Animal Excretory Systems

Animal Excretory Systems

• Osmoregulation balances water and salt levels in the body
• Excretion is the removal of metabolic wastes via an animal's osmoregulatory system

Ammonia

• Ammonia results from the breakdown of amino and nucleic acids
• Aquatic animals excrete ammonia directly due to its high solubility
• Terrestrial animals convert ammonia to less toxic urea or uric acid

Urea

• Urea is excreted in a moderately concentrated solution
• Mammals, sharks and amphibians produce urea
• Urea allows body water to be conserved
• Urea requires more energy than ammonia to produce

Uric Acid

• Uric acid requires less water to excrete nitrogen
• A long, complex series of enzymatic reactions synthesize uric acid
• Reptiles, birds, and insects produce uric acid
• Uric acid requires more energy to produce than urea
• Uric acid allows invasion of drier habitats far from water
• Uric acid is advantageous for shelled embryos
• Nitrogenous wastes are stored until hatching
• Gout in humans results from the buildup and precipitation of uric acid around joints

Nitrogenous Wastes

• Proteins hydrolyze into amino acids
• The breakdown results in carbon chains and amino groups
• Carbon chains can be used as an energy source
• Amino groups are excreted as ammonia, urea, or uric acid

Invertebrate Excretory Systems

• Most animals have tubular excretory organs
• These organs work to regulate water-salt balance
• These organs work to excrete metabolic waste into the environment
• Urine is a liquid that contains metabolic wastes, excreted salts, and water
• Planarians possess flame cells
• Earthworms possess nephridia
• Insects possess Malpighian tubules
• Earthworms secrete diluted urine through excretory pores
• Insects transport uric acid from hemolymph into Malpighian tubules
• Water reabsorption occurs at the rectum following active transport in insects
• Semisolid uric acid leaves insect's bodies through the anus

Specific Invertebrate Excretory Organs

• Aquatic crustaceans excrete nitrogenous wastes through their gills
• Green glands are excretory structures in the head region of some crustaceans
• Maxillary glands in maxillary segments function as excretory organs in shrimp and pill bugs
• Spiders, scorpions, and other arachnids use coxal glands near appendages for excretion

Animal Osmoregulation by Aquatic Vertebrates

• Kidneys are the most important organs involved in osmoregulation in most vertebrates
• Kidneys maintain critical functions of homeostasis
• Kidneys maintain the balance between water and different types of salts
• Ions like sodium, potassium, and chloride affect skeletal, nervous, and muscular systems
• Kidneys produce urine, which contains a number of different metabolic wastes
• Urine concentration varies based on the animal's environment as well as water and salt intake

Osmoregulation in Fish

• Cartilaginous fishes have blood that's nearly isotonic to seawater
• Their blood contains enough urea to match seawater tonicity
• Marine bony fishes live in a marine environment:
• High in dissolved salts
• Hypertonic to blood plasma
• Promotes osmotic water loss
• Causes the gain of ions by drinking water
• Freshwater bony fishes live in a freshwater environment
• This tends to promote a gain of water by osmosis
• This tends to create a loss of ions as excess water is excreted

Challenges of Osmoregulation in Fish

• Marine bony fish blood plasma are hypotonic to sea water
• Salt passively loses water through gills
• Salt must constantly drink seawater to compensate
• Excess salt ions are actively transported back into seawater through the gills
• Freshwater bony fish blood plasma are hypertonic to fresh water
• Water passively gains water through gills
• Freshwater fish eliminate excess water through copious hypotonic urine

Terrestrial Vertebrate Osmoregulation Strategies

• Terrestrial animals lose water via excretion and respiration
• Terrestrial animals drink water to compensate for water loss
• They reduce excretory water loss by excreting relatively insoluble uric acid
• Some animals (e.g., kangaroo rats) feature highly convoluted nasal passages
• These animals also feature a mucous membrane surface that captures condensed water from exhaled air
• A kidney structure, the loop of the nephron, is longer and more efficient

