Nephron physiology|How urine is produced actually by nephron

Nephron Physiology-Urine formation

In renal system filtration the blood is filtered by nephrons to produce urine, which is waste byproduct contains metabolic wastes and water.The rate at which different substances excreted in the urine is the sum of three renal processes;

1. Glomerular filtration
2. Reabsorption of substances from the renal tubules into the blood
3. Secretions of substances from the blood into renal tubules

Urine formation begins when a large amount of fluid that is virtually free of protein is filtered from the glomerular capillaries into the bowman's capsule

Let's discuss the three renal processes by which urine is actually formed.

Gromerular Filtration

• The mechanism by which glomerulus filters the blood, supplied by afferent arteriole to glomerulus,is based upon high glomerular hydrostatic pressure,which forces blood to leave the glomerulus through efferent arteriole resulting in the filtration of blood.
• Like most capillaries, the glomerular capillaries are relatively impermeable to proteins,so the filtered fluid is usually free of protein
• There are few low-molecular wight substances, such as calcium and fatty acids , that are not freely filtered because they are partially bound to plasma proteins
• The filtrate now contains large amount of water,glucose, aminoacids,urea, uric acid, electrolytes,the concentrations of which are similar to the concentrations in the plasma.
• The volume of filtrate formed by both kidneys each minute is called glomerular filtration rate, which is about 125ml/min when the net filtration pressure is 10mmHg.

Tubular Reabsorption

In reabsorption useful substances like most of water and solutes are reabsorbed to blood.

• Selective reabsorption occurs in renal tubules.
• As the filtrate moves to PCT,most of the water,organic nutrients such as glucose,amino acids, vitamins etc, are selectively reabsorbed into the blood.
• After reabsorption of  substances through PCT,in which some substances reabsorbed passively and some through active transport,only 60-70 % filtrate reaches loop of henle.
• Mostly sodium and chloride and some quantity of water absorbed in the loop of henle and the remaining filtrate moves to the DCT.
• Mostly sodium is reabsorbed here, with some electrolytes,so the filtrate entering the collecting duct is quite dilute
• The collecting ducts absorbs much water as the body needs

Tubular Secretion

• Tubular secretion plays an important role in determining the amount of potassium and hydrogen ions and a few other substances that are excreted in the urine
• Certain foreign substances  and drugs are poorly reabsorbed  but in addition are secreted from the blood into the tubules ,so their excretion rate is high.
• Excess k+ ion is secreted in the tubules and in exchange Na+ ion is reabsorbed to maintain homoeostasis
• Hydrogen ion secretion is very important in maintaining normal blood pH.

Now the fluid is mainly consist of water, urea, uric acid and inorganic salts which are constituents of urine.The diagram below demonstrate all steps mentioned above;

wikimedia commons