Question

Describe the process of the formation and concentration of urine.

Answer 1

Functional unit of the kidney is called nephrons. Urine is formed in the nephrons. Nephron has two main part renal capsule and renal tubule. Renal capsule consist of small capillary network called glomerulus and and a membraneous cover that cover the glomerulus called glomerular capsule or bowman capsule. Other part of the nephron is renal tubule that consist of mainly three parts Proximal convoluted tubules, loop of Henle, and distal convoluted tubule

Other than these two parts nephron also have a collecting region called collecting duct. End portion of renal tubule, distal convoluted tubule open to this collecting duct

Formation of urine

Urine formation occur mainly through three processes

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

First process of glomerular filtration occur in renal capsule whereas other two steps reabsorption and secretion take place in renal tubule

1. Glomerular filtration

It is the first step in urine formation. Water and almost all water soluble substances (except protein and RBC) in the plasma are forcefully filtered from the glomerulus to Bowman's capsular space through filtration membrane is called glomerular filtration. The hydrostatic pressure in glomerulus aid this filtration

Fluid that entered to capsular space after filtration is called filtrate. The amount of filtrate formed in a minute is called glomerular filtration rate

When this filtrate passes through renal tubule, other two step of urine formation occur

2. Tubular reabsorption

Water, glucose, amino acids, and ions (needed substances) are taken back into the blood from the filtrate into the blood is called tubular reabsorption

Water reabsorption take place by the process of osmosis and other solutes are reabsorbed by passive diffusion as well as active transport

Tubular reabsorption mainly occur in proximal convoluted tubule and loop of Henle

3. Tubular secretion

It is the final step in urine formation. Ions like H+, K+, other toxic substances like drugs, creatinine,a and other unwanted water soluble substances are secreted from the blood to tubular fluid is called tubular secretion. This process mainly occur in distal convoluted tubule

After these three steps urine is formed. Formed urine will be collected in the collecting duct from distal convoluted tubule

Process of urine concentration

This is usually occur during the time of dehydration. During dehydration more about if ADH is secreted from the posterior pituitary gland. This acts on medullary collecting duct and distal convoluted tubule of the nephron and increases water permeability in this area. Result in reabsorption of more water and retention of solute in the filtrate form concentrated urine.