Anatomy and Physiology of Urinary System
Introduction to the Urinary System
The Urinary System is responsible for filtering waste products, excess fluids, and toxins from the blood. These waste materials are removed from the body in the form of urine.
Urine is produced by the kidneys, collected in the bladder, and excreted through the urethra. This process helps remove excess minerals, vitamins, and blood corpuscles while maintaining the body’s internal balance or homeostasis.
The main organs of the urinary system are the kidneys, ureters, bladder, and urethra. Among these, the kidneys play the most crucial role in regulating acid-base balance and the water-salt balance of the blood.
Functions of the Urinary System
The primary function of the urinary system is excretion — eliminating metabolic waste products and excess substances from the body to maintain internal stability. The kidneys play the central role in maintaining homeostasis.
Major Functions of the Kidneys
- Regulation of Plasma Ionic Composition: The kidneys control ions like sodium, potassium, calcium, chloride, and bicarbonate by adjusting how much is excreted in the urine.
- Regulation of Plasma Osmolarity: The kidneys balance water and ion levels to maintain osmotic pressure in the blood.
- Regulation of Plasma Volume: By controlling water excretion, kidneys help regulate blood volume and blood pressure. Sodium plays a key role by drawing water into the bloodstream through osmosis.
- Regulation of Blood pH (Hydrogen Ion Concentration): The kidneys maintain acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate as needed.
- Removal of Metabolic Wastes and Foreign Substances: The kidneys eliminate nitrogenous wastes such as urea, creatinine, and uric acid. Urea is formed in the liver when ammonia combines with carbon dioxide, making it less toxic. Excess uric acid may form crystals, leading to gout.
- Secretion of Hormones: The kidneys produce hormones like renin (for blood pressure control), erythropoietin (stimulates red blood cell formation), and help activate vitamin D for calcium absorption.
Organs of the Urinary System
1. Kidneys
The kidneys are two bean-shaped organs, each about 10–12 cm long, located at the back of the abdominal cavity. They are protected by the ribcage and surrounded by a fibrous capsule and layers of fat.
Each kidney has three main regions:
- Renal Cortex: The outer granular layer.
- Renal Medulla: The inner striated layer containing pyramid-shaped tissues called renal pyramids.
- Renal Pelvis: The central cavity that collects urine before it passes to the ureter.
Renal Artery and Vein
The renal arteries branch from the abdominal aorta and supply blood to the kidneys, while the renal veins carry filtered blood to the inferior vena cava. Because of anatomical positioning, the right renal artery is usually longer than the left, while the left renal vein is longer than the right.
Nephrons – Functional Units of the Kidney
Each kidney contains about 1–2 million nephrons, which are the basic filtering units. A nephron begins with a glomerular capsule (Bowman’s capsule) surrounding a network of capillaries called the glomerulus. From there, the tubule has three main parts:
- Proximal Convoluted Tubule
- Loop of Henle
- Distal Convoluted Tubule (joins a collecting duct)
Blood enters each nephron through an afferent arteriole and leaves through an efferent arteriole. The difference in their diameters creates high pressure in the glomerulus, allowing filtration. The filtered fluid passes through the tubules, where useful substances are reabsorbed and wastes are excreted as urine.
2. Ureters
The ureters are muscular tubes about 25–30 cm long that carry urine from the kidneys to the bladder. They are continuous with the renal pelvis and enter the bladder at an oblique angle, preventing backflow during urination.
Structure of Ureters
- Outer fibrous tissue covering
- Middle muscular layer (smooth muscle for peristalsis)
- Inner mucosa with transitional epithelium
Function of Ureters
Peristaltic waves in the smooth muscles of the ureter move urine from the kidneys to the bladder in small spurts, ensuring one-way flow.
3. Urinary Bladder
The urinary bladder is a muscular, elastic organ located in the pelvic cavity. It stores urine until it is expelled from the body. The bladder can hold up to 500–530 mL of urine, though the urge to urinate begins at about 150–200 mL.
- The trigone is a triangular area on the bladder’s floor, formed by the openings of the ureters and the urethra.
- The internal urethral sphincter is involuntary and controls the release of urine into the urethra.
- Stretch receptors in the bladder wall trigger the micturition reflex when the bladder is half full.
4. Urethra
The urethra is a muscular tube that carries urine from the bladder to the outside of the body. Its structure differs between males and females.
Female Urethra
- About 3–4 cm long.
- Opens between the clitoris and vaginal opening.
- Shorter length makes females more prone to urinary tract infections (UTIs).
Male Urethra
- About 20 cm long, passing through the prostate and penis.
- Serves both urinary and reproductive functions.
- Contains two sphincters:
- Internal sphincter – involuntary
- External sphincter – voluntary control
Men generally have stronger sphincter muscles, allowing them to retain urine longer (up to 800 mL).
Detailed Notes:
For pdf style full-color notes, open the complete study material below: