Kidneys:

They play a crucial role in excretion by removing urea and other nitrogenous wastes from the bloodstream. They also eliminate excess water, ions, hormones, and drugs from the body. Through the process of filtration, reabsorption, and secretion, the kidneys help maintain the body’s internal balance and regulate its composition.

The kidneys are solid, oval structures with a red-brown colouration, encased in a transparent membrane, and located at the back of the abdominal cavity. 

• Oxygenated blood is supplied to the kidneys through the renal artery, which branches off from the aorta. 

• Deoxygenated blood is carried away from the kidneys by the renal vein to the vena cava. 

Each kidney is connected to the bladder in the lower abdomen by a tube called the ureter.

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.

Function of kidneys: 

Filtration:

• Blood pressure within the glomerulus causes plasma to filter through capillary walls.

• Large molecules like red blood cells and plasma proteins remain within the capillary.

• The filtered fluid, resembling tissue fluid, mainly consists of water with dissolved ions, glucose, and urea.

Components of filtrate: 

Water: Essential solvent for transporting substances and maintaining hydration.

Urea: Nitrogenous waste product resulting from protein metabolism, secreted to maintain nitrogen balance.

Salts: Electrolytes such as sodium, potassium, chloride, and bicarbonate, crucial for cellular function and osmotic balance.

Glucose: Primary energy source for cells, filtered out of the blood to be reabsorbed and utilized by the body.

Reabsorption:

Filtrate collected in Bowman’s capsule travels down the renal tubule.

Surrounding capillaries reabsorb essential substances back into the bloodstream.

Initially, all glucose is reabsorbed along with a significant amount of water.

Some ions are selectively reabsorbed to maintain proper blood concentration.

Formation of Urine:

• Urea, excess water, and ions not needed by the body pass down the renal tubule into the kidney’s pelvis.

• The fluid, now termed urine, flows through the ureter to the bladder.

Storage and Excretion:

• The bladder expands to hold approximately 400 cm3 of urine.

• A sphincter muscle contracts to prevent urine from escaping the bladder.

• When the sphincter relaxes voluntarily (controlled by adults) or reflexively (in babies), the bladder’s muscular walls expel urine through the urethra.

• Most children can voluntarily control the sphincter muscle by the age of 3.