4.2 Urinary System of Man

The human urinary system, also known as the excretory system, is responsible for filtering waste products from the blood and producing, storing, and eliminating urine.

1. Overview of the Urinary System

Components:

• Kidneys (2): Paired, bean-shaped organs that filter blood and produce urine.
• Ureters (2): Tubes that carry urine from the kidneys to the urinary bladder.
• Urinary Bladder: A muscular sac that stores urine.
• Urethra: A tube that carries urine from the bladder to the outside of the body.

2. Structure of the Kidney

The kidneys are the primary organs of the urinary system.

Location and External Structure

• Location: Lie on either side of the vertebral column, just below the liver in the peritoneal cavity. The right kidney is slightly lower than the left to accommodate the liver.
• Adrenal Glands: Located on top of each kidney.
• Protective Layers (from outermost to innermost):
  1. Renal Fascia: A tough outer layer of connective tissue.
  2. Perirenal Fat Capsule: A layer of fat that helps anchor the kidneys in place.
  3. Renal Capsule: The innermost layer that directly covers the kidney.

Internal Structure (Anatomy)

The kidney is divided into three main internal regions.

• Renal Cortex: The granular, outer region containing the nephrons (the kidney's functional units).
• Renal Medulla: The middle region, containing multiple cone-shaped tissue masses called renal pyramids.
  • Renal Columns: Spaces between the pyramids through which blood vessels pass.
  • Renal Papillae: The tips of the pyramids, which point toward the renal pelvis.
• Renal Pelvis: The central, funnel-shaped region located in the hilum (the concave side of the kidney).
  • It collects urine and channels it into the ureter.
  • Branches into major calyces, which further divide into minor calyces that receive urine from the renal papillae.

Relationship between Structure and Function

Functions of Kidney→

3. Structure of a Nephron

The nephron is the microscopic functional unit of the kidney responsible for forming urine.

• A. Renal Corpuscle: The initial filtering component.

  • Glomerulus: A tangled network of capillaries where blood filtration begins.
  • Bowman's Capsule: A double-walled, cup-shaped structure that surrounds the glomerulus and collects the filtrate.

• B. Renal Tubule: A long, coiled tube where the filtrate is processed into urine through reabsorption and secretion.

  1. Proximal Convoluted Tubule (PCT): The first, highly coiled section after Bowman's capsule.
  2. Loop of Henle: A U-shaped loop that dips into the medulla, consisting of a descending limb and an ascending limb.
  3. Distal Convoluted Tubule (DCT): The final, highly coiled section.
  4. Collecting Duct: Receives urine from multiple nephrons and carries it through the medulla to the renal pelvis.

4. Blood Circulation to the Nephron

A specialized blood supply is crucial for the nephron's function.

• Path of Blood Flow:
  1. The Renal Artery branches into smaller arteries.
  2. An Afferent Arteriole supplies blood to the glomerulus of each nephron.
  3. Blood is filtered in the Glomerulus.
  4. An Efferent Arteriole carries blood away from the glomerulus.
  5. This arteriole branches into the Peritubular Capillaries, a network surrounding the PCT and DCT.
  6. A specialized part of these capillaries, the Vasa Recta, runs alongside the Loop of Henle deep into the medulla.
  7. The capillaries merge, and blood drains into the Renal Vein, which exits the kidney.

Blood Pressure→