Body Fluid Regulation and

Excretion Chapter 36

Excretion • The excretory system monitors the concentration of

body fluids and removes harmful or waste substances

from the body

• Water, nutrients and minerals that exit the body through

excretion, exhaling, or evaporation must be replaced

in equal amounts

• Excretion occurs differently in aquatic and terrestrial

animals

Aquatic Excretion • The fluids of aquatic animals are nearly isotonic to

seawater

o Removing waste through osmosis is difficult

• Even if they live in salt water, fish have unusually low

amounts of salt

• Fish are at risk of constant dehydration (yep, fish get

dehydrated) unless they regulate their waste content

• Cartilaginous fish build waste as a mixture of ammonia

and urea, a combination so toxic it would be deadly to

terrestrial animals

• The high amount of solutes ensures the organisms are

isotonic with the high salt concentrations of their

environments

Terrestrial Excretion • Animals that live near the sea are usually able to drink

saltwater, reducing the likelihood of dehydration

• Some organisms prevent dehydration by secreting high

levels of nitrogenous waste

• Others, like camels, have large amounts of mucus

which collect air/water during exhalation

• Either by removing waste at a high rate or preventing

water loss, organisms are able to conserve water

• Humans accomplish water conservation by adjusting

the amount of water in their waste urine.

Waste Products • Ammonia is the main excretory chemical in fishes

o Ammonia is formed by Amino groups removed from amino acids in digestion.

o Ammonia requires high amounts of water to safely flush out, but little energy to form.

• Amphibians and mammals excrete urea as their main chemical component o Amino groups require more energy to convert to urea, but it

requires less water expenditure

• Reptiles, birds and insects excrete uric acid, o Uric Acid requires the most ATP for conversion but is released

with almost no water loss

• Thus, waste removal requires either lots of water or lots of energy, or a little of both. It all depends on your environment

Excretory System • Other non-human excretory systems

o Planarian organisms contain flame cells which

contain cilia that expel molecules from the body

o Annelids (such as earthworms) have nephridia in

each body segment

• Nephridia are similar to flame cells, but contain

capillaries that reabsorb any nutrients hiding in

the waste before excretion

o Insects contain malphighian tubules.

• MT’s are similar to flame cells, except they are

also able to reabsorb water in watery or humid

environments

Human Excretory System • Kidneys

o Kidneys filter out waste materials from blood and

produce a chemical called urine.

o Urine collects inside the renal pelvis inside the

kidney and is carried to the bladder through the

ureter.

o Each kidney contains millions of tubules called

nephrons, where the kidney forms urine.

o Nephrons are vascular tissue made up of multiple

filtering cells

Human Excretory System • Urine requires three distinct processes

• 1) Glomerular filtration at the glomerular capsule (GET

EVERYTHING OUT)

o Glomerular filtration is the movement of molecules from arteries

through the glomerular wall due to blood pressure

o This filtrate removes everything from the blood except large

plasma proteins and blood cells

o All the nephrons in the body filter 5 liters of blood in about 40

minutes

• 2) Tubular Reabsorption (BRING GOOD STUFF BACK)

o Water needs to be reabsorbed back into the blood

o Nephrons pump Na+ and Cl- ions into the blood, which osmotically attracts water out of the nephron and into the

blood.

Human Excretory System o At the same time, carrier proteins allow recognized nutrients

(glucose, amino acids, etc) to reenter the bloodstream

o Proteins monitoring the blood glucose level are selective about

the amount of glucose returning to blood or staying in urine.

• (Kidneys assume everything is bad and filter everything out

of the blood. Then, they receive information about which nutrients they need and re-filter it back in.

• 3) Tubular Secretion (GET OUT AGAIN!)

o Larger plasma molecules such as uric acid, hydrogen,

ammonia and penicillin are all removed in this tube.

o This is the last stop before urine. They won’t have a chance to

accidentally be reintroduced to blood in step two if they’re

removed here.

Urine • Antidiuretic hormone (ADH) released by the pituitary

affects water re-absorbtion

• ADH is released into the distal tubule of the nephron

when the pituitary senses a low amount of water in the

blood

• The ADH attracts water back into the collecting duct

and carries it to mix with urine

o ADH gives off a heavy greenish-yellow pigment. Hence, urine is

more colored when an organism is dehydrated

• Aldosterone is a hormone that triggers the return of 99%

of sodium back to the blood. o Sodium is needed for Na+/K+ exchange in nerve cells and for maintaining

osmosis in blood