Lecture 8 Kidney and Fluid Homeostasis 2

Introduction and Overview

The kidney is a vital organ involved in maintaining fluid homeostasis and regulating various aspects of the body's internal environment. The speaker emphasizes the importance of understanding kidney functions and their biological significance.

Immune System Note

The speaker mentions experiencing a cold and expresses a need to maintain distance from the audience, indicating a personal health issue.

Kidney Overview

Structure of the Urinary System

  • The urinary system is comprised of:

    • Two kidneys: Organs responsible for filtering blood and excreting waste.

    • Two ureters: Tubes transporting urine from the kidneys to the bladder.

    • Bladder: Stores urine.

    • Urethra: Conducts urine out of the body.

  • Urine Flow: Urine flows from the kidneys, through the ureters, into the bladder for storage, and out through the urethra when needed.

Functions of the Kidney

The kidneys perform several critical functions, including:

  1. Regulation of Blood Ionic Composition:

    • Maintains electrolyte balance essential for muscle contractions and nerve functions.

    • Electrolytes include sodium, potassium, chloride, and others crucial for the body's functionality.

  2. Control of Blood Osmolarity:

    • Normal blood osmolarity is maintained around 300 milliosmols.

    • Osmolarity influences cell volume, preventing cell shrinkage or explosion due to imbalanced fluids.

  3. Regulation of Blood Volume and Pressure:

    • Blood pressure is closely linked with the kidney's ability to filter and regulate fluid.

  4. Maintenance of Blood pH:

    • Normal pH is around 7.4.

    • The kidneys manage acid-base balance by excreting protons and retaining bicarbonate ions to counterbalance excessive acid.

  5. Regulation of Blood Glucose:

    • Healthy kidneys prevent glucose loss in urine, which is crucial for preventing metabolic disorders, including diabetes.

  6. Hormonal Production:

    • The kidneys activate vitamin D and produce erythropoietin, a hormone that stimulates red blood cell production.

  7. Waste Excretion:

    • The kidneys filter metabolic waste and toxins, including drug metabolites and environmental pollutants.

    • Average urine output is approximately 1.5 to 2 liters daily.

Anatomy of the Kidney

External Morphology

  • Kidneys can vary in shape across species:

    • Horse: Heart-shaped kidney.

    • Cow: Lobulated appearance.

    • Sheep: Similar morphology to human kidneys.

Internal Structure

  • Cortex and Renal Pyramids:

    • The renal cortex is the outer region, while renal pyramids are identified within, performing filtering tasks.

  • Renal Pelvis: The space through which urine drains, leading to the ureters.

  • Blood Supply:

    • The renal artery brings oxygenated blood, while the renal vein returns filtered blood.

Nephrons: Functional Units of the Kidney

Overview

  • The nephron is the basic functional unit of the kidney, filtering vast amounts of blood every day:

    • Average filtration: 180 liters/day, leading to urine production of only 1.5-2 liters.

    • Nephrons ensure waste removal and reabsorption of needed substances.

Components of Nephrons

Renal Corpuscle

  • Glomerulus: A network of capillaries where blood filtration occurs.

  • Bowman's Capsule: Envelops the glomerulus, receiving the filtered fluid.

Renal Tubule

  • Divided into:

    • Proximal Convoluted Tubule (PCT): Main site for reabsorption of ions, glucose, and water.

    • Loop of Henle: Involved in the concentration of urine, consisting of descending and ascending limbs.

    • Distal Convoluted Tubule (DCT): Further fine-tuning of filtrate before urine formation.

    • Collecting Duct: Last stage of urine formation; regulates water reabsorption controlled by hormonal signals.

Types of Nephrons

  1. Cortical Nephrons: Primarily located in the outer cortex, constituting about 80-85% of nephrons in humans.

  2. Juxtamedullary Nephrons: Have a long loop of Henle extending deep into the medulla, essential for concentrating urine. Comprise about 10-15%.

Filtration and Reabsorption Processes

Filtration Mechanics

  • Filtration is size-selective, where water, glucose, ions, and small molecules pass through while larger molecules like proteins and blood cells remain in circulation.

Reabsorption and Secretion Mechanisms

  • Reabsorption: Movement of substances from the tubule back into the bloodstream, around 99% of filtered substances.

  • Secretion: Transfer of substances from blood into the tubule for excretion.

  • Key Processes:

    • Active Transport: Requires energy (ATP) to move substances against their gradient (e.g., sodium reabsorption).

    • Passive Transport: Movement along concentration gradients without energy (e.g., aquaporins facilitating water movement).

Electrochemical Gradients

  • The kidneys maintain ion gradients essential for proper reabsorption and secretion.

  • Important in maintaining plasma and urine composition.

Regulation of Filtration Rate

Influencing Factors

  • Blood Pressure Regulation: Filtration rate can be modulated by changes in blood pressure through various physiological mechanisms, including muscular and neural responses.

  • Estimated GFR: Glomerular filtration rate assessed via urine components (creatinine and other markers).

Conclusion and Importance

The speaker emphasizes understanding the kidney's functions, morphology, and the nephron's physiology is critical for both academic success and practical applications in health and disease management.