The Kidney

The Kidney

  • Dr Shona McQuilken

Aims

  • After studying this topic, you should be able to:

    • Describe the macroscopic and microscopic anatomy of the renal system.

    • Describe the structure and functions of the different components of the nephron.

    • Discuss hormonal and nervous control of renal function.

Function of the Urinary System

  • Homeostasis

  • Disposal of waste from the body.

  • Osmoregulation: Control of water levels in the body.

  • Regulation of blood volume and pressure.

  • Regulation of blood pH.

  • Producing hormones.

Anatomy of the Renal System

  1. Ureters: Tubes transporting urine.

  2. Bladder: Holds 700 - 1000 ml of urine. To excrete waste:

    • Urethral sphincter relaxes.

    • Detrusor muscle contracts.

    • Urethra: Tube through which urine is expelled.

Internal Anatomy of the Kidney

  • Kidneys:

    • Located on the posterior wall of the abdomen, retroperitoneal.

    • Positioned on either side of the vertebral column around the level of the twelfth rib, with the left kidney slightly higher.

    • Blood Supply:

      • Receives blood directly from the aorta via renal arteries.

      • Blood returns to the inferior vena cava via renal veins.

      • Kidneys receive 20 - 25% of resting cardiac output (~1200 ml/min).

  • Capsule: Protective layer surrounding the kidney.

  • Cortex: Outer region containing nephron structures.

  • Medulla: Internal part comprising renal pyramids.

  • Renal Sinus: Area containing fat and blood vessels.

  • Renal Pelvis: Collects urine from the calyces and directs it to the ureters.

Nephrons

  • Components of Kidneys:

    • The cortex and medulla contain approximately 1.3 million nephrons each.

    • Functional unit of kidneys, responsible for:

      • Ultrafiltration of blood.

      • Reabsorption and excretion of solutes.

Renal Corpuscle

  • Structure:

    • Consists of the glomerulus (capillary network) and Bowman’s capsule (double-walled cup).

    • Inner layer (visceral) wraps around endothelial cells of glomerular capillaries.

    • Outer layer (parietal) forms the capsule’s outer wall.

    • Capsular Space: Located between the visceral and parietal layers, allowing filtrate capture.

Ultrafiltration

  • Process:

    • First step in urine production.

    • Water and most solutes pass from capillaries to the glomerular capsule, creating glomerular filtrate.

    • Filters 180 liters/day, eliminates about 2 liters/day as urine.

Renal Tubule

  • Components:

    • Proximal convoluted tubule: Major site for glucose, sodium, and solute reabsorption.

    • Loop of Henle: Involved in counter-current multiplication.

    • Distal convoluted tubule: Reabsorbs water from filtrate, adjusting urine concentration.

  • Collecting Ducts:

    • Distal convoluted tubules from multiple nephrons empty here.

    • Unite to form larger papillary ducts that drain into minor calyces.

Counter-current Multiplier Mechanism

  • Importance:

    • Establishes osmotic gradients within the kidney, crucial for urine concentration.

  • Osmotic Gradients:

    • Vary along the inner medulla, outer medulla, and cortex based on solute permeability.

Hormonal Regulation of Kidneys

  • Anti-diuretic hormone (ADH):

    • Stimulates the insertion of aquaporin channels, increasing water reabsorption and making urine more concentrated.

  • Renin-angiotensin-aldosterone system (RAAS):

    • Controls blood pressure by regulating fluid balance and vascular resistance.

    • Involves interactions between the liver, kidneys, and adrenal glands to increase blood pressure.

Other Hormonal Influences

  • Sympathetic Nervous System:

    • Can decrease sodium and water excretion, increase angiotensin II formation.

  • Parathyroid Hormone:

    • Increases calcium reabsorption in distal convoluted tubule.

  • Natriuretic Peptides (ANP, BNP, CNP):

    • Secreted in response to heart distension, promote diuresis and decrease blood volume.

Further Reading

  • Martini & Nath 8th ed, pp 966 - 997

  • Tortora & Derrickson 12th ed, pp1019 - 1046