Mechanism of urine formation: M echanism of urine formation Basic function of nephrons is to clear out the blood plasma from unwanted substances. Urea , creatininie, uric acids, hippuric acids and drug remains etc cleaned by nephrons. The fluid comings out from the nephrons, containing these unwanted substances is called urine . Principle Mechanism of urine formation includes three basic processes: : Principle Mechanism of urine formation includes three basic processes: Ultrafiltration in Bowman’s capsule. Selective reabsorption of wanted substances from tubular filtrate. Secretion of unwanted substances from PTCN to tubular filtrate. 1. Ultrafiltration: 1. Ultrafiltration Ultrafiltration means the filtration of blood present in golmerular capillary through glomerular capillary wall, a basement membrane and wall of Bowman’s capsule. The filtrate is called glomerular filtrate or tubular filtrate or ultrafiltrate , which reaches the tubular lumen . The unfiltered volume of the blood remains the glomerular capillary which is moved to efferent arteriole. This membrane is almost completely impermeable to all plasma proteins. Factor effecting GFR:: Factor effecting GFR: Change in renal blood flow – If BP rises than EFP would also rise. This will increase the GFR. Change in capillary ghdrostatic pressure . Change in capsular pressure. Change in the permeability of capillary wall or capsular wall. Composition of glomerular filtrate: Composition of glomerular filtrate The fluid that enters the PCT, is called glomerular filtrate. It contains no RBC and blood cells . It also does not have plasma proteins . But this filtrate contains other soluble substances, which are present normally in plasma. The substances present in filtrate are glucose, urea, amino acids, calcium ions, sodium ions, bicarbonate ions, potassium ions, protons and a large amount of water . 2-Tubular reabsorption:: 2-Tubular reabsorption : The glomerular filtrate contains some very useful substances like glucose, water and various ions. These substances are useful for the body, hence must not pass out with urine. Therefore these are to be reabsorbed from the tubular lumen . Water and some ions are reabsorbed from all parts of the nephrons . Almost all sugars , vitamins , amino acids and other organic nutrients are reabsorbed in PCT. About 99% of the filtered water is reabsorbed . Major amount of water is absorbed in PCT. Nephron carries out a ‘counter current system’ to absorb more and more water from tubular filtrate. 3- Tubular secretion:: 3- Tubular secretion : The blood remaining in the glomerular capillaries is sent to efferent arteriole . It means the blood contains some impurities. These impurities must be removed off from the blood. Most of the secretion occurs in DCT. Some of the H + ions are secreted in PCT. In DCT, for each Na + ion reabsorbed, either one K + or one H + ion is secreted. In DCT other substances like uric acid, amino hippuric acid, creatinine etc. are also secreted. Excess of water soluble vitamins, remains of drugs etc. are also secreted in DCT. URINE AND ITS COMPOSITION: URINE AND ITS COMPOSITION As a result of all the three processes, the fluid that comes out from collecting duct is called urine. It is a non viscous aqueous solution , containing various impurities. Volume The amount of urine passed out from the body varies from 800 ml to 2 litre . The average value is about 1.5 litre . Specific gravity Its specific gravity varies between 1.001 to 1.040. In ‘Diabetes mellitus’, the specific gravity becomes very high. Colour Urine has a pale lemon yellow colour. The colour is due to the presence of pigment called urochrome urobilin and uroerythrin . Abnomalities in urine:: Abnomalities in urine: Metabolic errors of kidney may severely affect the composition of urine. Occurrence of ketone bodies, glucose, albumin, blood cells, excess pigments, pus cells, calculi are some of the major abnormal constituents of urine . Some important abnormalities, have been listed below - Proteinuria – Excess protein level in urine. Albuminuria – Presence of albumin in urine . Glycosuria – Presence of glucose in urine, as in the case of Diabetes mellitus. Ketnuria – Presence of abnormally high ketone bodies. Hematuria – Presence of blood or blood cells in urine. Haemoglobinura – Presence of haemoglobin in urine. Micturation : Micturation Micturition is the act of voiding the urine. The release of urine is done by contraction of the smooth muscle of the urinary bladder wall and the relaxation of the skeletal muscle sphincter around the opening of the bladder. The gradual filling of the bladder stretches the walls of the bladder. The stretch receptors in the bladder wall in response generate impulses that are carried by afferent fibres to the spinal cord and also to, brain. Spinal cord stimulates the efferent fibres which cause the contraction of detrussor muscles, and simultaneously relaxation of internal sphincter.