Renal Circulation and Urine Formation:
Introduction : It is important to eliminate the wastes from the body ,as accumalation of the wastes products in the body causes harmful affect to the body and leads to imbalance and causes diseases . so for the elimination of these wastes from the body it has following ways, wastes may thrown out of the body by sweat mechanism and by formation of urine and faeces . Urine formation is the main function done by kidneys to purify the blood and expel wastes from body in the form of urine . so , we are going to discuss about the Renal circulation and Urine formation . Renal Circulation : In an adult human both the kidneys receive 13,00 ml blood/min which is 26% of the cardiac out put .Blood flow to the kidneys measured by using plasma clearance of para-aminohippuric acid . Renal blood flow is regulated by auto regulation , nerves innervating the blood vessels has no signifcant role in this mechanism . NOTE : auto regulation is an intrinsic ability of an organ to regulate it’s own blood . This mechanism is seen in the vital organs like brain ,heart and kidneys . RENAL BLOOD VESSELS : Auto Regulation : As it is the intrinsic ability of an organ to regulate it’s own blood flow , kidneys are more efficient in this mechanism. In glomerular filtration rate [GFR] the mean arterial blood pressure is maintained between 60 to 180mmHg . It is regulated by two mechanisms they are MYOGENIC RESPONSE : When there is increase in blood flow , it leads to stretching of afferent arteriole which in turn leads to increase in blood flow , it increases calcium ions and leads to influx of these calcium ions from extra cellular into cells . This influx of calcium leads to contraction of smooth muscle of afferent arteriole , which causes contraction of afferent arteriole and leads to decrease in blood flow. TUBULO GLOMERULAR FEEDBACK : This feed back is under control of the renal tubule and macula densa . In this macula densa acts as sensor which is sensitive to the sodium and chlorine ions which are detected through the Na+_K+_2Cl- cotransporters. When there is increase glomerular filtration rate there is increase in the concentration of the sodium and chlorine in the filtrate , this increase leads to release of adenosine from ATP by stimulating macula densa ,which causes contraction of afferent arteriole in presence of adenosine A1 receptors leads to decrease in blood flow and leads to decrease in glomerular filtration rate . When there is decrease in the glomerular filtration rate takes place there is decrease in the sodium and chlorine which stimulate macula densa to release PGE2 , Bradykinin and Renin . The PGE2 and bradyakin dilates the Afferent arteriole and the renin acts on angiotensin and leads to constriction of Efferent arteriole . in turn these mechanism leads to increase in GFR and blood flow . NOTE : Urine Formation : It is a blood cleansing function . This is the mechanism of the body through which the blood get purified and eliminate the wastes in the form of urine .In general the normal urine out put is 1 to 1.5 liters /day Glomerular Filtration : This is also called as the ultra filtration . In this process all substances get filtered except plasma proteins as they have large diameter than the filtration pores in filtration membrane from the glomerulus to Bowman’s capsule . Normally Glomerular Filtration Rate [GFR] is 125 ml/min or 180 L/day . FILTRATION FRACTION : Normal filtration fraction is 15 to 20% it is calculated as GFR/Renal plasma x 100 FACTORS FOR FILTRATION : There are three pressure factors , they are as follows : NET FILTRATIO PRESSURE : normal 20mmHg Net filtration pressure = Glomerular capillary pressure – [colloidal osmotic pressure + Hydrostatic pressure in bowman’s capsule ] Factors regulating Glomerular filtration rate : Tubular Reabsorption : This the process through which almost 99% of water and electrolytes get reabsorbed into the blood . This is also called as the selective reabsorption as in this only selected and necessary elements are only get reabsorbed . ROUTES OF REABSORPTION : There are two routes through which the substances get reabsorbed from the tubular lumen to the peritubular capillary . TRANSCELLULAR ROUTE : In this the substances from the tubular lumen get transported into the tubular cell through the apical surface of the cell membrane and from there to capillary through the interstitial fluid. PARACELLULAR ROUTE : Reabsorption of the substances from the tubular lumen to the interstitial fluid through the lateral intercellular spaces by tight junctions and then into the capillary . REGULATION OF TUBULAR REABSORPTION : Glomerulotubular balance : Balance due to the osmotic pressure in peritubular capillaries leads to to the increase in the GFR which in turn increase plasma protein in glomerulus which increases the osmotic pressure causes the increase in the reabsorption Hormonal Factor : Nervous Factor : Sympathetic system which indirectly stimulates the JG cells and release renin which acts on the angiotensin II which in turn leads to increase in reabsorption . SITE OF REABSORPTION : Tubular Secretion : Tubular secretion is a vital renal process in which the kidneys actively transport substances from the blood into the tubular fluid within the nephrons, facilitating the excretion of waste products and regulation of blood composition. This mechanism complements filtration and reabsorption, ensuring efficient waste removal and maintenance of homeostasis. This process is also called as tubular excretion. Primary Sites of Tubular Secretion: Mechanisms of Tubular Secretion: Functions of Tubular Secretion: CONCLUSION : Renal circulation and urine formation are fundamental processes that the kidneys utilize to maintain the body’s internal balance, remove waste, and regulate various physiological parameters. Renal Circulation: Blood enters the kidneys through the renal arteries, which branch off from the abdominal aorta. These arteries divide into smaller arterioles, leading to the glomeruli networks of capillaries within the nephrons, the functional units of
