Human Body and Health

The body systems and what they do

In plain words: each system has its own job, like workers in a restaurant — but they must work together.

In a restaurant, the cook, the server and the dishwasher each have a job. If one stops, the others feel it. Every body system has a main job, but none works alone. On the GED, match each job to the right system. Then see how two systems work together.

Systems work together. When you exercise, muscles need more oxygen and make more carbon dioxide. The respiratory system speeds up breathing. The circulatory system speeds up the heart to move that oxygen. The nervous system directs both. The skin sweats to release heat. Example: a graph shows breathing rate and heart rate rising together during a workout. The systems involved are respiratory and circulatory.

Homeostasis and feedback loops

In plain words: your body keeps things "just right" inside, no matter what happens outside.

Cells only work well within narrow ranges of temperature, water, sugar, and acidity. Homeostasis is how the body stays within those ranges even when the outside world changes. Most of this control uses negative feedback, which works like a home thermostat:

1. A sensor detects a change (blood is too warm).
2. A control center, often the brain, compares it to the set point.
3. An effector responds to reverse the change (sweat glands release sweat; blood vessels in the skin widen).
4. As the condition returns to normal, the response shuts off.

Key examples to know:

• Body temperature. Too hot → sweating and widened skin blood vessels. Too cold → shivering (muscles make heat) and narrowed skin blood vessels.
• Blood glucose. After a meal, glucose rises → the pancreas releases insulin → cells absorb glucose → levels fall. Between meals, glucose drops → the pancreas releases glucagon → the liver releases stored glucose.
• Water balance. When you are dehydrated, the kidneys save water. They make darker, more concentrated urine. You also feel thirsty.

When homeostasis fails, disease can follow. In type 1 diabetes, the immune system destroys the cells in the pancreas that make insulin. So blood glucose (sugar) stays high. In type 2 diabetes, the body still makes insulin, but cells respond to it poorly. A heat stroke happens when cooling cannot keep up and body temperature climbs dangerously.

Positive feedback is rarer. It pushes a change further until something ends it. During childbirth, contractions trigger the release of the hormone oxytocin. Oxytocin causes stronger contractions. This continues until the baby is born. Blood clotting also uses positive feedback.

Nutrition: what food does for you

In plain words: food is both fuel and building blocks for your body — like gas and spare parts for a car.

Food supplies energy (measured in Calories) and building materials. The six classes of nutrients are:

Deficiency diseases — illnesses from getting too little of a nutrient — are classic GED examples:

Too much of some nutrients also harms health. A diet high in sodium (salt) is linked to high blood pressure. Eating more Calories than you use, over time, leads to weight gain. Weight gain raises the risk of type 2 diabetes and heart disease.

Pathogens, disease, and the immune system

Infectious diseases are caused by pathogens:

• Bacteria — single cells; some cause strep throat, tuberculosis, and food poisoning. Treated with antibiotics.
• Viruses — not cells; they must enter a living cell to copy themselves. Cause colds, flu, measles, COVID-19, HIV. Antibiotics do not work on viruses; some antiviral drugs and vaccines help.
• Fungi — cause athlete's foot and yeast infections.
• Protists — cause malaria (spread by mosquitoes).
• Parasitic worms — such as tapeworms.

Pathogens spread in several ways:

• through the air (coughs, sneezes);
• by direct contact;
• in contaminated food or water, or body fluids;
• by vectors — animals such as mosquitoes and ticks that carry pathogens.

That is why handwashing, clean water, cooking food well, and covering coughs prevent disease.

The immune system has layers. First come barriers: skin, mucus, tears, and stomach acid. Next is the general (innate) response: white blood cells swallow invaders. Swelling (inflammation) and fever make the body less friendly to germs. Last is the specific (adaptive) response: white blood cells recognize a pathogen's antigens — its ID tags. They make matching antibodies. They also leave behind memory cells, like a "wanted" poster for next time. The second time the same pathogen appears, memory cells start a faster, stronger response, which is immunity.

Vaccines use this memory safely. They show the immune system a weakened, killed, or harmless piece of a pathogen. So memory cells form without the illness. When enough people are immune, a pathogen has trouble spreading. This also protects people who cannot be vaccinated. This is called herd immunity.

Antibiotic resistance: sometimes antibiotics are overused, or a course is stopped early. Then the few bacteria that happen to survive can multiply. Infections become harder to treat. This is natural selection in action.