Excretory Products And Their Elimination Revision notes For NEET


        Excretion is the process of removal of waste material from the body.

        Osmoregulation is the process of maintaining osmotic and ionic concentration of body fluid.

        Types of excretory products depends on the habitat, feeding habits and metabolism of an organism. It can be ammonia (ammonotelism), urea (ureotelism) and uric acid (uricotelism).

        Human excretory system consists of two kidneys, two ureters, a urinary bladder and a urethra.

        Human kidneys are mesodermal in origin, with retroperitoneal arrangement (present outside coelomic cavity). Externally, they are reddish brown, bean-shaped. Right kidney is smaller and placed lower compared to left one.

        Internally, human kidneys contain three protective layer, renal capsule, adipose capsule and renal fascia. Kidney is divided into two zones, i.e. an outer cortex and an inner medulla.

        Nephrons are the structural and functional unit of kidneys. They consists of renal corpuscle or Malpighian body and renal tubules.

        Renal corpuscles consists of Bowman’s capsule, i.e. a cup-shaped structure containing network of thin capillaries called glomerulus.

        Renal tubules are associated with process of reabsorption and secretion. It is formed of four parts, i.e. proximal convoluted tubule, Henle’s loop (descending, ascending and a main loop), DCT and collecting duct.

        Ureters are muscular tubes conveying urine from kidneys to the bladder. Urinary bladder, pear-shaped structure where urine is stored. Urethra is present in mammals only it is a canal extending from bladder to outside via external urethral orifice.

        Urea is main excretory product in humans, formed in the liver via deamination and ornithine cycle.

        Urine formation is called uropoiesis and occurs in three steps. These are ultrafiltration, selective reabsorption and tubular secretion.

        Ultrafiltration occurs when blood containing water and solutes enters the afferent arteriole in glomerulus. Glomerulus filtration rate (GFR) is the amount of filtrate formed by kidneys per minute (180 L/day).

        Selective reabsorption is the absorption of almost 99% of filtrate in renal tubules. 65% of filtrate reabsorbed in PCT (Na⁺, K⁺, glucose, amino acids, etc.) Reabsorption is minimum in Henle’s loop, absorption of Na⁺ occurs in DCT due to aldosterone. In collecting duct, water reabsorption occurs.

        Counter-current mechanism concentrates the urine produced in mammals in the loop of Henle.

        Tubular secretion is the phenomenon of secretion of metabolic wastes by tubular cells into the filtrate. It is an active process which occurs in PCT, DCT and collecting duct. It helps in maintenance of ionic and acid-base balance of body fluid.

        The blood supply to kidneys is maintained by the renal artery and renal veins.

        Regulation of kidney function occurs via.

  Juxtaglomerular apparatus, it works via Renin-Angiotensin Aldosterone System (RAAS).

  Antidiuretic hormone (ADH) or vasopressin secreted by neurohypophysis of pituitary gland. It is released when a change in blood volume, body fluid volume and ionic concentration by hypothalamus.

  Atrial natriuretic factor (ANF) works opposite to RAAS.

        Micturition is the release of urine from bladder. It occurs due to contraction and relaxation of urinary bladder and urethral sphincter. The signal for micturition originates from a stretched bladder.

        Urine is pale yellow (due to urochrome pigment), slightly acidic (pH-6), normally transparent and faintly aromatic.

        Accessory excretory organs are skin (sweat), lungs (CO₂), liver (bile pigments), intestine (inorganic ions), saliva, etc.

        Disorders of excretory system occurs due to failure of kidneys or malfunctioning of renal components, e.g. renal stones, nephritis (inflammation of kidney), renal failure, anuria, uremia, etc.

        Artificial kidney or dialysis is used when both kidneys get damaged.

        Renal transplantation is the ultimate cure for treating kidney failures. 

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