Question

Sequence the events that transpire in the kidneys during urine formation. MAKE SURE TO EXPLAIN THE CONCEPTS

Answer 1

The mechanism of urine formation involves three steps or processes

• Ultrafiltration or Glomerular filtration

• Selective tubular reabsorption

• Tubular secretion

ULTRAFILTRATION OR GLOMERULAR FILTRATION:

The first step in urine formation is the filtration of blood which is carried out by the glomerular capsule .this process occurs in the malpighian corpuscle of the nephron

The glomerular capsular membrane (Filtration membrane through which filtration of blood occur) consists of three layers

1. The endothelium of glomerulus blood vessels.

2. The epithelium of Bowman's capsule

3. A basement membrane between these two layers.

The epithelial cells of Bowman capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called as filtration sits or slit pores

The blood is filtered so finely through these membranes that almost all the constituents of the plasma except the proteins, pass on to the lure of the Bowman's capsule. Therefore it is considered as a process of ultra filtration:

The plasma fluid that filters out from glomerular capillaries is called as glomerular filtrate it is protein less plasma

About 20% of plasma fluid filters out into Bowmans capsule

The amount of the filtrate formed by the kidney per minute is called glomerular filtration rate (GFR), GFR in a healthy individual is approximately 125 ml/min ie. 180 litres per day.

SELECTIVE TUBULAR REABSORPTION

PCT:

Substances like glucose, amino acid , all of the essential nutrients and 70-80 Electrolytes and water are reabsorbed in this stage Fatty acids are completely reabsorbed by active transport in PCT.

Water & Na are reabsorbed passively Reabsorption in this segment is maximum

Henle's loop:

Reabsorption in this segment is minimum. This region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. The descending limb bf loop of Henle is permeable to water but most impermeable to electrolytes so here water is reabsorbed passively. This limb concentrate the filtrate as it move down.

DCT :

Conditional reabsorption of Na and water takes place in this segment in the presence aldosterone hormones , salts (Na) are reabsorbed actively and due to ADH water. Reabsorbed passively and also reabsorption of HCO3- takes place

Collecting duct:

Large amount of water could be reabsorbed from this region to produce concentrated urine in the presence of ADH. The distal part of collecting duct is permeable to urea so small amount of urea is also absorbed from the filtrate and it adds to the hyperosmolarity of medullary intertittium

TUBULAR SECRETION:

During urine formation the epithelial cells of renal tubules secrete excretory substance from the blood into the filtrate. This process is tubular secretion.

In PCT selective secretion of H, ammonia , Creatinine, Uric acid, Drugs, Hippuric and etc. occur . Collecting duct plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of k+ and H+ ions.

Tubular secretion is also an important step in urine formation as it helps in the maintenance ionic and acid base balance of body fluid