Excretory 1

Excretion

• Maintenance of internal environment is vital to survival (e.g. cell integrity)

• Needs the correct composition of water, solutes e.g. NaCl and low levels of waste

• Delicate balance is affected by diet, metabolic products and loss/gain of water and solutes

Selective excretion

It is crucial to internal fluid homeostasis and involves several system

Primary functions of excretory systems are:

• maintenance of inorganic solutes (e.g. Na+, K+, Cl-)

• maintenance of proper plasma water volume

• removal of metabolic waste product (e.g. urea)

• maintenance of water balance

The selective excretion involves the following systems:

• Respiratory (e.g. gills, lungs)

• Digestive (e.g. liver, intestinal epithelia)

• Skin and glands (e.g. salt glands)

• Renal (e.g. Malpighian tubules, kidneys)

Nitrogen metabolism

• Food produces CO2; protein and nucleic acid metabolism also produce nitrogenous waste (e.g. Proteins to Amino acids to Ammonia)

• NH3 is highly toxic and can bind H+ to form NH4+, which is also toxic to particular neurons

• 0.05mM NH4+ inhibits mammal neutral function; 2mM in fish.

• Thus NH3 and NH4+ must either be quickly excreted or converted to less toxic form

Nitrogenous waste products

Epithelium

Sheet of cells that covers a body surface or organ, or lines a cavity and forms a boundary between functionally different regions of the body or between the animal and the external environment.

Epithelial transport

• Numerous functional capacities

• Play major functional roles in animal physiology, particularly in excretion

Epithelial tissue diagram

Epithelial cells can form:

1. Sheets or layers e.g. skin

2. Tubules e.g. kidney nephron

3. follicles (hollow globes) e.g. sweat glands

Transcellular vs Paracellular transport

Salt Transport Only

Salt Transport diagram

Renal tubes

Produce urine and heavily rely on transport epithelia

Renal tubule processes and renal organs:

Involved processes:

• Filtration

• Secretion

• Reabsorption

• Osmoconentration

Major renal organs:

• Protonephridia

• Meso- and Metanephridia

• Malpighian tubules

Protonephridia

• In simple animals (e.g. flatworms)

• Ultrafiltration driven by cilia in ducts (‘‘flame’’ cells)

• Ducts projects into body cavity

• Process involves both secretion and absorption

Mesonephridia and Metanephdria

• In animals with 2 or more major fluid spaces (e.g. coelom and circulatory system)

• Basically consist of

  • filtering capsule/funnel-like opening

  • tubule segment for selective secretion + absorption

  • Canal or ureter leading to bladder

Verebrates tubules are called:

Nephrons

Invertebrates tubules are called:

Metanephric-type tubules also present in molluscs, annelids and some arhtropoda

Crustacean Antennal Gland

Diuretic Hormonee

e.g. mosquito

• Must excrete large fluid load

• Include serotonin and mosquito natriuretic peptide (MNP)

• Enhance secretion

Anti-diuretic hormone

e.g. mealworm beetle

• consume dry food and need to retain fluid

• tenmo-adfa reduces secretion and hence conserves water

Elasmobranch fish

• Retain urea and trimethylamine oxide (TMAO) as osmolytes, TMAO helps retain the urea for long periods of time to balance the creature.

• Osmoconformers

• Body fluids matches external environment (e.g. seawater); osmolality ~1000mOsm

• Use gills, kidneys and rectal glands for excretion and retention (e.g. sharks and rays)

Marine bony fish

• Use gills for most excretion and retention processes

• Osmoregulators

• e.g. blue fin tuna

Freshwater bony fish

• excrete water with their kidneys

• use gills to excrete wastes and take up salts

• Osmoregulators

• e.g. Mozambique tilapia

Terrestrial vertebrates

• Evolved new renal and external mechanisms to maintain ‘‘internal ocean’’

• Human kidney, functional unit = Nephron

Amphibians

• Use kidneys and bladder for excretion and retention

Non-avia reptiles

• use amphibian-like kidneys, hindgut and salt glands for excretion and retention (in marine and desert species.

Birds

• Have kidneys with both reptile-like and mammalian like nephrons, hindguts and salt glands for excretion and retention (in marine species)

Mammals

Kidneys have two different nephrons:

Functions of nephron regions

• Humans: filter ~180 litres of blood per day: ~99% of reabsorbed, ~1.5 litres of urine produced

The urinary system diagram

Rat Urinogenital Tracts (dissected)

Bladder storage and Micturition

• Mammals, amphibians and fishes. Urine is temporarily stored in the bladder

• Bladder wall = smooth muscle lined with special epithelial cells (umbrella cells)

• Exit of urine guarded by two urethral sphincters

  • Internal: smooth muscle, no control

  • External: skeletal muscle, under cortical control via motor neurons

• Emptied by process of micturition (e.g. urination)

Diagram of human bladders

Control of Micturition