Excretory system

• Animals accumulate substances like ammonia, urea, uric acid, carbon dioxide, water, and ions such as Na+, K+, Cl-, phosphate, and sulphate through metabolic activities or excess ingestion
• Ammonia, urea, and uric acid are the major forms of nitrogenous wastes excreted by animals
• Ammonia is the most toxic form and requires a large amount of water for elimination
• Urea and uric acid are less toxic forms of nitrogenous wastes
• Ammonotelism is the process of excreting ammonia, where it is excreted by diffusion across body surfaces or through gill surfaces in fish
• Terrestrial adaptation led to the production of less toxic nitrogenous wastes like urea and uric acid for water conservation
• Mammals, terrestrial amphibians, and marine fishes mainly excrete urea and are called ureotelic animals
• Reptiles, birds, land snails, and insects excrete nitrogenous wastes as uric acid with minimal water loss and are called uricotelic animals
• In humans, the excretory system consists of a pair of kidneys, ureters, a urinary bladder, and a urethra
• Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units
• Urine formation involves glomerular filtration, reabsorption, and secretion in different parts of the nephron
• Glomerular filtration is the filtration of blood through the glomerulus, where almost all constituents of plasma except proteins pass into the Bowman's capsule
• Nearly 99% of the filtrate formed by the kidneys has to be reabsorbed by the renal tubules
• Tubular cells secrete substances like H+, K+, and ammonia into the filtrate during urine formation
• Different segments of the nephron perform reabsorption either actively or passively
• Proximal Convoluted Tubule (PCT) reabsorbs essential nutrients, electrolytes, and water, and helps maintain pH and ionic balance
• Henle's Loop plays a significant role in maintaining high osmolarity of medullary interstitial fluid
• Distal Convoluted Tubule (DCT) allows conditional reabsorption of Na+ and water, and selective secretion of ions to maintain pH and balance in blood
• Collecting Duct extends from the cortex to the medulla, reabsorbs water to produce concentrated urine, and plays a role in maintaining pH and ionic balance
• Mammals can produce concentrated urine with the help of Henle's loop and vasa recta through a counter current mechanism
• The concentration gradient in the kidney ranges from about 300 mOsmolL –1 in the cortex to about 1200 mOsmolL –1 in the inner medulla
• The gradient is mainly caused by NaCl and urea
• NaCl is transported by the ascending limb of Henle’s loop and exchanged with the descending limb of vasa recta
• NaCl is returned to the interstitium by the ascending portion of vasa recta
• Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule
• The transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism
• The counter current mechanism helps to maintain a concentration gradient in the medullary interstitium
• Presence of the interstitial gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate (urine)
• Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed
• The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and to a certain extent, the heart
• Osmoreceptors in the body are activated by changes in blood volume, body fluid volume, and ionic concentration
• An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis
• ADH facilitates water reabsorption from latter parts of the tubule, preventing diuresis
• An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release
• ADH can also affect kidney function by its constrictory effects on blood vessels, causing an increase in blood pressure
• A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin, which converts angiotensinogen in blood to angiotensin I and further to angiotensin II
• Angiotensin II is a powerful vasoconstrictor that increases the glomerular blood pressure and GFR
• Angiotensin II also activates the adrenal cortex to release Aldosterone, causing reabsorption of Na+ and water from the distal parts of the tubule
• This complex mechanism is known as the Renin-Angiotensin mechanism
• An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF), which can cause vasodilation and decrease blood pressure