Urinary System Anatomy and Function

Urinary System Anatomy

• The kidneys are located on the posterior abdominal wall, outside the peritoneal cavity.
• The renal artery, vein, lymphatics, nerve supply and ureter enter the kidney at the hilum (medial side).
• The urinary system includes:
  • Hepatic veins (cut)
  • Esophagus. (cut)
  • Inferior vena cava
  • Adrenal gland
  • Aorta
  • Renal artery
  • Renal hilum
  • Renal vein
  • Kidney
  • Ureter
  • Urinary bladder
  • Urethra
  • Iliac crest
  • Rectum (cut)
  • Uterus

Overview of Renal Function

• Kidneys have several major regulatory functions, including:
  • Regulation of water and electrolyte balance
  • Excretion of metabolic waste products
  • Excretion of foreign chemicals (e.g., drugs, food additives)
  • Endocrine functions of the kidney:
    • Regulation of erythrocyte production (via erythropoietin)
    • Regulation of vitamin D3 production (crucial for calcium and phosphate homeostasis)
    • Renin secretion
  • Regulation of arterial blood pressure (short-term via renin-angiotensin-aldosterone system and long-term via sodium and water excretion)
  • Regulation of acid-base balance by eliminating acids from protein metabolism.

Physiologic Anatomy of the Kidneys

• The kidneys lie on the posterior wall of the abdomen.
• The nephron is the functional unit of the kidney.
• Each kidney has approximately 1.3 million nephrons.
• Blood vessels supply each nephron, and the nephron acts as a functional unit to form urine.

Nephron Structure

• Each nephron is composed of a glomerulus and a renal tubule.
• The glomerulus is a tuft of capillaries surrounded by Bowman's capsule.
• The capillaries are supplied by an afferent arteriole and drained by an efferent arteriole.
• The glomerulus has a high pressure (60 mmHg) which causes rapid filtration of fluids.
• The renal tubule is subdivided into:
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting ducts

Renal Blood Flow

• In a resting adult, the kidneys receive 1.2–1.3 liters of blood per minute (21% of cardiac output).
• Two capillary beds associate with each nephron:
  • Glomerular capillary bed ("high-pressure bed"):
    • Receives blood from afferent arteriole
    • Hydrostatic pressure of 60 mmHg causes filtration
  • Peritubular capillary bed ("low-pressure bed"):
    • Hydrostatic pressure of 13 mmHg facilitates reabsorption.

Urine Formation

• Each nephron forms urine through three processes:
  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
• The sum of these processes gives the urinary excretion rate.

Handling of Certain Important Solutes by the Renal Tubules

• Sodium (Na+) is heavily reabsorbed by the tubules, with 96–99% reabsorbed.
• Hormonal control of sodium reabsorption varies depending on several hormones.

Glucose Reabsorption and Glycosuria

• Normally, all filtered glucose is reabsorbed in the proximal convoluted tubule.
• The renal threshold for glucose is the plasma level exceeding which glucose appears in the urine.
• Glycosuria occurs when blood glucose is elevated and exceeds the renal threshold, or due to renal defects.

Water Reabsorption and Excretion

• Normally, 180 liters of fluid are filtered through the glomeruli daily, but only about 1 liter of urine is excreted.
• Water reabsorption includes an obligatory component (87%, independent of ADH) and a facultative component (13%, controlled by ADH).

Explore the anatomy and functions of the urinary system, focusing on the kidneys and their regulatory roles. This quiz covers key structures, including the renal artery and bladder, as well as essential functions like metabolic waste excretion and electrolyte balance. Test your knowledge on both the anatomy and physiological processes governing the urinary system.