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.

Terrestrial Vertebrates In mammals and land amphibians, ammonia is converted into urea, which is excreted in urine by the kidneys. The way kidneys operate results in some limitations. Most vertebrate kidneys cannot excrete concentrated salt. That’s why most vertebrates cannot survive by drinking seawater. All that extra salt would overwhelm the kidneys, and the animal would die of dehydration

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.