Transcript Excretory and Nervous Systems 2012

EXCRETORY AND NERVOUS SYSTEMS
How do animals manage toxic nitrogenous waste?
Animals either eliminate ammonia from the body quickly or convert it into other
nitrogenous compounds that are less toxic.
The breakdown of proteins by cells releases a nitrogen-containing, or nitrogenous,
waste: ammonia.
Ammonia is poisonous. Even moderate concentrations of ammonia can kill most cells.
Animals that cannot dispose of ammonia continuously, as it is produced, have evolved
ways to store nitrogenous wastes until they can be eliminated.
Insects, reptiles, and birds convert ammonia into a sticky white compound called uric
acid, which is much less toxic than ammonia and is also less soluble in water.
Mammals and some amphibians convert ammonia to a different nitrogenous
compound—urea. Urea is less toxic than ammonia, but unlike uric acid, it is highly
soluble in water
Maintaining Water Balance
Excretory systems are extremely important in maintaining the proper balance of
water in blood and body tissues.
In some cases, excretory systems eliminate excess water along with
nitrogenous wastes.
In other cases, excretory systems must eliminate nitrogenous wastes while
conserving water.
Many animals use kidneys to separate wastes and excess water from blood to
form a fluid called urine.
Kidneys separate water from waste products.
Kidney cells pump ions from salt to create osmotic gradients.
Water then “follows” those ions passively by osmosis.
Kidneys, however, usually cannot excrete excess salt.
Freshwater Animals
Many freshwater invertebrates lose ammonia to their environment by simple
diffusion across their skin.
Many freshwater fishes and amphibians eliminate ammonia by diffusion
across the same gill membranes they use for respiration.
The situation is more complex for some freshwater invertebrates and most
freshwater fishes. The bodies of freshwater animals, such as fishes, contain a
higher concentration of salt than the water they live in.
Water moves into their bodies by osmosis, mostly across the gills. Salt diffuses
out. Freshwater fish excrete water through kidneys that produce lots of watery
urine. They don't drink, and they actively pump salt in across their gills.
Saltwater Animals
Saltwater fish lose water through osmosis,
and salt diffuses in.
If they didn’t conserve water and eliminate
salt, they’d shrivel up like dead leaves.
Saltwater fish conserve water by producing
very little concentrated urine.
They drink, and they actively pump salt out
across their gills
Excretion in Terrestrial Animals
How do land animals remove wastes while conserving water?
Some terrestrial invertebrates, including annelids and mollusks, produce urine in
nephridia.
Other terrestrial invertebrates, such as insects and arachnids, convert ammonia
into uric acid. Uric acid is absorbed from body fluids by structures called
Malpighian tubules, which concentrate the wastes and add them to digestive
wastes traveling through the gut.
Mammals and land amphibians convert ammonia into urea, which is excreted in
urine. In most reptiles and birds, ammonia is converted into uric acid.
What happens in
different parts of
the nephron?
Functions of the Nervous System
The functions of the nervous system
are accomplished by the peripheral
nervous system and the central
nervous system.
The peripheral nervous system,
which consists of nerves and
supporting cells, collects information
about the body’s external and internal
environment.
The central nervous system, which
consists of the brain and spinal cord,
processes that information and creates a
response that is delivered to the
appropriate part of the body through the
peripheral nervous system.
Types of Neurons
Neurons can be classified into three types according to the
direction in which an impulse travels.
Sensory neurons carry impulses from the sense organs, such as
the eyes and ears, to the spinal cord and brain.
Motor neurons carry impulses from the brain and the spinal cord
to muscles and glands.
Interneurons process information from sensory neurons and then
send commands to other interneurons or motor neurons.
The Synapse
At the end of the neuron, the impulse reaches
an axon terminal, which may pass the impulse
along to another cell. A motor neuron, for
example, may pass impulses to a muscle cell,
causing the muscle cell to contract.
The point at which a neuron transfers an
impulse to another cell is called a synapse.
When an impulse arrives at the synapse,
neurotransmitters are released from the axon, diffuse
across the synaptic cleft, and bind to receptors on the
membrane of the receiving cell. This binding opens
ion channels in the membrane of the receiving
cell.
If the stimulation exceeds the cell’s threshold, a new
impulse begins.