Microscopic Structure of Kidneys: 

The kidney tissue is composed of numerous capillaries and tiny tubes known as renal tubules. When the kidney is sliced lengthwise, it reveals two distinct regions: the dark outer cortex and the lighter inner medulla. At the point where the ureter connects to the kidney, there is a cavity called the pelvis.

The renal artery branches into multiple arterioles and capillaries, primarily within the cortex. Each arteriole leads to a glomerulus, which is a network of capillaries enclosed by a cup-shaped structure called Bowman’s capsule. This capsule connects to a coiled renal tubule. The tubule then merges with a collecting duct, which travels through the medulla before emptying into the pelvis. The kidney cortex contains thousands of glomeruli, with a significant total surface area of their capillaries.

Nephron

A nephron comprises a single glomerulus with its Bowman’s capsule, renal tubule, and associated blood capillaries. Each nephron is responsible for the filtration, reabsorption, and secretion processes essential for kidney function.

Ultrafiltration:

Ultrafiltration is the initial stage of urine formation that occurs in the renal corpuscle, specifically in the glomerulus.

Process:

Blood Entry: Blood from the renal artery enters the glomerulus via the afferent arteriole under high pressure generated by cardiac output and arteriolar constriction.

Filtration: In the glomerulus, water, urea, salts, and small molecules like glucose are forced out of the blood and into the Bowman’s capsule.

Glomerular Structure: The glomerulus contains fenestrated capillaries that allow small molecules to pass through while retaining larger particles like blood cells and proteins.

Selective Reabsorption:

Selective reabsorption occurs in the renal tubule, which extends from the Bowman’s capsule.

Process:

Filtrate Processing: The filtrate from the Bowman’s capsule enters the renal tubule, where it undergoes further processing.

Reabsorption: Two-thirds of the filtered salt and water, along with all the filtered glucose, are actively transported out of the nephron and reabsorbed back into the bloodstream.

Active Transport: Active transport mechanisms utilize energy to move substances against their concentration gradient from the tubule into the surrounding blood capillaries.

Purpose: This process ensures retention of essential substances like glucose and most of the filtered water and salts in the body, preventing their loss in urine.