Urinary System Notes

Overview of the Urinary System

• Functions:
  • Maintains the composition, pH, and volume of body fluids within normal limits.
  • Removes metabolic wastes and excess substances.

Organs of the Urinary System

• Kidneys: Filter blood and form urine.
• Ureters: Transport urine from kidneys to urinary bladder.
• Urinary Bladder: Collects and stores urine.
• Urethra: Conveys urine from urinary bladder to outside of body.

Kidney Anatomy

• Size of Adult Kidney: Approximately 12 cm long, 6 cm wide, and 3 cm thick.
• Location: Positioned retroperitoneal (behind the peritoneum) on either side of the vertebral column.
• Protection: Enclosed in a tough, fibrous capsule surrounded by adipose tissues.

Key Structures of the Kidney

• Hilum: Concave region on the medial side where the ureter, renal artery, and renal vein enter/exit the kidney.
• Renal Pelvis: Funnel-shaped sac forming the superior end of the ureter.
• Renal Capsule: Fibrous capsule around the kidney.

Internal Structure of the Kidney

• Renal Cortex: Outer region where capillary beds and nephron initiation occur.
• Renal Medulla: Inner region containing renal pyramids where nephron tubules and collecting tubes are found.
• Renal Columns: Extensions of cortex into the medulla separating pyramids.

Nephron Anatomy

• Functional Units: Each kidney contains about 1 million nephrons.
• Nephron Components:
  • Renal Corpuscle:
  • Glomerulus: Cluster of capillaries that filter blood (first step in urine formation).
  • Glomerular (Bowman’s) Capsule: Receives filtrate from glomerulus.
  • Renal Tubule:
  • Extends from glomerular capsule to collecting duct.
  • Filtrate travels through: Glomerular Capsule → Proximal Convoluted Tubule → Loop of Henle (descending and ascending limbs) → Distal Convoluted Tubule → Collecting Duct.

Urine Formation Processes

• Glomerular Filtration: Water and small molecules move from blood into glomerular capsule, forming the filtrate. Large proteins remain in blood.
• Hydrostatic Pressure: Afferent arteriole has a larger diameter than efferent arterioles, creating pressure that favors filtration.

Tubular Reabsorption and Secretion

• Tubular Reabsorption: Substances move from renal tubules back into blood; occurs mostly in the proximal convoluted tubule.
  • Active transport and osmosis are crucial in sodium and water reabsorption.
• Tubular Secretion: Wastes move from capillaries into renal tubules. Important for regulating blood pH (H+ ions) and removing drugs.

Control of Urine Formation

• Regulation Factors:
  • Decreased blood volume lowers blood pressure affecting filtration rate.
  • Hormonal Regulation:
  • Renin-Angiotensin-Aldosterone System (RAAS): Regulates blood pressure; aldosterone increases sodium reabsorption and water retention.
  • Antidiuretic Hormone (ADH): Increases water reabsorption; high ADH levels lead to concentrated urine, while low levels lead to dilute urine.

Summary of Filtrate Journey

1. Glomerulus →
2. Glomerular Capsule →
3. Proximal Convoluted Tubule →
4. Nephron Loop (Descending & Ascending limbs) →
5. Distal Convoluted Tubule →
6. Collecting Duct →
7. Minor Calyx →
8. Major Calyx →
9. Renal Pelvis →
10. Ureters →
11. Urinary Bladder →
12. Urethra.