Animal Physiology

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

The excretory system plays a vital role in maintaining homeostasis through osmoregulation and waste removal. It balances water and solute concentrations in body fluids while eliminating metabolic byproducts. This complex system involves various organs, primarily the kidneys, which filter blood and produce urine. Urine formation occurs through filtration, reabsorption, and secretion in nephrons. Hormones like ADH and aldosterone regulate these processes, adapting to different environmental challenges. Understanding the excretory system is crucial for diagnosing and treating various disorders, from kidney stones to renal failure.

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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


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.