# 105 Summary of homeostasis, excretion, drugs

Homeostasis is the maintenance of a constant internal environment. It is achieved using negative feedback.

• Organisms that can control their internal body temperature are called homeotherms. Mammals and birds are homeotherms. All other animals are poikilotherms, meaning that they have only limited ways of controlling their temperature.

• The control of body temperature in humans involves the hypothalamus, the skin and muscles. When the body becomes too hot, sweating and vasodilatation increase the rate of heat loss from the skin. When the body becomes too cold, shivering increases heat production, and vasoconstriction reduces the rate of heat loss from the skin.

• The pancreas, working in conjunction with the liver, controls blood glucose concentration. When this rises too high, the pancreas secretes insulin which causes the liver to remove glucose from the blood and convert it to glycogen. When blood glucose concentration falls to low, the pancreas secretes glucagon which causes the liver to convert glycogen to glucose.

• Excretion is the removal from the body of waste products of metabolism. The main excretory products of mammals are CO₂, urea, salts and excess H₂O.

• Mammals excrete CO₂ from the lungs and urea from their kidneys.

• Urea is produced in the liver from excess amino acids. It is transported in solution in blood plasma to the kidneys, where it is excreted in urine.

• Urine is made in the nephrons of each kidney. First, blood is filtered. Then any substances to be retained in the blood are reabsorbed. The fluid that is left in the nephron flows into the ureters and then to the bladder, before leaving the body as urine through the urethra. 

• A drug is a substance that effects chemical reactions in the body.Many drugs are used in medicine. For example, antibiotics are used to kill bacteria that are causing disease in the body.

• Heroin is a depressant that is often addictive. Use of heroin often leads to crime and misery for the user’s family. People who inject heroin run a high risk of infection with HIV.

• Alcohol is also a depressant. Drinking alcohol lengthens reaction time, reduced self-control and may cause aggression, causing serious problems for friends and family. Some people become addicted to alcohol. Over time, the liver is damaged by excessive alcohol intake.