Excretory System

Excretion

Definition: The process of removal of metabolic waste products and excess substances from the body of an organism.

Organs and their Excretory Products

• Kidneys: Primary excretory organs, remove nitrogenous wastes (urea, uric acid, creatinine), excess salts, and water from the blood to form urine.
• Sweat Glands (Skin): Excrete excess water, salts, and a small amount of urea through sweat.
• Lungs: Excrete carbon dioxide and water vapor during respiration.

Renal Excretory System

The urinary system is the main excretory system in humans.

• Kidneys:
  • Location: A pair of bean-shaped organs located on either side of the spine, just below the rib cage.
  • Function: Filter blood to remove waste products and produce urine.
• Ureters:
  • Location: Two tubes extending from the kidneys to the urinary bladder.
  • Function: Transport urine from the kidneys to the urinary bladder.
• Urinary Bladder:
  • Location: A muscular sac located in the pelvis.
  • Function: Stores urine until it is ready to be expelled from the body.
• Urethra:
  • Location: A tube extending from the urinary bladder to the outside of the body.
  • Function: Carries urine from the bladder out of the body.

The Nephron: The Functional Unit of the Kidney

The nephron is the microscopic functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains approximately one million nephrons.

Structure of the Nephron

The nephron consists of two main parts: the renal corpuscle and the renal tubule.

• Renal Corpuscle:
  • Glomerulus: A tangled cluster of tiny blood capillaries.
  • Bowman's Capsule: A double-walled, cup-shaped structure that surrounds the glomerulus and collects the filtered fluid.
• Renal Tubule:
  • Proximal Convoluted Tubule (PCT): The first section of the renal tubule, where most of the reabsorption occurs.
  • Loop of Henle: A U-shaped tube that descends into the renal medulla and is responsible for concentrating the urine.
  • Distal Convoluted Tubule (DCT): The segment responsible for the fine-tuning of electrolyte balance.
  • Collecting Duct: The final segment of the nephron, where the final concentration of urine occurs.

[Diagram of a Nephron]

Function of the Nephron

The nephron performs three main functions to produce urine:

1. Glomerular Filtration: Blood pressure forces water, salts, glucose, and urea from the blood in the glomerulus into the Bowman's capsule, forming a fluid called filtrate.
2. Tubular Reabsorption: As the filtrate flows through the renal tubule, most of the water and many of the solutes are reabsorbed back into the blood.
3. Tubular Secretion: Waste products and excess ions that were not filtered in the glomerulus are actively transported from the blood into the filtrate.

Common Disorders of the Urinary System

• Urinary Tract Infection (UTI): An infection in any part of the urinary system, often caused by bacteria.
• Kidney Stone: Hard deposits made of minerals and salts that form inside the kidneys.