Osmoregulation by Kidney

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Osmoregulation is the process which regulates the concentration and osmotic pressure of blood by regulating the water contents of blood plasma.

Depending on the need of the water in the body, kidneys excrete hypotonic (dilute) or hypertonic (concentrated) urine. Osmoregulation is controlled by the hormones ADH and aldosterone.

(i) Water content of the body is more, leading to low osmotic pressure, less ADH (anti-diuretic hormone) is released, so the wall of the DCT and collecting tubules remain less permeable and as a result plenty of dilute urine is released.

(ii) Water content of the body is less so posterior pituitary secretes more of ADH. So water is reabsorbed into the blood and reduced volume f concentrated urine is excreted.

(iii) Urine concentrated by the counter current system of the descending and ascending limbs of Henle’s loop. About 5% of the water from the filtrate is absorbed in this part.

(iv) Aldosterone is produced in the cortex of the adrenal glands, which are located above the kidneys. Aldosterone affects the body's ability to regulate blood pressure. It sends the signal to organs, like the kidney and colon, that can increase the amount of sodium the body sends into the bloodstream or the amount of potassium released in the urine.

Haemodialysis and Kidney

The blood urea level rises abnormally (uremia) in patients suffering from renal failures, in uremia patients, an artificial kidney is used for removing accumulated waste products like urea from the blood by a process called Haemodialysis. It carried following steps; (1) blood is taken out from an artery of the patient, cooled at , mixed with anticoagulant and then pumped into artificial kidney. (2) Blood flows through the channels bounded by cellophane membrane. The blood flowing in the channels is kept separate from the dialyzing fluid outside the membrane. (3) Outside the cellophane tube is the dialyzing fluid, which has some solutes like those in blood plasma but no nitrogenous molecules like urea, uric acid. (4) The nitrogenous compounds from within the cellophane tubes flow into the dialyzing fluid by diffusion. (5) Blood coming out of the artificial kidney is warmed to the body temperature and returned to the vein of the patient.

Kidney transplantation

If the kidney failure cannot be otherwise treated by drug and dialysis, the patients are advised for kidney transplantation. Grafting a kidney from a compatible donor to restore kidney function in a recipient suffering from kidney failure is called kidney transplantation. Donated kidney may come from a living person or a donor who has recently died. The genetic makeup of the donor should be as close to the patient as possible, that is, if it is donated by a close relation, it reduces the chances of rejection. Drugs are, however, used to prevent rejection of the transplanted kidney by the body.

Role of liver in excretion

  • It excretes bile pigments, cholesterol, drugs and some vitamins.

  • Production of bile juices that help in the breakdown of lipid molecules.

  • It excretes all the above mentioned in bile, which flows into the small intestine and from there these get removed with the faeces.

  • Formation of urea and uric acid (from ammonia) also takes place in liver. These are removed from the body by the kidneys.