Excretory Products and Their Elimination notes

Excretion is the removal of nitrogenous wastes and excess substances produced during metabolism. In humans, kidneys are the chief excretory organs and maintain water, salt and acid-base balance along with waste elimination. This chapter focuses on the human excretory system, structure of kidney and nephron, formation of urine by glomerular filtration, selective reabsorption and tubular secretion, and concentration of urine by the counter-current mechanism. It also explains osmoregulation, hormonal control by JGA, RAAS, ADH and ANF, and important disorders such as uremia, renal failure, kidney stones and glomerulonephritis. Dialysis is studied as an artificial method of removing wastes when kidneys fail.

The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are the main organs that filter blood and form urine. Each kidney has an outer cortex, inner medulla and renal pelvis. The medulla is organized into renal pyramids that drain into calyces and then into the renal pelvis. The nephron is the functional unit and includes the glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. Ureters carry urine to the bladder, the bladder stores it temporarily, and the urethra releases it. Blood reaches kidneys through renal arteries and leaves through renal veins.

Urine formation occurs in nephrons through four linked processes: glomerular filtration, selective reabsorption, tubular secretion and concentration of urine. In glomerular filtration, blood pressure forces water and small solutes into Bowman’s capsule while cells and most proteins remain in blood. The filtrate then passes through PCT, loop of Henle, DCT and collecting duct. Useful substances such as glucose, amino acids, water and ions are reabsorbed into blood. Additional wastes and ions like hydrogen, potassium and ammonia may be secreted into the tubule. The loop of Henle and vasa recta operate a counter-current mechanism that maintains a medullary osmotic gradient, allowing concentrated urine formation under ADH influence.

Osmoregulation is the maintenance of water and solute concentration in body fluids. Human cells function properly only when extracellular fluid osmolarity remains within a narrow range. Kidneys are the major organs of osmoregulation because they adjust the amount of water, sodium, potassium, chloride, bicarbonate and other ions excreted in urine. When water is excess, kidneys excrete dilute urine. When water is scarce, they conserve water by producing concentrated urine. Salt balance is regulated through reabsorption and secretion in nephron segments, especially under hormonal control. Osmoregulation is a part of homeostasis, ensuring stable internal conditions despite changes in diet, sweating, water intake or environmental temperature.

Kidney function is precisely regulated to maintain GFR, blood pressure, blood volume and osmolarity. The juxtaglomerular apparatus, located near the contact between the distal convoluted tubule and afferent arteriole, senses changes in blood pressure, sodium level and filtrate flow. When GFR or blood pressure falls, JGA releases renin, activating the renin–angiotensin–aldosterone system. Angiotensin II constricts blood vessels and stimulates aldosterone, which increases sodium and water reabsorption. ADH from the posterior pituitary increases water permeability of collecting ducts, conserving water. Atrial natriuretic factor from heart atria acts oppositely by promoting sodium and water excretion, reducing blood pressure and volume.

Excretory disorders arise when kidneys or urinary passages fail to remove wastes, regulate fluids or maintain normal urine flow. Uremia is the accumulation of urea and other nitrogenous wastes in blood due to poor kidney function. Renal failure is a severe reduction or loss of kidney function and may be acute or chronic. Renal calculi, commonly called kidney stones, are hard deposits formed from salts such as calcium oxalate. Glomerulonephritis is inflammation of glomeruli, often affecting filtration and causing blood or protein in urine. Urinary tract infections involve microbial infection of urethra, bladder or upper urinary tract. Understanding symptoms, causes and consequences helps in clinical NEET questions.

Dialysis is an artificial method of removing nitrogenous wastes, excess salts and excess water from blood when kidneys fail. It is based on diffusion of small solutes across a semipermeable membrane from higher concentration to lower concentration. In haemodialysis, blood is taken from the patient, passed through a dialyser or artificial kidney containing cellophane-like tubing, and surrounded by dialysing fluid. The fluid has normal plasma-like composition but lacks nitrogenous wastes, so urea and toxins diffuse out of blood. Heparin prevents clotting and the cleaned blood is returned to the body. Peritoneal dialysis uses the peritoneum as the membrane. Dialysis supports life in renal failure but does not fully replace all kidney functions.