This unit covers how your organ systems keep you alive by bringing in materials, moving them around, sending signals, removing waste, and defending against pathogens. By the end, you should be able to trace a molecule of food or oxygen through the body and explain how feedback keeps conditions like blood glucose and water balance stable.
Read the overview first, then work through each topic note and answer its checkpoint before you look at the answer.
Human anatomy is the study of body structures, and physiology is the study of how those structures work. This unit pairs organ systems that cooperate closely, so you learn each system along with the partner it depends on.
Honors Biology Unit 16 covers four groups of human body systems. You study the digestive and excretory systems (enzymes, absorption, kidneys and nephrons), the immune system (innate and adaptive defenses, antibodies, vaccines), the nervous and endocrine systems (neurons, synapses, hormones, feedback loops), and the cardiovascular and respiratory systems (heart circuits, blood vessels, alveoli, gas exchange). Many courses also expect you to read data on things like blood pressure, antibody levels, or blood glucose and to calculate values such as cardiac output.
Villi in the small intestine and alveoli in the lungs both pack huge surface areas into small spaces, which speeds up absorption and diffusion. Capillary walls are one cell thick so materials can cross quickly. The left ventricle has the thickest wall because it pumps blood to the whole body.
When a value like blood glucose or body water drifts, sensors detect the change and the body responds in the opposite direction. Insulin lowers high blood glucose, and ADH helps the kidneys save water when you're dehydrated. Positive feedback, like contractions during childbirth, is rarer and pushes a change further until an endpoint is reached.
Nutrients absorbed in the intestine are useless unless the blood carries them to cells. Most cells use oxygen from the lungs to release much more energy from nutrients than they can without oxygen. The nervous and endocrine systems adjust heart rate, breathing, and digestion so supply matches demand.
Each system handles part of the job of keeping your cells supplied and your internal environment stable. If one system fails, others feel it fast. Kidney failure, for example, lets wastes and excess fluid build up and raises blood pressure. When you study any organ, ask what it brings in, removes, moves, or controls.
Digestion turns large food molecules into small ones your cells can absorb, and excretion removes the wastes your cells make.
Your immune system works in layers.
Both systems carry messages, but they use different signals.
The heart and lungs work as one delivery system.
Open this guide for a closer review of the topic.
Open this guide for a closer review of the topic.
Digestion turns large food molecules into small ones your cells can absorb, and excretion removes the wastes your cells make. Mechanical digestion, like chewing and stomach churning, breaks food into smaller pieces. Chemical digestion uses enzymes to break bonds. Amylase in saliva starts starch digestion in the mouth. Pepsin breaks down proteins in the stomach at a pH near 2. Pancreatic lipase splits fats in the small intestine.
Most digestion and absorption happen in the small intestine, which is about 6 meters long and lined with villi and microvilli that multiply its surface area. The large intestine mainly absorbs water. The liver makes bile, which emulsifies fat into small droplets, and the gallbladder stores it.
Each kidney filters blood through about a million nephrons. In the glomerulus, blood pressure pushes water, salts, glucose, and urea into the nephron. Most of that filtrate is then reabsorbed into the blood. When you're dehydrated, the hormone ADH tells the kidneys to reabsorb more water.
Worked example. Your kidneys filter about 180 L of fluid per day but produce only about 1.5 L of urine. The fraction reabsorbed is , so about 99% of filtered fluid returns to the blood.
Check yourself: Healthy urine contains almost no glucose, even though glucose enters the nephron during filtration. Why? (Answer: The nephron tubule reabsorbs nearly all filtered glucose back into the blood. Glucose can appear in urine when blood glucose is so high, as in untreated diabetes, that reabsorption can't keep up. Some kidney conditions and medications can also cause glucose to appear in urine.)
Your immune system works in layers. Innate defenses recognize common signs of infection or tissue damage and can respond within minutes to hours. Adaptive defenses take days to respond the first time, but they target specific antigens and can remember them.
Innate defenses include skin, mucus, stomach acid, lysozyme in tears, and phagocytes such as neutrophils and macrophages that engulf invaders. Inflammation is also innate. Histamine widens blood vessels and makes capillaries leaky, so fluid and white blood cells reach the injured tissue. That's why an inflamed area turns red, warm, swollen, and painful.
Adaptive immunity recognizes antigens, which can be molecules on pathogens or other substances. B cells make antibodies, Y-shaped proteins that bind one antigen shape. Antibodies can block a virus from entering cells or tag a bacterium for phagocytes. Helper T cells release signals that activate both B cells and cytotoxic T cells. Cytotoxic T cells kill infected body cells, which can limit the spread of a virus.
After an infection, memory B and T cells stay in your body. A second exposure produces a faster, larger antibody response, often before you feel sick. Vaccines use this by exposing your immune system to an antigen or providing instructions for cells to make one. They aim to create immune memory without causing the disease, although protection varies by vaccine and person. HIV infects helper T cells, which weakens both antibody and T cell responses.
