Physiology of Excretion | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Physiology of Excretion

Water's Vital Role

• Water is closely tied to all aspects of life, forming chemical and physical connections with living matter.
• Its high specific heat allows it to absorb heat without a significant temperature rise, serving as the matrix for vital processes.

Body Water Distribution

• Total body water varies among species, ages, sexes, and nutritional statuses.
• In lean adult non-herbivores, about 70% of the body is water, with newborns having the highest content.
• Fat tissue has low water content (10% or less), and very fat animals have around 40% total water.

Fluid Compartments

• Water within cells is Intracellular fluid, while water outside cells is Extracellular fluid.
• Extracellular fluid is divided into interstitial fluid or plasma by vascular system walls.
• Approximately 50% of water is within cells, 15% in interstitial fluid, and 5% in blood plasma.
• Alimentary canal water is termed extracellular, and specific fluids like CSF, aqueous humor, synovial fluid, urine, and bile are transcellular fluids.

Water Movement Dynamics

• Water moves between extracellular and intracellular compartments, maintaining balance.
• Excessive water added to extracellular fluid without proper pressure involvement can disrupt metabolic functions, leading to water intoxication.

Effect of Sodium Chloride Solutions

• Administering hypotonic sodium chloride solutions can cause water to flow into the plasma.
• Isotonic sodium chloride has little impact on cellular water.
• The capillary wall freely allows water and electrolyte movement, while the cell membrane restricts NaCl movement, causing cellular dehydration with increased extracellular osmo-concentration.

Water Balance in the Body

Constant Total Water Amount

• The total body water remains relatively constant.
• Water is gained through ingestion, cellular metabolism, and lost through urine, skin, expired gases, and feces.
• Lactating animals lose water in milk.

Water Deprivation and Thirst

• Continuous water loss creates a deficit corrected by periodic water ingestion.
• Water deprivation triggers the sensation of thirst and a behavioral drive to drink water.
• Thirst can also be stimulated by a drop in blood pressure.

Urinary Excretion and Hormonal Control

• Depriving animals of water decreases urine excretion, controlled by ADH (Antidiuretic Hormone).
• ADH, also known as Vasopressin, increases water reabsorption in the kidneys, reducing urine output.
• Blood concentration of ADH rises during water deprivation and falls when over-hydrated.

ADH Release and Osmotic Concentration

• ADH release is influenced by changes in plasma osmotic concentration.
• Rising osmotic concentration stimulates ADH release, while low concentration limits or halts ADH release.

ADH in ECF Volume Regulation

• ADH regulates Extracellular Fluid (ECF) volume.
• ADH release can be stimulated by pain, exercise, stress, and variations in Glomerular Filtration Rate (GFR).

Water Sources

• Food, in addition to drinking water, serves as a water source.
• Metabolic water is produced during the oxidation of food, contributing to water balance.

Water Loss by Different Routes

• Water is lost through urine, feces, expired gases, and skin surface.
• Internal disturbances, like thermal polypnea in some animals, can lead to significant and rapid water loss.
• Water loss through skin diffusion and respiratory tract evaporation is termed Insensible Water Loss.

Water Loss and Dehydration

Sweating and Heat Stress

Sweating is a significant water loss method, especially under heat stress in humans and horses.

Dehydration Issues

• Intermittent water intake with continuous loss leads to slow dehydration.
• Dehydration involves loss of water and electrolytes.
• Signs include increased thirst, decreased urine output, and reduced food intake.

Electrolytes

• Extracellular Electrolytes:
  • Sodium (Na), Chloride (Cl), Bicarbonate ions (HCO₃).
  • Blood plasma contains proteins influencing osmotic balance.
• Intracellular Electrolytes:
  • Potassium (K), Magnesium (Mg), small amounts of Sodium.
  • Organic phosphates, proteinates, sulfates, and some bicarbonates.

Sodium Excretion and Aldosterone

• Kidney excretes sodium through glomerular filtration and tubule reabsorption.
• Aldosterone release increases sodium reabsorption when plasma Na falls.
• Renin, released due to low plasma Na, triggers aldosterone release.
• Another hormone (Natriuretic hormone) increases sodium excretion.

Potassium Handling

• Major intracellular cation exchanged with extracellular potassium.
• Renal handling complex, inversely related to sodium.
• Inverse relationship means when sodium excretion increases, potassium excretion decreases.

Renal Transport Processes

• Glomerular Filtration:
  • Polar substances like hexamethonium excreted via glomerular filtration.
  • Exist in urine in their ionic or dissociated forms.
• Active Transport:
  • Some organic compounds actively transported by tubular cells.
  • Requires metabolic energy, maintains concentration gradients.
  • Examples: Glucose, amino acids, para-amino hippurate, N-methyl nicotinamide.

Uric Acid Elimination

• End product of purine metabolism.
• Eliminated through a combination of filtration, secretion, and reabsorption.
• Converted to allantoin in most mammals, except for humans and higher apes.