🐅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.
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