Kidney Function and Homeostasis HL

  • Homeostasis and Kidney Function

    • Overview

    • The kidneys play a twofold role in homeostasis.

    • Function 1: Osmoregulation

      • Definition: Controlling osmotic concentration in the body.

      • Osmotic concentration defined as osmoles per liter.

      • Conceptualization: Similar to total solute concentration in a cell.

      • Importance of osmotic concentration:

      • Controls solute concentration, water amounts, and pressure inside cells.

      • Water movement can affect pressure; maintenance is crucial within narrow limits.

    • Function 2: Excretion

      • Definition: Removal of toxic waste produced by metabolism.

      • Example: Urea, product of protein digestion.

  • Structure of the Kidney

    • Each kidney contains approximately one million nephrons, the filtering units.

    • Major structures within a nephron:

    • Bowman's Capsule:

      • C-shaped structure at the start of nephron.

      • Collects filtrate from glomerulus.

    • Proximal Convoluted Tubule (PCT):

      • Twisted structure that leads to loop of Henle.

    • Loop of Henle:

      • Ascending and descending limbs involved in concentration of urine.

    • Distal Convoluted Tubule (DCT):

      • Follows the loop of Henle.

    • Collecting Duct:

      • Final opportunity for water reabsorption before urine is excreted.

  • Blood Supply to the Nephron

    • Blood enters the nephron through the afferent arteriole.

    • Efferent arteriole: Slightly smaller than the afferent, resulting in high pressure in the glomerulus.

    • Glomerulus: A tangled network of fenestrated capillaries; facilitates ultrafiltration.

    • Definition of fenestrated: Capillaries with holes allowing for filtration.

  • Ultrafiltration Process

    • Blood enters glomerulus through afferent arteriole.

    • High pressure in the glomerulus forces substances out of the blood into Bowman's capsule.

    • Size and charge dictate filtering:

    • Small molecules and water pass through while blood cells and proteins remain.

  • Selective Reabsorption

    • Takes place primarily in the proximal convoluted tubule (PCT).

    • Returned substances from filtrate to the blood include glucose and some ions.

    • Nephron cells are adapted for this process:

    • Microvilli: Increases surface area for absorption.

    • Mitochondria: Provide ATP for active transport.

  • Mechanisms of Reabsorption

    • Sodium Ions: Actively transported from filtrate to blood.

    • Chloride Ions: Follow sodium ions due to opposite charges.

    • Water: Reabsorbed via osmosis in response to osmotic changes.

    • Glucose: Reabsorbed through facilitated diffusion or by glucose cotransporters.

  • Excretion of Waste Products

    • Molecules that are not reabsorbed include:

    • Urea and other metabolic byproducts that are toxic.

    • Excess salt or water that needs to be eliminated.

  • Gross Anatomy of the Kidney

    • Regions of the kidney:

    • Cortex: Outer region containing Bowman's capsule and PCT.

    • Medulla: Inner region where the loop of Henle is located.

    • Renal Pelvis: Collects urine before it flows to the ureter.

  • Loop of Henle Functionality

    • The descending limb:

    • Permeable to water; water exits via osmosis driven by osmotic concentration in the medulla.

    • The ascending limb:

    • Impermeable to water; actively transports sodium ions out to maintain high solute concentration in the medulla.

    • Prevents dilution of the medulla, allowing continued effective osmosis.

  • Antidiuretic Hormone (ADH) and Water Reabsorption

    • ADH increases the permeability of the collecting duct to water.

    • Released by the hypothalamus based on osmotic concentration in the blood.

    • High solute concentration triggers ADH release, leading to greater reabsorption of water and concentration of urine.

    • Without ADH, the collecting duct remains impermeable, resulting in diluted urine.

  • Influence of Activity on Kidney Blood Flow

    • Changes in blood flow occur in response to activity levels:

    • Sleep: Low kidney blood flow; conserving water to reduce urination during sleep.

    • Resting: Moderate kidney blood flow to facilitate efficient filtration.

    • Exercise: Low kidney blood flow; blood directed to skeletal muscles, reducing urine production.

  • Conclusion

    • Kidneys demonstrate an intricate relationship between structure and function, adjusting blood flow and solute levels to maintain homeostasis and physiological balance.