Micturition and Role of Other Organs in Excretion
சிறுநீர் கழித்தலும் கழிவுநீக்கத்தில் பிற உறுப்புகளின் பங்கும்
Urea is made in the liver, so the vessel leaving the liver — the hepatic vein — carries the most urea, not the renal artery and certainly not the renal vein.
The idea
Urine formed by the nephrons is carried to the urinary bladder, where it is stored until a voluntary signal is given by the central nervous system. Stretch receptors on the bladder wall send signals to the CNS when the bladder fills; the CNS passes on motor messages to initiate contraction of the smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, causing release of urine. This process of release of urine is called micturition, and the neural mechanism causing it is the micturition reflex. An adult human excretes on average 1 to 1.5 litres of urine per day; it is light yellow, slightly acidic (pH 6.0) and has a characteristic odour; on average 25-30 g of urea is excreted per day. Other than the kidneys, lungs, liver and skin also help in eliminating wastes. Lungs remove large amounts of CO2 (around 200 ml/minute) and significant quantities of water every day. The liver, the largest gland, secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, most of which pass out with digestive wastes. Sweat glands eliminate NaCl, small amounts of urea and lactic acid; sebaceous glands eliminate sterols, hydrocarbons and waxes through sebum. Small amounts of nitrogenous wastes can be eliminated through saliva too.
Why it matters
Storing urine and releasing it under control lets the body excrete continuously but empty only at convenient times; other organs share the load for wastes the kidney does not handle well.
NEET lens
Urine facts: 1-1.5 L/day, pH 6.0, 25-30 g urea/day. Other organs: lungs ~200 ml CO2/min; liver — bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, drugs via bile; sweat — NaCl, small urea, lactic acid; sebum — sterols, hydrocarbons, waxes. Highest urea: hepatic vein (urea made in liver).
Clinical significance
Spinal cord injury can disrupt the micturition reflex, leaving patients unable to control the bladder. Yellow skin (jaundice) appears when the liver cannot clear bilirubin into bile.
Introduction
Urine is stored in the bladder and released by the micturition reflex. Lungs, liver, skin and even saliva also remove some wastes.
Core concept
Micturition: bladder fills → stretch receptors → CNS → motor signals → bladder smooth muscle contracts + urethral sphincter relaxes → urine released (micturition reflex). Urine: 1-1.5 L/day; light yellow; slightly acidic (pH 6.0); 25-30 g urea/day. Lungs: ~200 ml CO2/min + water. Liver: bile with bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, drugs → mostly out with faeces. Skin: sweat — NaCl, small urea, lactic acid (main job: cooling); sebum — sterols, hydrocarbons, waxes (protective oily covering). Saliva: small amounts of nitrogenous wastes.
Go deeper
Urea is made in the liver from ammonia (ornithine/urea cycle), enters blood through the hepatic vein, and is removed when blood passes through the kidneys — so the renal vein carries the least urea.
From Class 10
Class 10 introduced the bladder storing urine. NEET adds the reflex and the excretory roles of lungs, liver and skin.