Question

Define excretion. Describe the structure of nephron. Write a note on the physiology of urine formation.

Answer 1

Excretion can be defined as the process of removal of nitrogenous waste produced by the metabolic activity of the cells. In humans, urea is the major nitrogenous waste. Uric acid and creatinine are also expelled in the process of excretion.

Nephrons are the structural and the functional units of kidneys. They perform the function of blood filtration, ionic absorption, and the formation of concentrated urine. A nephron is made up of two main parts as Malpighian Body and Renal Tubules.

Malpighian body is made of:

1. Glomerulus: coiled capillaries through which blood enters for the filtration.

2. Bowman's capsule: the cup-like structure that encloses the glomerulus. Blood plasma is diffused into the Bowman's capsule from the glomerulus.

Renal Tubules are made up of:

1. Proximal Convoluted Tubule: it is situated close to the Bowman's Capsule and performs the absorption of water, electrolytes, glucose from the filtrate.

2. The loop of Henle: it is a U-shaped tubule and has a descending limb and an ascending limb. It performs a role in counter-current mechanism.

3. Distal Convoluted Tubule: it is the distal tubular portion which performs the absorption of electrolytes and also secretes the excessive electrolytes.

Physiology of urine formation: It occurs in three steps as follows:

1. Glomerular Filtration- the water, minerals, ions, and solutes are diffused out of the blood and move into the Bowman's capsule forming a filtrate.

2. Tubular reabsorption: the necessary substances such as water, glucose, amino acids, vitamins, mineral ions in the filtrate are reabsorbed by the tubules of the nephrons. The loop of Henle regulates the water reabsorption.

3. Tubular secretion: K${}^{+}$, H${}^{+}$, creatinine and excessive water are secreted out from the tubules into the filtrate. Such a filtrate is called the urine.