Renal Physiology and Kidney Functions

Renal Anatomy and Physiology

Overview of the Kidneys

  • Kidneys: Pair of bean-shaped structures, posterior wall of the abdomen, outside peritoneal cavity, just below the liver.

  • Dimensions: Each adult human kidney weighs approximately 115g to 170g, measures about 11cm in length, 6cm in width, and 3cm thick (about the size of a clenched fist).

Body Fluid Compartments

  • Body Compartments:

    • Retroperitoneal organs: Organs outside the peritoneum, e.g., kidneys, rectum, pancreas.

    • Intraperitoneal organs: Organs within the peritoneum, e.g., liver, stomach, gall bladder, spleen.

External Anatomy of the Kidneys

  • Position: The right kidney is often situated inferior to the left.

  • Vertebral Locations: Left kidney: T12 to L3 vertebrae; right kidney: lower due to liver displacement.

Structural Components

  • Hilum: The indented region on the medial side of each kidney; passage for renal artery, renal vein, ureter, lymphatics, and nerve supply.

  • Renal Capsule: Tough fibrous protective layer surrounding each kidney.

Internal Structure of the Kidneys

Regions of the Kidney

  • Cortex: Granular appearance due to nephron presence.

  • Medulla: Contains renal pyramids, which house the loops of Henle and collecting ducts.

  • Renal Pelvis: Funnel-shaped continuation leading to ureters; branches into major and minor calyces that collect urine.

Nephrons: The Functional Unit

  • Nephrons: The basic structural and functional units of kidneys, comprising renal corpuscles and tubules. Each kidney contains approx. 1 to 1.5 million nephrons.

  • Functional Mechanisms: Filtration of blood, reabsorption, secretion, and excretion of waste products.

Kidney Functions

Overview of Functions

  1. Excretion of metabolic waste products: Urea, creatinine, uric acid, bilirubin, etc.

  2. Regulation of water and electrolyte balance.

  3. Regulation of body fluid osmolality.

  4. Regulation of arterial pressure.

  5. Regulation of acid-base balance.

  6. Hormonal secretion: Erythropoietin, renin, and active vitamin D.

  7. Gluconeogenesis: Glucose synthesis during prolonged fasting.

Detailed Functions
1. Excretion of Waste Products

  • Urea: Byproduct of amino acid metabolism.

  • Creatinine: Derived from muscle metabolism.

  • Uric acid: Byproduct of nucleic acid breakdown.

  • Elimination of toxins: Pesticides, drugs, food additives.

2. Regulation of Water and Electrolytes

  • Homeostasis: Excretion must match intake; excess intake leads to body accumulation, while excess excretion reduces body levels.

3. Regulation of Arterial Pressure

  • Sodium and water excretion: Changes in these levels affect blood volume and pressure.

  • Renin secretion: Produces angiotensin II, influencing blood pressure.

4. Regulation of Acid-Base Balance

  • Kidneys vs. Lungs: Control acidity/alkalinity through hydrochloric and phosphoric acid excretion.

5. Regulation of Erythrocyte Production

  • Erythropoietin Secretion: Stimulates red blood cell production, especially under hypoxic conditions.

6. Conversion of Vitamin D

  • 1,25-Dihydroxyvitamin D3 (Calcitriol): Active form produced by hydroxylation, crucial for calcium metabolism.

7. Gluconeogenesis

  • The process by which kidneys synthesize glucose from amino acids during fasting.

Renal Blood Supply

  • Blood Flow: Both kidneys receive about 1,300 mL of blood/min (about 26% of cardiac output).

    • Renal arteries: Arise from the abdominal aorta and enter each kidney through the hilum.

Renal Artery Branching

  • Segmental Arteries: Divide into interlobar arteries running between renal pyramids.

  • Arcuate Arteries: Formed at the base of renal pyramids, giving rise to interlobular arteries.

  • Afferent Arterioles: Branch from interlobular arteries and lead into glomerular capillaries.

Function of Renal Blood Vessels

  • Glomerular Capillaries: The only capillaries that drain into arterioles (efferent arterioles) and are key to ultrafiltration.

  • Peritubular Capillaries: Surround tubular portions of nephrons; facilitate reabsorption and secretion.

Vasa Recta

  • Characteristics: Long, straight capillaries surrounding loops of Henle; play a role in concentrating urine and maintaining osmotic gradients.

Renal Autoregulation

  • Importance: Maintains GFR despite blood pressure variations (60 mmHg to 180 mmHg).

Mechanisms

  1. Myogenic Response: Constriction of afferent arterioles in response to increased blood pressure.

  2. Tubuloglomerular Feedback: Macula densa cells regulate GFR by sensing sodium chloride in distal tubules, altering afferent/efferent arteriolar resistance.

Glomerular Filtration Rate (GFR)

  • Typical Value: Approximately 180 L/day.

  • Regulation: Influenced by hormones like angiotensin II and overall blood pressure.

Filtration Barrier and Mechanism

  • Composition: Multiple layers (endothelium, basement membrane, podocytes) regulate passage of materials from blood into glomerular filtrate.

    • Fenestrations: Allow passage of small solutes but prevent large proteins and cells from filtering.

Tubular Reabsorption

  • Location: Primarily occurs in proximal convoluted tubule (PCT) but also in the loop of Henle and distal convoluted tubule (DCT).

  • Types of Reabsorption: Active and passive transport mechanisms vary based on nephron segment.

Regulatory Factors

  • Hormonal Regulation: ADH and aldosterone modulate water reabsorption rates.

  • Glomerulotubular Balance: Ensures proportional reabsorption irrespective of GFR changes.

Renal Threshold and Transport Maximum

  • Concept: The renal threshold for glucose = 180 mg/dL; exceeds leads to glucosuria.

Summary of Key Takeaways

  • Kidney Functions: Multifaceted roles from waste management to fluid homeostasis.

  • Anatomy Importance: Structure reflects function; over 1 million nephrons in each kidney are critical for maintaining bodily fluid balance.

  • Regulatory Mechanisms: Renin-angiotensin-aldosterone system (RAAS) plays a crucial role in responding to changes in blood pressure and volume.