Describe the structure and functioning of nephrons.

STRUCTURE - a nephron consists of a renal corpuscle and a long complicated renal tubule.

• Renal corpuscle/ Malpighian corpuscle is a rounded structure consisting of glomerulus and Bowman’s capsule. A glomerulus is a rounded tuft of anastomosing blood capillaries which is invested by the Bowman’s capsule. The blood enters this tuft through afferent arterioles and leaves through an efferent arteriole.

• Renal tubule is divided into three parts- proximal convoluted tubule (PCT), loop of Henle and distal convoluted tubule (DCT) which ends by joining a collecting tubule.

PCT arises from Bowman’s capsule and traverses within the cortex. The cells of PCT are large, columnar and have brush borders in their apical surface. The loop of Henle consists of a thin descending limb, a loop and a thick ascending limb. The DCT has a straight part continuous with ascending limb of loop of Henle and a convoluted part lying in the cortex. The terminal part of DCT is straight and is called connecting tubule which ends by joining a collecting duct.

FUNCTIONING - Blood having wastes like urea are brought by renal artery into the glomerulus. Small molecules like glucose, urea, creatinine, amino acids and minerals salts are filtered into the Bowman’s capsule in concentration similar in the plasma. This filtrate is known as glomerular filtrate. As it courses through the renal tubules, its composition, pH and osmotic pressure change because of reabsorption of water and solutes in the tubules. Glucose and amino acids are rapidly reabsorbed actively & urea, ammonia, creatinine and ketone bodies are reabsorbed slowly from the tubules by diffusion until their concentrations in glomerular filtrate exceed their concentrations in the blood (which means that the substances can never be completely re-absorbed from urine). The filtrate passes through PCT, loop of Henle, DCT and the collecting duct from where it is drained into the pelvis and eliminated as urine outside the body.