Internal structure of Mammalia Kidney

Internal structure of mammalian kidney

The kidney is a complex organ with a well-defined internal structure designed to carry out essential functions like filtration, reabsorption, and excretion. The kidney consists of three main regions:

i. Cortex

The cortex is the outer zone of the kidney, which is dark in color due to its dense network of blood capillaries. This area contains the glomeruli of the nephrons, which are the functional units of the kidney.

ii. Medulla

The medulla lies between the cortex and the pelvis. Its surface facing the pelvis is folded to form projections called pyramids. These pyramids are essential for the filtration process.

iii. Pelvis

The pelvis is a collecting space where urine accumulates before being transported to the ureter. The pelvis narrows and leads to the ureter, which takes the urine to the bladder for storage.

The nephron

The nephron is the functional unit of the kidney responsible for both osmoregulation (maintaining constant osmotic pressure of body fluids) and excretion. Each nephron consists of a long tubule that is closed at one end and open at the other. It is divided into four parts:

1. Bowman's Capsule
2. Proximal Convoluted Tubule
3. Loop of Henle
4. Distal Convoluted Tubule

i. Bowman's capsule

This is a round, cup-shaped structure at the closed end of the nephron tubule that encloses the glomerulus, a network of blood capillaries formed from the afferent blood vessels (a branch of the renal artery).

ii. Proximal convoluted tubule

The proximal convoluted tubule is coiled and located next to Bowman's capsule. It lies in the cortex of the kidney and is involved in the reabsorption of substances from the filtrate.

iii. Loop of Henle

The Loop of Henle is a U-shaped part of the nephron tubule. It extends from the proximal convoluted tubule into the medulla and then bends back into the cortex. This part is crucial for water and salt balance.

iv. Distal convoluted tubule

The distal convoluted tubule is another coiled section of the nephron located near the open end. It joins the collecting duct, which leads to the ureter. This part further regulates the composition of urine.

Excretion in the kidney

Excretion in the kidney occurs in three stages: filtration, reabsorption, and removal.

i. Filtration

1. Blood enters the kidney at high pressure through the renal artery (a branch of the aorta).
2. The blood is rich in nitrogenous wastes like urea, dissolved substances, plasma proteins, minerals, hormones, and oxygen.
3. The afferent vessels entering the glomerulus are wider than the efferent vessels leaving it. This causes resistance to blood flow and generates high pressure in the glomerulus.
4. Due to the high pressure, small molecules like urea, glucose, salts, and amino acids are forced out of the blood and into the Bowman's capsule. This is called ultrafiltration, and the filtrate formed is called glomerular filtrate.
5. Larger molecules like proteins and blood cells are not filtered because the capillary walls of the glomerulus and Bowman's capsule have very small pores.

ii. Reabsorption

1. As the glomerular filtrate moves along the nephron tubules, useful substances like water, glucose, and amino acids are reabsorbed back into the bloodstream.
2. Most reabsorption occurs in the proximal convoluted tubule through active transport.
3. The proximal convoluted tubule is well-adapted for reabsorption due to:
   1. Presence of mitochondria providing energy in the form of ATP.
   2. Microvilli on the cells to increase surface area for reabsorption.
   3. Being coiled to provide a larger surface area.
   4. Having a rich network of blood capillaries for efficient exchange.
   5. Slowing down the speed of filtrate flow for more efficient reabsorption.