WEEK 6 URINARY SYSTEM

Urinary System Functions

• Excretion: Blood is filtered through the kidneys, allowing urine formation through a complex process including filtration, reabsorption, and waste elimination. The kidneys process about 180 liters of fluid daily, yet most is reabsorbed, resulting in only about 1 to 2 liters of urine.

• Regulation of Blood Volume and Pressure: Kidneys balance extracellular fluid volume by adjusting urine concentration. This involves intricate mechanisms controlling sodium and water reabsorption, leading to either a small volume of concentrated urine or a large volume of dilute urine to maintain blood pressure.

• Regulation of Solute Concentration: Maintains major ion concentrations (e.g., Na^+, K^+, Cl^- Ca^{2+}). The kidneys ensure these ions are kept within a narrow physiological range crucial for nerve and muscle function.

• pH Regulation: Kidneys secrete H^+ to manage acidity levels in body fluids. This process involves the excretion of acids and the reabsorption of bicarbonate to balance blood pH.

• Red Blood Cell Synthesis Regulation: Secretion of erythropoietin stimulates erythrocyte production. This hormone targets the bone marrow to increase red blood cell production in response to hypoxia.

Anatomy of the Kidneys

• Location: Situated on either side of the vertebral column in the posterior abdominal cavity (T12-L3). Protected by the rib cage, kidneys are retroperitoneal, lying behind the peritoneum.

• Structural Layers:

  • Renal Capsule: Fibrous connective tissue providing a barrier against trauma and infection.

  • Adipose Tissue: Surrounds the kidneys, providing cushioning and insulation.

  • Renal Fascia: Connective tissue layer encasing the kidneys, anchoring them to the abdominal wall.

• Kidney Structure: Comprises an outer cortex and inner medulla.

  • Hilum: Entry point for renal artery and nerves; exit for renal vein and ureter. This area also connects to the lymphatic vessels.

  • Renal Sinus: Cavity behind the hilum filled with connective and adipose tissue, housing the renal vessels and nerves.

  • Renal Pyramids: Form the boundary between cortex and medulla; contain renal papillae that drain into minor calyces, which then converge into major calyces.

Nephron Anatomy

• Functional Unit: Approximately 1.3 million per kidney; each nephron is capable of forming urine.

• Components:

  • Renal Corpuscle: Includes glomerulus (capillary network) and Bowman’s capsule (site of filtration). The glomerulus filters blood, and Bowman’s capsule collects the filtrate.

  • Proximal Convoluted Tubule (PCT): Absorbs filtered substances back into blood (e.g., glucose, amino acids, sodium, chloride, potassium, and water). The PCT is highly folded and lined with microvilli to increase surface area for reabsorption.

  • Loop of Henle: Conserves water and solutes; has descending and ascending limbs which create a concentration gradient in the medulla, essential for water reabsorption.

  • Distal Convoluted Tubule (DCT): Adds waste to filtrate, including urea, ammonia, and certain drugs.

  • Collecting Duct: Links DCT and empties into the papillary duct. Multiple nephrons drain into a single collecting duct.

Filtration Process

• Formation of Filtrate: Non-selective pressure forces small molecules and water from blood into Bowman’s capsule; the filtrate composition changes as it travels through nephron.

• Filtration Pressures:

  • Glomerular Capillary Pressure: High pressure forces fluid into Bowman’s capsule, driven by the heart's pumping action.

  • Capsular Hydrostatic Pressure: Opposes filtration due to fluid in Bowman’s capsule, creating back pressure.

  • Blood Colloid Osmotic Pressure: Related to plasma proteins in glomerulus resisting filtration, pulling fluid back into the capillaries.

Regulation of Glomerular Filtration Rate (GFR)

• Intrinsic Mechanisms:

  • Myogenic and Tubuloglomerular Feedback: Adjust vessel diameter to regulate blood flow and maintain GFR within a constant range, despite changes in blood pressure.

  • Juxtaglomerular Apparatus: Monitors blood pressure and solute concentration, secreting renin when necessary to regulate blood pressure and GFR.

Urine Formation Stages

1. Filtration: Formation of filtrate from blood in the glomerulus.

2. Tubular Reabsorption: Reabsorbing water and solutes back into blood within renal tubules (PCT, loop of Henle, DCT) to maintain fluid balance and prevent loss of essential nutrients.

3. Tubular Secretion: Transporting drugs and toxins from blood into the filtrate for excretion.

Hormonal Regulation

• ADH (Anti-Diuretic Hormone): Promotes water reabsorption, affecting urine concentration and volume. ADH increases the permeability of the collecting ducts to water.

• RAAS (Renin-Angiotensin-Aldosterone System): Regulates blood pressure and blood volume through sodium retention and vasoconstriction, affecting both afferent and efferent arterioles.

• Atrial Natriuretic Peptide (ANP): Released in response to increased blood volume, reducing blood pressure by promoting sodium/water excretion and vasodilation.

pH Regulation

• Importance: pH affects enzyme function; regulated by excretion of H^+ ions to maintain optimal physiological conditions.

• Buffer Systems:

  • Chemical Buffers: Act within seconds by neutralizing acids/bases in blood, including bicarbonate, phosphate, and protein buffers.

  • Respiratory Regulation: Adjusts blood pH by modifying CO_2 levels through changes in breathing rate and depth.

  • Renal Mechanisms: Adjusts H^+ reabsorption or secretion over hours to days, providing a long-term pH control mechanism.