Urine formation takes place in three stages-
i) Ultrafiltration
ii) Selective reabsorption
iii) Tubular secretion
i) Ultrafiltration :
In takes place in Malpighian body. It is a physical process. Glomerulus and Bowman's capsule acts as filtering unit. Capillaries of glomerulus are extremely porous or perforated. These pores are 10 to 1000 times more permeable to water and small molecules than other capillaries. Blood cells, plasma proteins and large fat molecules are unable to filter through glomerular capillaries.
The diameter of afferent arteriole is larger than that of the efferent arteriole. Blood enters the glomerulus at a faster rate than it leaves it. This creates a hydrostatic pressure within the glomerulus.
Glomerular membrane is semi permeable. It allows only low molecular weight substances along with water. Bowman's capsule cup-like structure. Outer layer of Bowman's capsule is called parietal layer lined by squamous epithelium. Inner layer of Bowman's capsule id called visceral layer lined by squamous epithelium with specialized cells called podocytes. Podocytes are connected to the basement membrane by several foot like processes. The gaps between these processes are filtration slits.
The substances that can pass through the pores of endothelium can also pass through these slits.
The filtrate can reach the urinary space of the Bowman's capsule. High pressure of blood in the glomerular capillaries actually forces certain substances to get out. About 1/5 th of the volume of blood get filtered.
The force is called effective filtration pressure (EFP).
EFP is produced by
a) The glomerular hydrostatic pressure in the glomerular capillaries which is about 55 mmHg.
b) The osmotic pressure of blood which is 30 mmHg due to the presence of plasma proteins it opposes a capiilary hydrostatic pressure.
c) The hydrostatic pressure of glomerular capsule is caused by filtrate that reaches into the Bowman's capsule. It is about 15 mm Hg.
The net filtration pressure is = Capillary hydrostatic pressure - (osmotic pressure + filtrate hydrostatic pressure)
55-(30+15) = 10mmHg
About 125 ml per minute (180 liters per day) of filtrate is formed. It is called glomerular filtrate rate (GFR). It takes only four minutes for the entire blood to pass through glomeruli once. Amount of blood passes through glomerulus varies form 600 - 650 ml per minute (1200ml).
About 99% of filtrate is reabsorbed and only 1- 1.5 liters of urine is formed.
The composition of the filtrate is more or less similar to that of body fluid. It is blood plasma except proteins (deproteinised plasma) Glucose, amino acid, salts, urea etc. are readily filtered.
ii) Selective reabsorption :
By comparing the rate of production of filterate with rare of production of urine, we understand the rate of reabsorption. About 99% of filtrate is reabsorbed.
This process includes two processes depending upon concentration gradient. I) passive transport or osmosis - along the concentration gradient.
Active Transport - against the concentration gradient by using ATP molecules.
As the filtrate moves through renal tubule, it comes in contact with blood found in peritubular capillaries. So, exchange occurs between blood and filtrate are altered. High therehold substances are completely reabsorbed foe eg; glucose and amino acids. Low threshold substances are uric acid and urea.
Water is reabsorbed by osmosis in PCT, DCT and descending limb of loop of Henle, everywhere except in ascending limb of loop of Henle. It is called obligatory absorption of water.
PCT pumps out glucose, amino acid and ions like potassium, calcium and chloride.
The kidneys maximum capacity for reabsorption of a substance is transport maximum or renal threshold. For eg; 45 to 95 mg per 100 ml of blood is the normal blood glucose level. If the level rises above the transport maximum i.e; 160 mg per 100 ml of blood, then glucose appears in the urine. This is called glucosuria which leads to diabetes mellitus. Other substances reabsorbed by active transport include amino acids, sodium, calcium, potassium and chloride ions. Potassium and chloride ions are reabsorbed in DCT.
Urea us reabsorbed because urea molecule is very small and tubules are partially permeable to it.
iii) Tubular secretion :
It is wrongly termed. The tubular cells do not synthesize any material and add to the filtrate of urine. They merely let certain substances pass from the blood to the lumen of the tubule. The substances that are routinely secreted are creatinine and potassium and hydrogen ions. The secretion of hydrogen which takes place in DCT and collecting tubule is important for homeostatic regulation of acidity of blood. Hydrogen ions are actively transported by the tubular cells to the extent necessary to bring down its concentration in blood to the regulation level. Some abnormal substances found in the blood are also secreted into urine The antibiotics like penicillin, iodine containing compound iodopyracet are secreted out into urine.
aldosterone from adrenal gland (cortex) maintains sodium ion concentration. Sodium ion conc. is maintained by calcitonin and parathormone.
Composition of urine :
Composition of urine produced depends upon the food and fluid consumed by the individual. About 1.2 to 1.5 liters of urine per day is produced. It is yellow in colour due to the presence of pigment called urochrome. It shows presence of 95% of water, organic substances like urea (2.5%), uric acid, creatinine (formed in muscles).