The kidneys form urine, and while carrying out this process, perform its main functions, namely:
(i) The excretion of nitrogenous waste products and
(ii) "Homeostasis" or maintaining stable, constant conditions of internal environment. Some of the homeostatic functions of the kidney are:
* Maintenance of pH (or acid-base balance). It is brought about mainly by phosphate buffers (you must have learnt about buffers in your chemistry section.)
** Maintenance of ion concentration (Na+, K+, H+, C1₋, HPO2/4-, etc), and
*** Maintenance of Osmotic pressure
The figure shows a diagrammatic representation of the relationship between the vascular and tubular comportment of a nephron. If the direction of the movement of fluid and dissolved solutes is from the tubule to blood stream ( capilary network), it is called as tubular reabsorption and if it occurs in the opposite direction, i.e from the blood stream to tubular lumen, then it is known as tubular secretion. These two processes help in urine formation.
The process of urine formation by kidneys essentially involves three basic processes:
(i) Glomercular Filtration : This refers to be movement of fluid and dissolved solutes by force or pressure (blood pressure) from the glomerulus to Bowan's capsule, through small pores or openings called as "filtration slits".
the process is somewhat similar to "straining of tea" through "tea strainer". Here, tea is equivalent to "blood";
Pores in the strainer =filtration slits.
Strainer = Bowman's capsule
& tea leaves are = proteins & blood cells which are not allowed to pass.
Only small molecules of substances present in the blood viz. glucose, amino acids, urea, water, salts, etc. can pass through the openings of the glomerulus and collect as filtrate in the bowman's capsule. The fluid finally resent in the capsule after filtration is known as ultra -filtrate and the process is known as Glomerular Filtration. the glomerular filtration rate (GFR) in adult man is 120ml/ mt or 180 liters/ day.
