Renal System Physiology

Overview of the Renal System

• The renal system plays a crucial role in maintaining homeostasis by regulating body fluid composition and excreting waste products.

Specific Learning Objectives

1. Organs of the Urinary System:

   • Kidneys

   • Ureters

   • Urinary Bladder

   • Urethra

2. Functions of the Kidneys:

   • Excretion of waste products.

     • urine production in kidneys via filtration of blood and reabsorption of nutrients

   • Regulation of blood volume and pressure.

   • Control of solute concentration and pH.

     • producing large amounts of dilute urine or small amounts of concentrated urine

   • Regulation of red blood cell and vitamin D synthesis.

3. Location and External Anatomy of the Kidneys:

   • Positioned on either side of the spine, behind the peritoneum.

   • Protected by a layer of fat and a fibrous capsule.

4. Structure of the Nephron:

   • The nephron consists of a renal corpuscle (glomerulus and Bowman’s capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule).

5. Urine production processes:

   1. Kidneys sort chemicals in blood for removal or return into blood - regulates the body fluid composition

   2. Nephrons are the structural components that sorts the blood

      

      • Filtration:

        • Blood plasma is filtered to form filtrate.

      • Tubular Reabsorption:

        • Useful substances are reabsorbed into the blood.

      • Tubular Secretion:

        • Additional wastes are secreted into the nephron.

6. Filtration Membrane Components:

   • Fenestrae in endothelial cells - highly permeable

   • Basement membrane.

   • Filtration slits formed by podocytes.

7. Nephron Regions and Their Roles:

   • Proximal Convoluted Tubule: Mainly reabsorbs water, ions, and nutrients.

   • Loop of Henle: Reabsorption of water and ions; has thin and thick segments.

   • Distal Convoluted Tubule: Further reabsorption, influenced by hormones.

   • Collecting Duct: Final adjustments to urine concentration.

8. Movement Across Cell Membrane of Nephron:

   • Substances move via active transport for ions and passive transport for water and small molecules.

   • Sodium is actively transported to create gradients for reabsorption.

9. Movement of Glucose:

   • Glucose is reabsorbed via co-transport with sodium in the proximal convoluted tubule.

10. Waste Products in Urine:

    • Urea, uric acid, ammonia, creatinine, various ions (H+, K+), and any drugs or toxins.

Urine Production Process

• Filtration:

  • Occurs in the glomerulus.

  • Driven by blood pressure

  • Filtration pressure - includes of pressure gradient

  • 99% of filtrate is reabsorbed.

    • consists of water, glucose, fructose, amino acids, urea, urate ions, creatine, Na+, K+, Ca+2, Cl-

  • Renal fraction: total blood output by the blood (varies from 12-30%)

  • Glomerular filtration rate: amount of filtrate produced each minute (~125ml/minute; 180L/day)

  1. Small molecules + ions + H20 (now known as filtrate) are pushed into Bowman’s capsule (doesn’t include RBCs, proteins or large molecules)

  2. FORMATION + REGULATION OF FILTRATE AND BP

     

     • juxtaglomerular apparatus

     • next to glomerulus - juxtaglomerular cells surrounds afferent arteriole

     • section of DCT - macula densa (specialised cells)

     • both of these cells secrete renin - important for formation of filtrate and regulation of BP

• Tubular Reabsorption (Process 2):

  

  • Return of good, usable substances (filtrate) into blood for cellular function

  • Reabsorption occurs mainly in the renal tubules, PCT (majority), DCT and in descending limb of loop of Henle.

    • peritubular capillaries surround tubules - low pressure so substances will diffuse out of renal tubules (~65%) and into capillaries

  • Active transport of Na+ allows for the reabsorption of glucose and other solutes

    • HOWEVER both active and passive transport can occur

  • Under influence of Anti-Diuretic Hormone (ADH), the distal convoluted tubule and collecting duct adjust permeability to water.

• Tubular Secretion (Process 3):

  • Non-filtered substances and toxins are transported into the nephron, typically in the distal convoluted tubule.

  • Facilitates the excretion of ammonia, H+, K+, and certain drugs.

Urine Movement

• Urine moves through ureters to the bladder via peristalsis, with frequency influenced by sympathetic and parasympathetic nervous systems.

• Prevention of Backflow:

  • Achieved by the trigone area of the bladder.

Composition of Urine

• Urine is composed of:

  • 1-2L produced daily, varying water content based on body needs.

  • Waste products include urea, uric acid, creatinine, and ions (H+, K+).

Micturition Reflex

• Flow from Bladder to Urethra:

  • Controlled by stretching of bladder walls, signaling the CNS for voluntary control over the external urethral sphincter (EUS).

  • Full bladder capacity is around 1L.

  • Urination occurs when contraction of bladder muscles overcomes sphincter control.