Marine Mammals and Seabirds

• Marine mammals have evolved on land; their kidneys are good at conserving water
• Some have secondarily adapted to living in or near the sea
• Some possess specialized salt glands to actively transport salt from the blood
• Salt-excreting glands are located near the eyes in seabirds
• In sea turtles, the salt gland is a modified tear gland
• Gland activity are regulated by the nervous system

Human Urinary System

• Human kidneys are bean-shaped, reddish-brown organs, about the size of a fist
• Kidneys are located on each side of the vertebral column, just below the diaphragm
• Each kidney is connected to a ureter
• Ureters conduct urine from the kidneys to the urinary bladder
• The urinary bladder stores urine
• Urine is voided through the single urethra

Kidneys

• Kidneys are composed of three major parts
• Renal cortex: the outer region with a granular appearance
• Renal medulla: contains 6-10 cone-shaped renal pyramids
• Renal pelvis: the hollow-chambered innermost part

Nephron

• Each kidney contains over 1 million tiny tubular nephrons that produce urine
• Each nephron is made of several parts:
  • Glomerular capsule (Bowman’s capsule)
  • Glomerulus
  • Proximal convoluted tubule
  • Loop of the nephron (loop of Henle)
  • Distal convoluted tubule
  • Collecting duct

Urine Formation

• Urine production requires three distinct processes
  • Glomerular filtration at the glomerular capsule:
    • Blood pressure causes small molecules to move from the glomerulus to inside the glomerular capsule
    • The glomerular filtrate is the same as plasma, but without proteins
  • Tubular reabsorption at the convoluted tubules entails:
    • Active transport of ions
    • Passive water movement
    • Selective reabsorption of nutrients
  • Tubular secretion at the convoluted tubules entails:
    • Active secretion of certain molecules (e.g. penicillin) into the tubules
    • Getting rid of potentially harmful, non-filtered compounds

Kidneys and Homeostasis

• Kidneys perform four homeostatic functions:
  • Excretion of metabolic wastes
  • Maintenance of water-salt balance
  • Maintenance of acid-base balance (pH balance)
  • Secretion of hormones like erythropoietin to stimulate red blood cell production

Water-Salt Balance

• Excretion of hypertonic urine is dependent upon water reabsorption from the loop of the nephron and collecting duct
• An osmotic gradient within the renal medulla causes water to leave the descending limb along its entire length
• This is a countercurrent mechanism
• Water passes through water channels called aquaporins
• Antidiuretic hormone (ADH)
  • Released by the pituitary gland
  • Promotes water reabsorption in the collecting duct
  • Allows excretion of hypertonic urine with diminished fluid
• Diuresis increases the amount of urine
• ADH causes aquaporins to be inserted into the DCT and collecting duct cells

Hormones and Salt Reabsorption

• Hormones control salt reabsorption
  • The renin-angiotensin-aldosterone system:
    • Low blood volume causes the kidneys to secrete renin
    • Renin changes angiotensinogen into angiotensin I
    • Angiotensin I is converted to angiotensin II
    • Angiotensin II stimulates the adrenal glands to release aldosterone
    • Aldosterone promotes sodium excretion and potassium reabsorption at the distal convoluted tubule
    • Reabsorption of ions is followed by water reabsorption
    • Blood volume and pressure increase
  • Atrial natriuretic hormone (ANH):
    • Secreted by the atria when cardiac cells stretch due to high blood volume
    • ANH inhibits renin and aldosterone secretion
    • This promotes excretion of sodium and water which follows
    • Blood volume and pressure decrease

Acid-Base Balance

• Cell functions are influenced by pH
• pH is regulated by
  • The bicarbonate buffer system:
    • Adjusted by either the reabsorption of bicarbonate ions or the secretion of hydrogen ions
• Breathing:
  • Excretion of carbon dioxide by the lungs affects bicarbonate levels in the blood