The structure of the kidney

The kidneys are vital organs involved in homeostasis, primarily responsible for filtering waste products from the blood and regulating the body's water and ion content.

Gross Structure of the Kidney

Each human kidney is a solid, oval, red-brown organ typically found attached to the back of the abdominal cavity.

• Fibrous capsule: The entire kidney is covered by a tough fibrous capsule.

• Cortex: The outer region of the kidney.

• Medulla: The inner region, lighter in color, beneath the cortex.

• Renal pelvis: An area where the ureter joins the kidney, collecting urine before it leaves the kidney.

• Ureter: A narrow tube that carries urine from each kidney to the bladder.

• Renal artery: Brings oxygenated blood to the kidney from the aorta.

• Renal vein: Carries filtered blood away from the kidney to the vena cava.

The Nephron: Functional Unit

The kidney tissue is made up of thousands to over a million tiny tubules called nephrons. Each nephron is a thin-walled tubule, approximately 3 cm long, with parts located in both the cortex and the medulla. Blood vessels are closely associated with each region of the nephron.

Each nephron consists of several distinct parts:

• Bowman's capsule (Renal capsule): A cup-shaped structure in the cortex that surrounds a network of capillaries. This is where ultrafiltration begins.

• Glomerulus: A bundle of capillaries looped inside the Bowman's capsule. Blood enters the glomerulus via the afferent arteriole and leaves via the narrower efferent arteriole. This difference in diameter creates high hydrostatic pressure, forcing fluid and small molecules out of the blood and into the Bowman's capsule.

  • The filtration barrier consists of three layers: the capillary endothelium (with pores), a basement membrane (acting as the primary filter for large proteins), and the epithelium of the Bowman's capsule (made of podocytes with tiny slits).

• Proximal convoluted tubule (PCT): A twisted region leading from Bowman's capsule, located in the cortex. Its epithelial cells have numerous microvilli (forming a brush border) to increase surface area for selective reabsorption of useful substances like glucose, amino acids, and water, and are rich in mitochondria to provide ATP for active transport.

• Loop of Henle: A long hairpin loop that extends down into the medulla and then back up into the cortex. It consists of a descending limb (permeable to water but impermeable to ions) and an ascending limb (impermeable to water, actively pumps out sodium and chloride ions). This counter-current multiplier mechanism creates a low water potential (high solute concentration) in the medulla's tissue fluid, which is crucial for water conservation.

• Distal convoluted tubule (DCT): Another twisted region in the cortex, leading from the loop of Henle to the collecting duct. It is involved in final adjustments to blood pH and ion reabsorption.

• Collecting duct: A tube that runs down through the medulla into the renal pelvis, receiving fluid from several nephrons. Its permeability to water is regulated by Antidiuretic Hormone (ADH), which controls water reabsorption via aquaporins (water channels) in its cell membranes, thus determining urine concentration and volume.

This intricate structure, from the macroscopic regions to the microscopic details of the nephron and its associated blood supply, enables the kidneys to perform their essential homeostatic functions of excretion and osmoregulation.