process of urine formation

process of urine formation

Human is ureotelic i.e their excretory product is urea.the urea produced by liver is sent to kidney by blood and is excreated out in the form of urine.the process of urine formation involves: 

ultrafilteration

selective reabsorption

tubular secretion

Ultra filteration : the process of ultrafilteration takes place in malphigian body.since the diameter of afferent arteriole is more than that of efferent arteriopne,the blood in glomerulus experiences greater hydrostatic pressure.it is about 70mmhg.this pressure forces the blood toward the endothelium of capillaries of glomerulus.the glomerular hydrostatic pressure is opposed by osmotic pressure of blood i.e 30 mmhg and hydrostatic pressure of nephric filterate i.e 10 mmhg.thus net filteration pressure is about 70-(30+10) mmhg i.e 30mmhg.since the wall of endothelium and epithelium is permeable,basement membrane acts as filter.due to resultant pressure all component of blood except blood cells,proteins and fats get filtered.this filtered fluid is called nephric filterate which contain urea,water,ammonia,uric acid,ammino
acid,glucose,electrolytes,hormone etc.the average glomerular filteration rate is 180 litre per day.

Selective reabsorption : nephric filterate contains many essential substances to the body.they are thus reabsorped into efferent arteriole from tubule.this process is called selective reabsorption.glucose,urea.ammino acids are absorped in proximal convulated tubule , electrolytes in distal convulated tubule and 99 percent water are absorped in loop of henle and collecting duct.urea and water are absorped by passive diffusion and others by active transport against concentration gradient with utilisation of energy.selectve reabsorption rate is 124 ml per minute.

Tubular secretion :the substances which couldnot be filtered in glomerulus are actively secreted by efferent arteriole into distal convulated tubule. this process is called tubular secretion.nh3,hydrogen and creatinine are secreted during this process.
thus the substance formed after selective reabsorption and tubular secretion is called urine.it passes through collecting duct to pelvis and finally passes to urinary bladder through ureter.