Function Of The Kidney And The Urine Formation

Function of the kidney

• It regulate the volume of the body fluids, as the volume of urine is adjusted according to the needs of water balance

• Important for elimination of inorganic ions to keep the total crystalloid osmotic pressure of extracellular fluid and plasma within normal value

• The kidney helps to preserve a normal plasma concentration of non-electrolytes a glucose and urea.

• Important in regulation of pH through:

- Elimination of non volatile acids and basic radicals in urine.
- Formation of ammonia which neutralize acid radicals of blood.

• Helps to get rid of wastes product as nitrogen and sulfur containing substances derived from metabolic process of the protein.

• Kidney carries out detoxicating reaction ,when benzoic acid combine with glycine to form the hippuric acid, it pass out through the kidneys.

• Helps in maintenance of normal arterial blood pressure.

Mechanism Of Urine Formation

Urine formation happen with 3 organules ( nephrons, glomerulus, tubules ) functions

1-Function of Nephron:
The function of a single nephron reflects the function of kidney it self .in mammals kidney, urine is formed by filtration from the glomerioli of non colloidal constituent of plasma, followed by re-absorption both active and passive ,in addition to some tubular secretions.

2-Function of The Glomerulus:
The glomerulus acts as a filter , the membrane which separate the capillaries from the cavity of bowman capsule is semipermeable membrane, with very minute pores, it allows water and crystalloids to pass through but prevent colloids and proteins.
the fluid which pass to the bowman's capsule is known as the glomerular filtrate , its rate is about 170- 180 L/day. its content resemble blood plasma except the absence of plasma protein.

Function of Tubules:
The tubules change the glomerular filtrate to normal urine by the following steps:
a-re-absorption of water and solutes
b-secretion of some substances to concentrate the filtrate.

Mechanism of re-absorption in tubules:

About 90% of the filtrate water are reabsorbed , and all glucose also absorbed in the proximal tubules. chloride is reabsorbed in the distal tubules and acidification also occur in this part by re-absorption of NaHCO3.The mechanism of re-absorption is either simple diffusion mechanism or active transport mechanism.

Three divisions of substances are reabsorbed:
1-Higher threshold substances as glucose, chloride, amino acids and vitamin C. These substances are reabsorbed completely and nothing of them is excreted in urine except if their concentration exceeds than threshold value.
The renal threshold of glucose is 150 mg/ 100 ml blood, thus if the glucose ratio at this level or below its completely reabsorbed, if its more than this level it will appear in urine , a case know as renal glucoseria.

2-Low threshold substances as urea , uric acid , sulfate and phosphate, which simply diffuse when their concentration in the filtrate increase than that in plasma.

3-No threshold substances as waste product of metabolic process as creatinine, they are excreted in urine continuously and not reabsorbed in normal kidney.

Hormones that affect re-absorption process:

Aldosterone, anti-diuretic hormone and parathyroid hormone.