Biology 12 Notes

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The Excretory System

Main functions of the excretory system are:
oto concentrate wastes and expel them from the body
oto regulate fluids and water within the body
Most metabolic wastes and toxins are dissolved in the body’s internal environment, so the maintenance of the body fluids is essential for keeping the body free of waste products enabling it to function properly.

Excretion in Invertebrates and Non-mammaliam Vertebrates

Single celled organisms and simple multicellular organisms produce the same metabolic wastes and toxic compounds as more complex organism •However wastes are excreted directly from their cells since they have constant contact with the external environment. •The greater challenge for simple organisms is maintaining a fluid balance with their external environment •An example is paramecium:

oIf they are not able to maintain a fluid balance they would continuously absorb water from the environment and eventually burst oTo expel excess water, these protozoans have contractile vacuoles, which pump out water to maintain osmotic balance •More complex organisms face different challenges

Some invertebrates such as earthworms have excretory organs called matanephridia that expel wastes from the body. oA fluid that serves as both interstitial fluid and bood known as hemolymph flows into a pair of metanphridia. oIons and wastes are reabsorbed from the hemolymph and secreted with water into a saclike organ called the bladder. From there, the excess water and waste products are secreted to the external environment through a poke in the side of the worm’s body. •Insects like grasshoppers excrete wastes uses a set of organs called malpighian tubules. oThe closed end of the organs are surrounded with hemolymph while open ends empty into intestines. oSubstances, such as uric acid, and potassium and sodium ions are secreted into the tubules. oWhen concentration of the substances increases, water movies osmotically from the hemolymph into the tubule to form a dilute waste solution and then it travels to the intestine of the insect where the cells reabsorb most of the K+ and Na+ back into hemolymph oWater moves back and forth using osmosis.

oUric acid is left behind and forms crystals and is then expelled •Terrestrial reptiles and most birds conserve water by excreting nitrogenous wastes in the form of an almost water free paste of uric acid crystals. oIt’s excreted into the cloaca (end of the digestive system) and removed from the body along with the digestive wastes oThe white substance in bird droppings is uric acid while the darker substance is feces. oThose that live in or around salt water take in large quantities of salt and rarely drink fresh water therefore they excrete excess salt through specialized salt glands in the head. oSalt glands remove salts from the blood using active transport. oSalt is secreted to the environment as a water solution

oThe concentration of this is two to three times more than that in the body fluids. oSecretion exits through the nostril of birds and lizards and as salty tears from sea turtles and crocodilians.

The Human Excretory System

All vertebrates used specialized tubules called nephrons to regulate water balance in the body and conduct excretion. •They are located in the kidneys which are the major organs of excretory. •The kidneys, ureters, bladder, and the urethra together make up the human excretory system. Kidneys

Play a critical role in
oRemoving wastes
oBalancing blood pH
oMaintaining the body’s water balance
Mammals have two kidneys on each side of the vertebrate column •Human kidneys:
o150g
oreceives 25% of cardiac output (1.25 L/min.)
Blood is supplied through renal artery.
“Renal” refers to the kidneys
Kidney filters the wastes from the blood and clean blood exits the kidney through the renal veins. •Outer layer of the kidney is called the renal cortex
Inner...
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