Human Body Systems

This is the highest-volume topic in General Science biology, and it is pure recall. There is nothing to reason out, which means the return on twenty minutes of study is unusually good.

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Every system below serves one larger goal: homeostasis, keeping the body's internal conditions - temperature, water, blood sugar, oxygen - steady while the outside world changes. When a question asks what two systems are working together to do, homeostasis is very often the answer.

The systems

The pancreas appears twice, in digestive and endocrine, because it does two jobs: it releases digestive enzymes and it produces insulin. That is a favorite question.

Circulatory

The heart has four chambers: two atria on top receiving blood, two ventricles below pumping it out.

The path, in order:

1. Body, returning oxygen-poor
2. Right atrium
3. Right ventricle
4. Lungs, where the blood picks up oxygen
5. Left atrium
6. Left ventricle
7. Body, leaving oxygen-rich

The right side sends blood to the lungs. The left side sends it to the body. The left ventricle has the thickest wall because it pushes blood around the whole body rather than just to the lungs next door.

The vessels:

Arteries carry blood away - both start with A. This holds even for the pulmonary artery, which carries oxygen-poor blood, which is the exception the test likes: arteries are defined by direction, not by oxygen content.

Blood components: red cells carry oxygen using hemoglobin, white cells fight infection, platelets clot, plasma is the liquid.

A normal resting heart rate is 60 to 100 beats per minute. Well-trained athletes often sit below 60.

Respiratory

Air goes nose or mouth, then pharynx, larynx, trachea, bronchi, bronchioles, and finally alveoli.

Gas exchange happens in the alveoli, tiny sacs wrapped in capillaries. Oxygen diffuses into the blood and carbon dioxide diffuses out.

The diaphragm is the muscle that drives breathing. It contracts and flattens to draw air in, and relaxes to push it out.

The trachea is the windpipe; the esophagus is the food tube. The epiglottis covers the trachea when you swallow so food goes down the right one.

Digestive

The order, and where the work happens:

The small intestine is where absorption happens, not the stomach. Its lining is covered in villi, folds that multiply the surface area available.

Two accessory organs that are not part of the tube:

• Liver produces bile, which emulsifies fats, and processes nutrients from the blood.
• Pancreas releases digestive enzymes into the small intestine.

The gallbladder stores bile; the liver makes it.

The enzymes each break down one kind of food, and their names give it away:

Starch digestion starts in the mouth, protein digestion in the stomach, and fat digestion waits for the small intestine - which matches the order the table was already in.

Nervous and endocrine

Both control the body, and the contrast is what gets asked.

The central nervous system is the brain and spinal cord. The peripheral nervous system is everything else.

The nerve cell is the neuron. Its branching dendrites receive signals, the long axon carries the impulse away, and the gap between one neuron and the next is the synapse, crossed by chemical messengers.

A reflex does not reach the brain. The signal goes to the spinal cord and back, which is why a reflex is faster than a decision.

Brain parts worth knowing: cerebrum for thought and voluntary movement, cerebellum for balance and coordination, brain stem for breathing and heart rate, and the hypothalamus, which links the nervous system to the endocrine system by controlling the pituitary.

The glands and their hormones

This is asked far more than any other part of the endocrine system, so it is worth learning as a table.

Diabetes is the blood-sugar system failing. Type 1 diabetics make no insulin and must inject it; type 2 make insulin but respond to it poorly. A thyroid short of iodine swells into a goiter, which is why salt is iodized.

Skeletal and muscular

Bones support, protect organs, store calcium, and produce blood cells in the bone marrow.

The femur, the thigh bone, is the longest and strongest bone in the body.

Joints are where bones meet. Ligaments join bone to bone; tendons join muscle to bone. Both connect, and the test asks which connects what.

Three muscle types:

Muscles pull, they do not push, which is why they work in opposing pairs - the biceps bends the arm and the triceps straightens it.

Excretory and immune

The kidney's filtering unit is the nephron, about a million per kidney. Blood is filtered, useful material is taken back, and what is left becomes urine.

The immune system's white blood cells include lymphocytes, which make antibodies - proteins that lock onto a specific invader and mark it for destruction. The lymphatic system is a second network of vessels that drains excess fluid from the tissues and filters it through lymph nodes; the spleen filters the blood.

A vaccine gives active immunity without the illness, by showing the immune system a harmless version of the invader so it is ready for the real one.

The senses

The eye: the iris is the colored ring that opens and closes the pupil; the lens focuses light onto the retina, which detects it.

The ear: sound passes the eardrum to three tiny bones in the middle ear - the smallest, the stapes, is the smallest bone in the body - and on to the inner ear, which also controls balance. The eustachian tube connects the middle ear to the throat and equalizes pressure, which is what your ears are doing when they pop on a plane.

What you can skip

The body is where the bank has its longest tail of one-off terms. Across all 2,104 General Science questions, each of these appears once or twice:

• The minor hormones. Prolactin, oxytocin, erythropoietin and leptin each come up once or twice. Learn the table above; it covers the questions that recur.
• Microanatomy. The glomerulus, osteoclasts, the pyloric sphincter and the mitral valve are one question each. Know the organ and its job, not the parts of the parts.
• The cochlea never appears. "The inner ear handles hearing and balance" is enough.

A one-question term is a poor use of study time when the same hour on the hormone table or the heart's path pays several times over.

Where people lose points

Saying the stomach absorbs nutrients. The small intestine does.

Defining an artery by oxygen content rather than by direction.

Swapping ligaments and tendons.

Giving the liver the kidneys' job.

Saying the right side of the heart pumps to the body. It pumps to the lungs.

Confusing trachea and esophagus.

Work one in under a minute

Which system does the pancreas belong to?

Both the digestive and the endocrine systems - it releases digestive enzymes into the small intestine and it produces insulin, which regulates blood sugar.

If a question forces one answer, read which function it describes. Enzymes into the intestine is digestive; insulin into the blood is endocrine.

Where this leads

The respiratory and circulatory systems deliver the oxygen that cellular respiration needs, and nutrition is what the digestive system is processing.