Question

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

Answer 1

The kidneys are the organs of renal system that is responsible for formation of urine. The nephrons are the main site of urine formation located within the kidneys. They form the functional unit of kidney.
Each nephron is made up of two parts :

1. renal corpuscle

2. renal tubules

Renal corpuscle contain a network of capillaries arranged like a tuft known as glomerulus. The glomerulus is contained is a cup shaped Bowman's capsule.
The Bowman's capsule is attached to the renal tubule which is made up of epithelial cells. The renal tubules contains a lumen through which the filtrate passes.
The renal tubule is divided into :

1. proximal renal tubule consisting of a convulated and straight proximal tubule.
2. Loop of henle consisting of descending and ascending parts.
3. distal convulated tubule.

4. collecting tubule

5. collecting duct

The formation of urine takes place in the nephron through filteration, reabsorption, secretion and excretion.
The afferent arterioles receive blood from renal artery which is then passed into the glomerulus.
The blood from the glomerulus is collected into the Bowmans capsule. Here the blood the filtred out through the endothelial cells, basement membrane and podocytes. Large molecules such as wbc, rbc and proteins are filtered out and transported back to the blood whereas small molecules pass through and forms the filtrate that passes in to the proximal tubule.

The proximal tubules absorb water, ions and important nutrients such as amino acids, vitamins, glucose and sends them to peritubular capillaries.
The filtrate then passes to descending loop of henle. The descending limb is more permeable to water than ions. It also causes secretion of urea. It makes the filtrate hypertonic. This filtrate is passed on to ascending limb of henle. The ascending limb is imperpeamble to water. It helps in reabsorption of sodium, potassium and chloride ions. This causes the filtrate to become hypotonic.
The hypotonic filtrate passes on to distal convulated tubule (DCT). DCT reabsorbs and secretes ions depending upon the hormonal regulation. Parathyroid hormone causes reabsorption of calcium and secretion of phosphate. Aldosterone increases sodium reabsorption in exchange of potassium secretion. ANP increases sodium secretion. Under the influence of different hormones DCT reabsorps sodium, water, bicarbonate and chloride ions. DCT also secretes different drugs, acids, metabolites and toxins.
The filtrate then passes on to collecting duct. This is impermeable to water but in presence of ADH reabsorps water.
The filtrate then passes on to the kidneys through ureter to the urinary bladder. Here urine is stored till excreted out of the body through urethra and external urinary orifice.