KD

5.3 bio notes

Osmoregulation Part 2

Osmoregulation Principle

  • Terrestrial animals face challenges in osmoregulation due to water and electrolyte loss.
  • The principle of retention is key: absorbing substances back into the body before excretion.
  • Retention relies on filtration and reabsorption.
  • Filtration: separating water-based bodily fluids from cells and large molecules.
  • Reabsorption: selectively absorbing desired components from the filtered material.

Mammalian Osmoregulation

  • The kidney is the primary organ for filtration and reabsorption.

  • Blood enters the kidney via the renal artery.

  • Blood plasma is separated from the blood within the kidney.

  • The kidney reabsorbs water, electrolytes, and non-waste molecules back into the blood.

  • Two fluids exit the kidney:

    • Freshly processed blood via renal veins.
    • Urine (non-reabsorbed material) via the ureter.

  • Urine is stored in the urinary bladder before excretion.

Kidney Regions

  • Three major regions in cross-section (outer to inner):
    1. Renal Cortex
    • Contains nephrons (filtering units).
    1. Renal Medulla
    • Concentrates filtered blood plasma into urine.
    1. Renal Pelvis
    • Collects urine and funnels it into the ureter.

Nephron and Medullary Gradient

  • Each kidney has approximately 1-1.5 million nephrons (tube-shaped filtering units).
  • Nephrons utilize active transport, diffusion, and osmosis for reabsorption.
  • The medullary gradient, an osmotic gradient of electrolytes, facilitates reabsorption.
  • Osmolarity increases from the cortex to the medulla.
    • Blood osmolarity is about 300 milliosmoles.
    • Deepest medulla reaches approximately 1200 milliosmoles.
  • The medullary gradient allows the kidney to remove large volumes of water and electrolytes from the blood.

Nephron Sections

  • Five major sections:

    1. Renal Corpuscle
    • Plasma filtration occurs here.
    1. Proximal Tubule
    2. Loop of Henle
    3. Distal Tubule
    4. Collecting Duct

  • The latter four sections reabsorb filtered blood plasma.

Renal Corpuscle

  • The renal corpuscle filters plasma from the blood.
  • Filtered material is called filtrate or pre-urine.
  • The glomerulus, a network of blood vessels with tiny holes, is located inside the corpuscle.
  • The holes allow blood plasma, electrolytes, and small molecules to leave.

Filtration

  • Blood pressure drives filtration in the renal corpuscle by pushing plasma through the glomerulus holes.
  • Blood pressure imbalances can lead to kidney issues:
    • Hypertension (long-term high blood pressure) damages the corpuscle, potentially causing kidney failure.
    • Hypotension (short-term low blood pressure) halts filtration, leading to toxic waste buildup.

Obligatory Reabsorption

  • The body filters the total blood volume about 30 times per day.
  • With 5-6 liters of blood, approximately 150-180 liters of pre-urine are produced daily.
  • 90% of pre-urine is automatically reabsorbed, known as obligatory reabsorption.
  • Obligatory reabsorption occurs in the proximal tubule and loop of Henle.

Proximal Tubule

  • The proximal tubule reabsorbs about 2/3 of all materials in the pre-urine through active transport, diffusion, and osmosis.
  • It reabsorbs water, electrolytes, and all nutrients (glucose, amino acids, fatty acids, etc.)
  • The pre-urine leaving the proximal tubule consists of water, electrolytes, and waste products.

Loop of Henle

  • The loop of Henle reabsorbs water and electrolytes using diffusion and osmosis, driven by the medullary gradient (no energy required).
  • Descending Limb:
    • Permeable to water only.
    • Water exits via osmosis; electrolytes cannot leave.
  • Ascending Limb:
    • Permeable to electrolytes only.
    • Electrolytes exit via diffusion; water cannot leave.

Loop of Henle and Osmolarity

  • In the descending limb, pre-urine encounters a high osmolarity environment causing water to leave by osmosis.
  • In the ascending limb, pre-urine encounters a low osmolarity environment causing electrolytes to leave by diffusion.

Regulated Reabsorption

  • Of the initial 150-180 liters of pre-urine, 90% is reabsorbed through obligatory reabsorption in the proximal tubule and loop of Henle.

  • The remaining 15-18 liters are subject to homeostatic regulation via regulated reabsorption.

  • Regulated reabsorption takes place in the distal tubule and collecting duct.
    Note: Normally, most of this is reabsorbed, resulting in 1-2 liters of urine per day (99-99.5% total reabsorption), but this can change due to homeostatic imbalances or disease.

Distal Tubule

  • The distal tubule reabsorbs sodium and chloride electrolytes through active transport.
  • Aldosterone regulates the amount of reabsorption.
    • Low sodium levels trigger high aldosterone release, increasing sodium and chloride reabsorption.
    • High sodium levels trigger low aldosterone release, decreasing sodium and chloride reabsorption.

Collecting Duct

  • The collecting duct reabsorbs water.
  • Anti-diuretic hormone (ADH) regulates the amount of reabsorption.
    • Low water levels trigger high ADH release, increasing water reabsorption.
    • High water levels trigger low ADH release, decreasing water reabsorption.

ADH and Water Permeability

  • High ADH levels: The collecting duct is highly permeable to water, leading to water reabsorption, resulting in a small volume of concentrated urine.
  • Low ADH levels: The collecting duct is not very permeable to water, leading to less water reabsorption, resulting in a large volume of dilute urine.