Check yourself: A graph shows antibody levels after two exposures to the same virus. The second peak is higher and arrives sooner. What explains this? (Answer: Memory cells formed during the first exposure recognize the antigen right away, so B cells produce more antibodies more quickly the second time.)
| Feature | Innate immunity | Adaptive immunity |
|---|---|---|
| Speed | Minutes to hours | Days on first exposure, faster after that |
| Specificity | Recognizes common patterns shared by groups of invaders | Targets one specific antigen |
| Memory | None | Memory B and T cells last for years |
| Examples | Skin, stomach acid, neutrophils, macrophages, inflammation | B cells, antibodies, helper and cytotoxic T cells |
Both systems carry messages, but they use different signals. Neurons send electrical signals called action potentials along their axons. At a synapse, they release neurotransmitters that pass the message to the next cell. Hormones travel through the blood and affect only target cells that have the matching receptor.
A neuron receives signals on its dendrites, combines them at the cell body, and sends an action potential down the axon. Myelin speeds this up by letting the signal jump between gaps in the sheath. That's why myelin damage in multiple sclerosis slows nerve signals. The central nervous system (brain and spinal cord) processes information, and the peripheral nervous system carries signals to and from the rest of the body. Within the autonomic division, the sympathetic side speeds up the heart during stress (the fight-or-flight response), and the parasympathetic side slows it down for rest and digestion.
Blood glucose shows how the endocrine system uses negative feedback. After a meal, glucose rises, so the pancreas releases insulin. Insulin helps cells take up glucose and helps the liver store it as glycogen. When glucose drops, the pancreas releases glucagon, and the liver breaks glycogen back down. The hypothalamus links the two systems by controlling the pituitary gland, which releases hormones that regulate other glands.
Check yourself: In type 1 diabetes, the pancreas makes little or no insulin. What happens to blood glucose after a meal, and why? (Answer: It stays high because muscle and fat cells take in less glucose without insulin, and the liver stores less glucose as glycogen. Some cells can still take in glucose without insulin.)
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Signal | Action potentials and neurotransmitters | Hormones |
| Pathway | Along neurons to specific cells | Through the blood to any cell with the right receptor |
| Speed | Milliseconds | Seconds to hours |
| How long effects last | Short | Often minutes to days |
| Example | Pulling your hand off a hot stove | Insulin lowering blood glucose after a meal |
The heart and lungs work as one delivery system. Blood travels in two loops. In the pulmonary circuit, the right ventricle pumps oxygen-poor blood to the lungs. In the systemic circuit, the left ventricle pumps oxygen-rich blood through the aorta to the body, which is why the left ventricle has the thickest wall.
Arteries carry blood away from the heart, and veins carry it toward the heart, whatever its oxygen level. Capillaries have walls one cell thick, so oxygen, nutrients, and wastes diffuse across them easily.
Air moves through the trachea, bronchi, and bronchioles into about 300 million alveoli. Oxygen diffuses from the alveoli into the blood, and diffuses out, each moving down its concentration gradient. Each hemoglobin molecule in a red blood cell carries up to four molecules. Most travels in plasma as bicarbonate ions. Your breathing rate is set mainly by . When blood rises, blood pH drops, and the brainstem speeds up breathing.
Worked example. Cardiac output equals heart rate times stroke volume. If your heart pumps 70 times per minute and moves 70 mL each time, the output is mL/min, or about 4.9 L/min. During exercise, at 150 pumps per minute and 100 mL each, the output is mL/min, about three times higher.
Check yourself: Which vessel carries the most oxygen-rich blood, the pulmonary artery or the pulmonary vein? (Answer: The pulmonary vein, because it carries blood that just picked up oxygen in the lungs back to the left atrium. The pulmonary artery carries oxygen-poor blood away from the heart to the lungs.)
| Term | Definition |
|---|---|
| Homeostasis | The maintenance of a stable internal environment, like steady body temperature and blood glucose, despite outside changes. |
| Negative feedback | A control loop where the body responds to a change by pushing conditions back toward a set point. |
| Enzyme | A protein that speeds up a specific chemical reaction, like amylase breaking starch into sugars. |
| Nephron | The kidney's filtering unit, which filters blood, reabsorbs useful substances, and forms urine. |
| Antigen | A molecule that the immune system can specifically recognize. Antigens can come from pathogens, other substances, or the body's own cells. |
| Antibody | A protein made by B cells that binds a specific antigen to block a pathogen or mark it for removal. |
| Neurotransmitter | A chemical released at a synapse that carries a signal from a neuron to the next cell. |
| Hormone | A chemical messenger released by a gland into the blood that acts on target cells with matching receptors. |
| Alveolus | A tiny air sac in the lung where oxygen enters the blood and carbon dioxide leaves it. |
| Hemoglobin | The protein in red blood cells that binds up to four oxygen molecules for transport. |
Most arteries in the body do, so the pattern feels like a rule. Arteries are defined by direction, though, because they carry blood away from the heart. The pulmonary arteries carry oxygen-poor blood to the lungs, and the pulmonary veins carry oxygen-rich blood back.
Bile helps with fat digestion, so it's easy to lump it in with lipase. Bile breaks fat into tiny droplets without breaking any chemical bonds. Pancreatic lipase chemically splits fats mainly into fatty acids and monoglycerides.
Antibiotics target structures that bacteria have, like cell walls and bacterial ribosomes. Viruses don't have those structures, so antibiotics do nothing against a cold or the flu. Vaccines prevent many viral diseases, and some antiviral drugs treat specific viruses.
It seems logical because breathing brings in oxygen. In a healthy person, the main trigger is rising , which lowers blood pH. Sensors in the brainstem detect the pH change and increase the breathing rate.