Urinary System – Anatomy and Physiology of Urinary System
Introduction
The urinary system, also known as the excretory system, plays a vital role in maintaining the internal chemical balance of the body.
It removes waste products formed during metabolism, regulates water and electrolyte levels, and maintains acid-base balance.
The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra.
Original Notes:
Anatomy of the Urinary System
1. Kidneys
- The kidneys are two bean-shaped organs located on either side of the vertebral column in the posterior abdominal wall.
- Each kidney measures about 10–12 cm in length, 5–7 cm in width, and weighs about 150 grams.
- The right kidney is slightly lower than the left due to the position of the liver.
Structure of Kidney
- Renal Capsule: Outer protective covering of the kidney.
- Cortex: Outer region containing nephrons and blood vessels.
- Medulla: Inner region with renal pyramids that lead into minor and major calyces.
- Renal Pelvis: Funnel-shaped area that collects urine and continues as the ureter.
- Nephron: The functional unit of the kidney responsible for urine formation. Each kidney contains about one million nephrons.
Structure of a Nephron
Each nephron consists of:
- Bowman’s Capsule: A cup-like sac that surrounds the glomerulus.
- Glomerulus: A cluster of capillaries where filtration of blood occurs.
- Proximal Convoluted Tubule (PCT): Reabsorbs water, glucose, and ions.
- Loop of Henle: Helps concentrate urine.
- Distal Convoluted Tubule (DCT): Further regulates ion balance.
- Collecting Duct: Collects urine from many nephrons and delivers it to the renal pelvis.
2. Ureters
- Two narrow tubes (about 25–30 cm long) that carry urine from the kidneys to the urinary bladder.
- They move urine through peristaltic movements (wave-like contractions).
3. Urinary Bladder
- A hollow, muscular sac located in the pelvic cavity.
- Stores urine temporarily before excretion.
- The bladder wall has detrusor muscles that contract during urination.
- Normal capacity is about 400–500 mL.
4. Urethra
- A thin-walled tube that carries urine from the bladder to the outside of the body.
- In males, it is about 18–20 cm long and passes through the penis.
- In females, it is about 3–4 cm long and opens in front of the vaginal opening.
Physiology of the Urinary System
1. Urine Formation
The process of urine formation occurs in three main steps:
a. Glomerular Filtration:
Blood pressure forces water and solutes from the glomerulus into Bowman’s capsule.
This filtrate contains water, salts, glucose, and urea.
b. Tubular Reabsorption:
Useful substances like glucose, amino acids, and most of the water are reabsorbed back into the blood from the tubules.
c. Tubular Secretion:
Additional wastes like hydrogen ions, potassium, and drugs are secreted into the tubule from the blood, forming final urine.
2. Urine Composition
Normal urine contains:
- Water: 95%
- Solutes: 5% (urea, uric acid, creatinine, sodium, potassium, chloride, etc.)
Normal urine is pale yellow due to urochrome pigment and has a slightly acidic pH (around 6).
3. Micturition (Urination)
- It is the process of emptying the bladder.
- Controlled by both voluntary (somatic) and involuntary (autonomic) nervous systems.
- When the bladder is full, stretch receptors send signals to the brain to trigger urination.
Functions of the Urinary System
- Excretion of waste products like urea, uric acid, and creatinine.
- Regulation of water and electrolyte balance.
- Maintenance of acid-base balance in blood.
- Regulation of blood pressure through renin secretion.
- Erythropoietin secretion, which stimulates red blood cell production.
- Vitamin D activation for calcium metabolism.
Common Disorders of the Urinary System
- Urinary Tract Infection (UTI): Bacterial infection affecting bladder or urethra.
- Kidney Stones (Renal Calculi): Solid deposits formed from salts and minerals.
- Glomerulonephritis: Inflammation of glomeruli in kidneys.
- Renal Failure: Loss of kidney function leading to waste buildup.
- Proteinuria: Presence of protein in urine indicating kidney damage.
