Homeostasis - An Introduction to the Kidney

Homeostasis

“A constant internal environment is a necessary condition for life under varying external conditions”

  • Importance of maintaining fluid around cells within narrow limits to ensure proper body function.

  • Small fluctuations can disrupt biochemical activities; larger fluctuations can result in cell death.

  • Maintenance of a constant internal environment - Homeostasis

    • Greek for 'staying the same.'

Key Features of Internal Environment

  • Chemical composition: Ions, glucose.

  • Blood pH: 7.35 to 7.45

    • CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

  • Osmotic pressure: Osmoregulation.

  • Temperature regulation:

    • Endotherms (birds, mammals) maintain temperature.

    • Ectotherms (most fish, amphibians) not regulated tempreture

Temperature Regulation Mechanism

  • regional heterothermy - tuna

    • some parts for swimming are warmer than other parts

  • preflight warm up - moths/bees

    • start shivering to get energy and warm, them up

  • Behavioural regulation in an ectotherm - lizard

    • radiates heat on rocks

  • regulation and conforming

    • river otter - regulator

    • largemouth bath - conformer

Control System

  1. Stimulus: Temperature change.

  2. Receptors: Skin and hypothalamus.

  3. Control Centre - set point: Hypothalamus.

  4. Effectors: Skin blood vessels, sweat glands, hair (fur), muscles.

  5. Responses: Constriction/dilation of blood vessels, sweat production, piloerection, shivering thermogenesis.

    negative feedback loop

Osmosis and Solutions

  • Hyperosmotic Solution

    • More solutes than other solution - less water.

  • Hypo-osmotic Solution

    • Less solutes than other solution - more water.

  • Water moves by osmosis from low to high concentration

    • into a hyperosmotic solution, out of a hypo-osmotic solution

  • A solution with few solutes in it will have lower osmoticpressure than one withmany solutes

  • Osmolarity : Number of osmoles of solute per liter of solution.

  • analogy - squash in high concentrations has more solute, less solution and less water = hyper. squash in low concentrations has less solute, more solution, more water = hypo.

  • Water will move from a low to a high osmotic pressure solution (opposite)

Functions of the Kidney

  • Nitrogenous waste removal: Metabolism of proteins/nucleic acids.

  • Water content regulation: Osmoregulation.

  • Salt balance regulation: Concentrations of Na+, K+, Cl-, Ca2+, Mg2+ (ionic regulation).

Nitrogenous Waste Products

  • Ammonia (NH3): Highly toxic waste product.

  • Urea: Less toxic, excreted mostly by mammals.

  • Uric Acid: Very low toxicity, excreted by birds and reptiles.

Comparison of Nitrogenous Wastes

Waste Type

Toxicity

Solubility (ml/g N)

Metabolic Cost

Examples

Ammonia

High

500

None

Fish, Aquatic Amphibians

Urea

Low

50

Some

Mammals

Uric Acid

Very Low

1

High

Birds, Terrestrial Reptiles

Structure of the Kidneys

  • Two bean-shaped organs, each the size of a clenched fist, located against the back wall of the abdomen.

  • Kidneys comprise only 1% of body weight but receive 25% of cardiac output (1.25 liters/min).

  • Control chemical composition of blood.

  • connects to bladder with ureters which prevent urine from reentering the kidney

Kidney Anatomy

  • Parts: Renal pelvis, ureter, cortex, medulla.

  • Nephron: Over 2 million nephrons per kidney; approximately 60 km in total length.

Nephron Structure

  • Two types of nephron

    • Juxta-medullary (concentrated urine)

    • cortical (less concentrated urine).

      peritubular capillaries*

  • Key Components:

    • Vasa recta (blood vessels)

    • Afferent arteriole

    • Efferent arteriole

    • Peritubular capillaries.

  • 85% of nephrons are cortical; 15% are juxta-medullary (only found in birds and mammals for concentrated urine).

Bowman’s Capsule

  • Fluid moves via ultrafiltration.

  • Hydrostatic pressure of blood plasma forces fluid and solutes through glomerular capillaries into Bowman’s capsule.

  • The process is passive (no energy needed).

Substances Passing Through Bowman’s Capsule

  • Molecules <1.8 nm filter freely (water, glucose, urea, amino acids).

  • Molecules between 1.8 – 4.2 nm filter but less freely.

  • Molecules >4.2 nm do not pass (e.g., proteins).

Filtration Mechanism and Forces

  • Hydrostatic Force: Blood pressure in glomerular capillaries is high due to:

    • Low resistance input pathway (large diameter arteries).

    • Constriction of arteriole increases pressure.

    • High resistance due to numerous thin capillaries.

  • Promoting Filtration:

    • Glomerular hydrostatic pressure = 55 mm Hg (promotes fluid movement out of plasma and into Bowman’s capsule).

  • Opposing Filtration:

    • Capsular hydrostatic pressure = 15 mm Hg (resists filtration).

    • Glomerular colloid osmotic pressure = 30 mm Hg (osmotic pressure in capillaries).

  • Net Filtration Pressure: 10 mm Hg (calculated as 55 - 15 = 40, 40-30 = 10).