Functions of the Kidney, Renal physiology

This article is to discuss the renal physiology, describing the functions of the kidney as a whole, then illustrating the function of each part of the nephron, which is the functional unit of the kidney.
Functions of the kidney :
1- Excretory   : excretion of waste products and drugs.
2- Regulatory  : control of body fluids volume & composition.
3- Endocrinal  : production of erythropoietin, renin & P.G.
4- Metabolic   : metabolism of vitamin D.

Endocrine functions of the kidney :

- Renin-angiotensin system.
- Production of PG & erythropoietin.
- It is a site of action of ANP and parathormone.
- Vitamin D metabolism.

Function of the kidney tubules:

1- Proximal Convuluted tubules (P.C.T)
- Reabsorption of 65% of Na and most K and Ca.
- Reabsorption of all HCO3.
- Reabsorption of 65% of water (obligatory water reabsorption).
- Reabsorption of all A.A and glucose.

2- Loop of Henle
- Reabsorption of Na, CI, Ca, K in ascending limb and reabsorption of H2O in descending limb.
- It increases the medullary tonicity which is important in the process of urine concentration in collecting tubule as follows :
• Na outflow from ascending limb ===> ++ Na in medulla ===> ++ medullary tonicity.
• ADH ===> open H2O channels of collecting tubules through cAMP ===> This will subject the fluid in the collecting tubules to the high medullary tonicity ===> H2O reabsorption ===> urine concentration.

3- Distal Convuluted Tubules DCT
- Na reabsorption & k excretion through aldosterone.
- Also there is reabsorption of H2O and Ca.
- Secretion of H+ and ammonia.
4- Collecting tubules
- Concentration of urine as above.
- The amount of reabsorbed water is variable (facultative water reabsorption) depending on the blood level of ADH.

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