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Biology Unit 9: Human body systems and health

Organ systems work together to keep the body in balance, and the immune system defends it against disease.

Covers: Homeostasis and feedback, organ systems working together, the immune system and vaccines, disease and transplants, reproduction and development, effects of drugs and the environment

Study guide

Homeostasis is keeping a stable internal environment despite outside changes. The body uses feedback mechanisms: in negative feedback, a change triggers a response that reverses it, as when rising blood sugar causes the pancreas to release insulin, which lowers blood sugar (glucagon does the opposite). Body temperature, water balance and blood pH are also controlled by negative feedback. Positive feedback amplifies a change, as in childbirth and blood clotting. Failure of homeostasis can lead to disease, such as diabetes when insulin isn't made or used properly.

Organ systems cooperate. The digestive system breaks food into molecules that the circulatory system carries to cells; the respiratory system supplies oxygen for cellular respiration and removes carbon dioxide; the excretory system (kidneys, lungs, skin) removes wastes; the nervous and endocrine systems coordinate responses using nerve signals and hormones. Hormones travel in the blood and act only on target cells with matching receptor proteins.

The immune system defends against pathogens like bacteria and viruses. White blood cells engulf invaders and make antibodies that recognize specific antigens (molecules on the pathogen's surface). A vaccine contains weakened or killed pathogens or parts of them, which stimulate the body to make antibodies and memory cells without causing disease, so the body responds faster to future infection. Antibiotics kill bacteria but not viruses, and overuse leads to antibiotic resistance. Transplanted organs may be rejected because the immune system recognizes their antigens as foreign, so patients take drugs that suppress the immune system. Allergies are immune overreactions to harmless substances, and in autoimmune diseases the immune system attacks the body's own cells. In human reproduction, fertilization usually occurs in the oviduct, the embryo develops in the uterus, and the placenta lets nutrients, oxygen and wastes pass between mother and embryo. Alcohol, tobacco and many drugs can cross the placenta and damage the developing embryo, especially early in development when organs form.

Key ideas

• Negative feedback keeps conditions stable; positive feedback amplifies change.
• Insulin lowers blood sugar; glucagon raises it.
• Organ systems work together to supply cells and remove wastes.
• Vaccines produce antibodies and memory cells without causing disease.
• Transplants can be rejected because the immune system sees foreign antigens.

Formulas and rules

Common mistakes

• Thinking antibiotics cure viral infections; they don't.
• Thinking vaccines cause the disease; they use weakened, killed or partial pathogens.

Words to know

Homeostasis

Keeping a stable internal environment.

Negative feedback

A response that reverses a change.

Positive feedback

A response that amplifies a change.

Insulin

Hormone that lowers blood sugar.

Glucagon

Hormone that raises blood sugar.

Diabetes

A disease where blood sugar isn't properly controlled.

Target cell

A cell with receptors for a specific hormone.

Pathogen

A disease-causing organism or virus.

Antigen

A molecule that the immune system recognizes as foreign.

Antibody

A protein that binds a specific antigen.

White blood cell

An immune cell that fights infection.

Vaccine

A preparation that triggers immunity without causing disease.

Memory cell

An immune cell that remembers a past pathogen.

Antibiotic resistance

Bacteria evolving to survive antibiotics.

Transplant rejection

The immune system attacking a transplanted organ.

Allergy

An immune overreaction to a harmless substance.

Autoimmune disease

The immune system attacking the body's own cells.

Placenta

Organ that exchanges materials between mother and embryo.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.