY PRODUCTS AND THEIR ELIMINATION MADE BY S RATH PGT BIO K V III BBSR http://students-learn.blogspot.com

DEFINITION It is a process of waste removal in the form of ammonia, urea or uric acid produced during metabolic activities.

AMMONIA-THE WASTE Highly toxic Readily soluble in water Excreted by diffusion across body surface or through gill surface.

AMMONIA-THE WASTE Released as ammonium ions Kidney has no significant role in its removal. E.g. bony fishes, aquatic amphibians and aquatic insects

UREA-THE WASTE Ammonia produced in liver is converted into urea Less toxic waste than ammonia, so needs less water for its removal. The waste is released into blood and excreted out by kidneys Some amount of urea is retained in kidney matrix to maintain osmolarity. E.g. mammals, marine fish, terrestrial amphibians

URIC ACID-THE WASTE Less toxic than urea. Less soluble in water, so animals conserve water with minimum water loss. Waste is released in the form of paste or pellet. Reptiles, birds, land snail, land insects

EXCRETORY ORGANS PLATYHELMINTHES (PLANIRIA)-Protonephridia or flame cells ANNELIDA (EARTH WORM)- Nephridia INSECTS- Malpighian Tubules PRAWN- Antennal Gland Or Green Glands VERTEBRATES- Kidney

HUMAN EXCRETORY ORGANS A pair of kidney A pair of ureters Urinary bladder Urethra

L.S OF KIDNEY HILUM RENAL PELVIS CALYCES RENALCAPSULE CORTEX MEDULLA MEDULLARY PYRAMIDS RENAL COLUMN OR COLUMNS OF BERTINI RENAL ARTERY RENAL VEIN URETER

STRUCTURE OF NEPHRON Bowman’s capsule Glomerulus Malpighian body PCT Henel’s loop-ascending limb & descending limb DCT Collecting duct Afferent arteriole Efferent arteriole

PARTS OF NEPHRON

CORTICAL JUXTA MEDULLARY Glomeruli are in the outer cortex. Glomeruli are close to inner margin of the cortex. Short loops of henle. Long loops of henle Henle’s loops extend only to a short distance in the medulla. Henle’s loops are found deeper in the medulla. Vasa rectae are absent. Vasa rectae are present They are more common (85%) They are less common (15%)

GLOMERULAR FILTRATION A protein free fluid is filtered from the blood of glomerular capillaries into the lumens of bowman’s capsule. The filtration is caused by the blood pressure in the glomerular capillaries. Filtration occurs through three layers which form filtration membrane. They are endothelium of Glomerulus, epithelium of bowman’s capsule & basement membrane between the two layers. The epithelial cells (podocytes) of bowman’s capsule are arranged in an intricate manner to leave some filtration slits through which blood is filtered finely.

GLOMERULAR FILTRATION The glomerular filtration rate (GFR) is about 125ml/minute or 180L/day. JGA is a specialised cellular apparatus located where dct passes close to bowman’s capsule between afferent and efferent arteriole. A fall in GFR activates the cells of the JGA to release renin which through a series of reactions, brings the GFR back to normal.

REABSORPTION Nearly 90% of the filtrate is reabsorbed by epithelial cell lining of renal tubule. Glucose, amino acids, Na+, Ca++,K+ are reabsorbed actively. Other substances like Cl- are absorbed passively.

TUBULAR SECRETION It is the process in which certain substance or ions like K+ and ammonia are directly secreted into the lumen of nephron. This step is important because it helps to maintain ionic balance and pH of the body fluids.

ROLE OF DIFFERENT SEGMENTS OF NEPHRON Malpighian body- ultrafiltration of blood from glomerular capillaries to lumen of Bowman’s capsule. PCT- reabsorption of 70- 80% of electrolytes & water. Maintain pH and ionic balance of body fluids, by selective secretion of H+, NH4+, K+ into the filtrate and by absorbing HCO3 ions.

ROLE OF DIFFERENT SEGMENTS OF NEPHRON Henle’s loop- maintain high osmolarity of medullary interstitial fluid. Descending limb makes filtrate hypertonic and ascending limb makes filtrate hypotonic. DCT- Reabsorption of Na+ and water, reabsorbs HCO3-, secretes NH3, H+, K+ into filtrate. Maintains pH and sodium and potassium balance in blood. Collecting duct- Reabsorption of water, transport of small amount of urea into interstitial fluid, maintain pH and ionic balance of body fluids.

MECHANISM-URINE CONC.

COUNTER CURRENT MECHANISM Loop of henle and vasa rectae responsible for concentrating urine by a mechanism called counter-current system. The flow of filtrate in two loops of henle is in opposite direction & forms a counter-current system. The flow of blood in opposite direction in vasa rectae forms a counter-current system.

COUNTER CURRENT MECHANISM The two factors responsible for increasing the osmolarity towards the medullary interstitium are (i) the proximity between the loop of henle and vasa rectae (ii) the counter current system in them. The osmolarity in cortex is about 300mos/l and medulla 1200 mos/l, this gradient is maintained by NaCl & urea. Interstitial gradient of NaCl is maintained by loop of henle.

COUNTER CURRENT MECHANISM Concentration of NaCl increases in descending due to loss of water. In ascending limb the salt passes actively and passively into interstitium making filtrate dilute. Urea diffuse out from collecting duct reenters the ascending limb by diffusion. The remaining urea is excreted in the urine. In collecting duct water moves out by osmosis and urine becomes concentrated.

REGULATION OF KIDNEY FUNCTION BY HYPOTHALAMUS Change in osmolarity and volume of blood & osmoreceptors are stimulated. Stimulation of hypothalamus to release ADH from pituitary. ADH signals DCT and collecting duct to reabsorb water from filtrate. This increases blood volume which switch off osmoreceptors

REGULATION OF KIDNEY FUNCTION BY JGA Glomerular blood flow/volume or pressure decreases. JGA releases renin which converts angiotensinogen in the blood into angiotensin I and then angiotensin II. Angiotensin II is a powerful vasoconstrictor and increases the glomerular blood pressure and maintains the glomerular filtration rate.

REGULATION OF KIDNEY FUNCTION BY JGA Angiotensin II also activates the adrenal cortex to release aldosterone. Aldosterone stimulates the reabsorption of Na+ and water from PCT. This leads to an increase in the blood volume and pressure and brings back the GFR to normal.

REGULATION OF KIDNEY FUNCTION BY ATRIAL WALL An increase in the blood flow (volume) and pressure to the atria of the heart causes the release of Atrial Natriuretic Factor (ANF). ANF causes vasodilatation and decreases the blood pressure. Thus ANF functions antagonistically to the renin- angiotensin mechanism by inhibiting the release of renin.