🐅Animal Physiology Unit 11 – Excretory System: Osmoregulation & Waste

Key Concepts and Terminology

• Osmoregulation maintains optimal concentrations of water and solutes in body fluids
• Excretion removes metabolic waste products and excess substances from the body
• Osmolarity measures the total solute concentration in a solution (mOsm/L)
• Osmosis passive movement of water across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration
• Diffusion net movement of particles from an area of high concentration to an area of low concentration
• Urea nitrogenous waste product formed from the breakdown of amino acids in the liver
• Antidiuretic hormone (ADH) regulates water reabsorption in the kidneys to maintain blood osmolarity
• Aldosterone mineralocorticoid hormone that regulates sodium and potassium balance in the body

Osmoregulation Basics

• Osmoregulation crucial for maintaining homeostasis in animals
• Involves balancing water and solute concentrations in extracellular and intracellular fluids
• Osmotic pressure drives the movement of water across cell membranes
  • Water moves from areas of low solute concentration to areas of high solute concentration
• Osmolarity of body fluids tightly regulated within a narrow range
  • Deviations can lead to cell swelling or shrinkage and disrupt cellular functions
• Osmoconformers (marine invertebrates) maintain internal osmolarity equal to their surroundings
• Osmoregulators (most vertebrates) maintain internal osmolarity different from their surroundings
• Osmotic challenges faced by animals in different environments (freshwater, marine, terrestrial)

Excretory System Structure

• Excretory system consists of organs and tissues involved in removing waste products and regulating water and solute balance
• Kidneys primary excretory organs in vertebrates
  • Contain functional units called nephrons
  • Each nephron consists of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule
• Renal corpuscle site of filtration where blood is filtered to form an ultrafiltrate
• Renal tubule divided into proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct
  • Reabsorption and secretion of substances occur along the renal tubule
• Ureters transport urine from the kidneys to the urinary bladder for storage
• Urethra carries urine from the urinary bladder to the external environment during urination

Urine Formation Process

• Urine formation involves three main processes: filtration, reabsorption, and secretion
• Filtration occurs in the renal corpuscle (glomerulus)
  • Blood pressure forces plasma and small solutes through the glomerular capillaries into Bowman's capsule
  • Larger molecules (proteins) and blood cells are retained in the bloodstream
• Reabsorption selective process in which essential substances (water, glucose, amino acids, ions) are transported from the tubular lumen back into the bloodstream
  • Occurs primarily in the proximal convoluted tubule and loop of Henle
• Secretion process by which substances (hydrogen ions, potassium, ammonia) are actively transported from the peritubular capillaries into the tubular lumen
  • Helps maintain acid-base balance and eliminates certain waste products
• Countercurrent multiplication system in the loop of Henle creates a concentration gradient that facilitates water reabsorption
• Collecting duct permeability to water regulated by antidiuretic hormone (ADH)
  • ADH increases water reabsorption, leading to concentrated urine

Waste Products and Their Elimination

• Metabolic waste products include urea, uric acid, ammonia, and creatinine
• Urea main nitrogenous waste product in mammals
  • Formed in the liver through the ornithine cycle (urea cycle)
  • Transported to the kidneys for excretion in urine
• Uric acid nitrogenous waste product in birds, reptiles, and some terrestrial invertebrates
  • Less toxic than ammonia and requires less water for excretion
• Ammonia toxic nitrogenous waste product excreted by aquatic animals (fish)
  • Directly released into the surrounding water
• Creatinine waste product of muscle metabolism
  • Filtered by the glomerulus and excreted in urine
• Other waste products (bilirubin, excess hormones, drugs) eliminated through the excretory system

Adaptations in Different Animals

• Excretory systems adapted to the specific needs and environments of different animals
• Freshwater animals (fish) face the challenge of water influx and ion loss
  • Produce large volumes of dilute urine to eliminate excess water
  • Active uptake of ions through gills and kidneys to maintain ionic balance
• Marine animals (sharks, marine mammals) face the challenge of water loss and salt gain
  • Produce small volumes of concentrated urine to conserve water
  • Salt glands (seabirds, marine reptiles) secrete excess salt
• Desert animals (kangaroo rats, camels) have adaptations to conserve water
  • Produce highly concentrated urine
  • Efficient water reabsorption in the kidneys
• Insects have Malpighian tubules as excretory organs
  • Tubules absorb waste products and ions from the hemolymph and produce urine
• Flame cells excretory structures in flatworms (planaria)
  • Collect and eliminate waste products from the body

Hormonal Control of Excretion

• Hormones play a crucial role in regulating excretory functions
• Antidiuretic hormone (ADH) secreted by the posterior pituitary gland
  • Increases water permeability in the collecting duct, promoting water reabsorption
  • Released in response to increased blood osmolarity or decreased blood volume
• Aldosterone mineralocorticoid hormone secreted by the adrenal cortex
  • Stimulates sodium reabsorption and potassium secretion in the distal convoluted tubule and collecting duct
  • Helps maintain blood volume and pressure
• Atrial natriuretic peptide (ANP) secreted by the atria of the heart
  • Increases sodium excretion and reduces blood pressure
  • Inhibits the release of ADH and aldosterone
• Parathyroid hormone (PTH) regulates calcium and phosphate homeostasis
  • Increases calcium reabsorption and phosphate excretion in the kidneys
• Renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and fluid balance
  • Renin (enzyme) released by the kidneys in response to decreased blood pressure or flow
  • Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II
  • Angiotensin II stimulates aldosterone secretion and vasoconstriction

Clinical Applications and Disorders

• Urinalysis diagnostic tool that assesses the physical, chemical, and microscopic properties of urine
  • Helps detect kidney disorders, urinary tract infections, and metabolic diseases
• Renal failure occurs when the kidneys lose their ability to filter waste products effectively
  • Acute renal failure sudden onset, often reversible
  • Chronic renal failure gradual, progressive loss of kidney function
• Nephrotic syndrome characterized by excessive protein loss in the urine (proteinuria) due to increased glomerular permeability
• Renal calculi (kidney stones) hard deposits of minerals and salts that form in the kidneys or urinary tract
  • Can cause pain, obstruction, and infection
• Urinary tract infections (UTIs) bacterial infections affecting the urethra, bladder, or kidneys
  • More common in females due to shorter urethra
• Diabetes insipidus disorder characterized by excessive thirst and urination
  • Caused by insufficient ADH production (central diabetes insipidus) or kidney unresponsiveness to ADH (nephrogenic diabetes insipidus)
• Dialysis medical procedure that filters waste products and excess fluids from the blood when the kidneys are not functioning properly
  • Hemodialysis uses an artificial kidney machine
  • Peritoneal dialysis uses the lining of the abdominal cavity (peritoneum) as a natural filter