ExcretionBrianna & Keonna

OverviewThe balance of water in the body

depends on the rate of exchange of solute between internal body fluids and the external environment.

Excretory Systems are key to homeostasis because the dispose of metabolic wastes, and control body fluid composition.

Nitrogenous Wastes Animals excrete nitrogenous wastes

as ammonia, urea or uric acidAnimals that excrete ammonia need

access to lots of water because ammonia can only be tolerated in low concentrations

Urea combines ammonia with carbon dioxide, and it has a very low toxicity◦Excreted by vertebrate mammals, turtles,

amphibians, sharks, etc.

Insects, birds, snails, and many reptiles uric acid which is essentially non toxic.

Its non toxicity makes it hydrophobic, thus it can be excreted without much water loss.

Requires more ATP to the synthesize from ammonia.

Excretory ProcessesMost animals produce fluid waste

known as Urine◦Body fluid (i.e. Blood) comes in

contact with a selectively permeable membrane of the transport epithelium

◦Filtration is driven by hydrostatic pressure (blood pressure) Water, small salts, sugars, nitrogenous

wastes, and amino acids are removed from body fluids to create filtrate

Filtrate is converted into fluid waste when reabsorption places useful materials from the filtrate back into the blood stream by active tranport.

Selective Secretion pumps nonessential solutes and wastes into the filtrate by active transport.

Excretory SystemsExcretory systems vary widely

among animal groups◦Flatworms that lack a coelom or

body cavity have protonephridia which form network of dead end tubules connected to external openings

◦Annelids (ex: earthworms) have metanephridia, excretory organs that open internally to the coelom

Insects and terrestrial anthropods have Malpighian tubules which extend from dead end tips to the digestive tract

Kidneys are present in vertebrates and chordates◦Consist of highly organized tubules

associated with a network of capillaries◦Ducts and other structures carry urine

from the tubules and out of the body

Structure of Mammalian Excretory SystemThe average kidney is about 10

cm longBlood is supplied to the kidney

through the renal artery and drained by the renal vein ◦take up less than 1% of the body’s

mass but receive ¼ of the blood that leaves the heart

Urine exits the kidney through two ducts called ureters

Ureters drain into the urinary bladder

Urine is expelled through a tube called the urethra

The sphincter muscles regulate urination near urethra and bladder