Human Physiology: Body Systems
High SchoolA human is trillions of cells organised into tissues, organs and systems that cooperate to keep one internal environment stable. Physiology is the study of how those systems work — and how they are integrated so exquisitely that we rarely notice them at all.
Organisation and homeostasis
Life is organised in a hierarchy: cells → tissues → organs → organ systems → organism. The systems are integrated by two great control networks — the fast, electrical nervous system and the slower, chemical endocrine (hormonal) system — to maintain homeostasis: the keeping of a stable internal environment (temperature, blood glucose, pH, water balance) despite external change.
Homeostasis works by negative feedback: a change is detected by receptors, a control centre compares it to a set point, and effectors act to reverse the change, returning the system toward normal.
The major systems (survey)
| System | Core function | Key organs |
|---|---|---|
| Circulatory | Transport of O₂, CO₂, nutrients, hormones, heat | Heart, blood vessels, blood |
| Respiratory | Gas exchange: O₂ in, CO₂ out | Lungs, trachea, alveoli, diaphragm |
| Digestive | Break down & absorb food | Mouth, stomach, small intestine, liver, pancreas |
| Nervous | Rapid electrical signalling & control | Brain, spinal cord, neurons |
| Endocrine | Slow chemical (hormone) control | Pituitary, thyroid, pancreas, adrenals |
| Excretory (urinary) | Remove wastes; osmoregulation | Kidneys, bladder |
| Immune | Defence against pathogens | White blood cells, lymph nodes, spleen |
Two systems in focus
Circulatory: humans have a double circulation — the heart pumps deoxygenated blood to the lungs (pulmonary circuit) and oxygenated blood to the body (systemic circuit). Arteries carry blood away from the heart at high pressure (thick, elastic walls); veins return it at low pressure (valves prevent backflow); capillaries are one cell thick for exchange.
Nervous: a neuron transmits an electrical action potential along its axon; at the synapse, a chemical neurotransmitter crosses the gap to the next cell. A reflex arc (receptor → sensory neuron → relay neuron → motor neuron → effector) gives rapid, automatic protection.
Homeostasis: the thermostat principle everywhere
The single most unifying idea in physiology is negative feedback. It is the same logic as a home thermostat: sense the temperature, compare to the set point, and act to oppose any deviation. Your body runs dozens of these loops at once.
Take blood glucose control by the pancreas: after a meal, blood glucose rises; the pancreas detects this and releases insulin, which makes the liver and muscles take up glucose and store it as glycogen — glucose falls back to normal. Between meals, glucose falls; the pancreas releases glucagon, which makes the liver break glycogen back down to glucose — glucose rises to normal. Two opposing hormones, one stable set point. When this loop fails (diabetes), the danger of losing homeostasis becomes starkly clear. Thermoregulation works the same way: too hot → sweating and vasodilation shed heat; too cold → shivering and vasoconstriction conserve it.
Negative feedback opposes change and restores stability — the norm in homeostasis. Positive feedback amplifies change and is rarer, used for decisive "all-or-nothing" events like childbirth (oxytocin intensifies contractions until delivery) and blood clotting. Positive feedback drives a process to completion; negative feedback holds a value steady.
Worked example: tracing a reflex and calculating cardiac output
Part A — the knee-jerk reflex. Trace the pathway when a doctor taps below your kneecap and your leg kicks.
- Stimulus & receptor. The tap stretches the thigh muscle; a stretch receptor detects it.
- Sensory neuron. An impulse travels along a sensory neuron into the spinal cord.
- Integration. In the spinal cord the signal passes (via a synapse) directly to a motor neuron — a fast, spinal-level reflex that bypasses conscious thought in the brain.
- Motor neuron & effector. The motor neuron carries the impulse to the thigh muscle (effector), which contracts, kicking the lower leg.
- Point. The brain is informed after the event — reflexes are fast and protective precisely because they skip the brain's decision-making.
Part B — cardiac output. How much blood does the heart pump per minute?
- Formula. cardiac output = heart rate × stroke volume, where stroke volume is the blood ejected per beat.
- Insert resting values. Heart rate = 70 beats/min; stroke volume = 70 mL/beat.
- Compute. 70 × 70 = 4900 mL/min ≈ 4.9 L/min — roughly your whole blood volume every minute at rest.
- Exercise. During hard exercise, rate may reach 180 and stroke volume 120 mL: 180 × 120 = 21 600 mL/min ≈ 21.6 L/min — over four times higher, delivering far more oxygen to working muscles. This shows how the circulatory system flexes to meet demand.
Test yourself
Q1 Explain how the body restores normal temperature when you become too cold.
Temperature receptors (in skin and the hypothalamus) detect the drop; the hypothalamus acts as control centre and triggers effectors to generate and conserve heat: shivering (muscle contraction releases heat), vasoconstriction of skin blood vessels (less heat lost from the surface), erection of hairs to trap insulating air, and release of hormones (e.g. thyroxine, adrenaline) that raise metabolic rate. These reverse the fall in temperature — negative feedback — restoring the ~37°C set point.
Q2 Compare the structure of arteries, veins and capillaries with their functions.
Arteries carry blood away from the heart at high pressure: they have thick, muscular, elastic walls to withstand and smooth the pressure, and a narrow lumen. Veins return blood at low pressure: thinner walls, a wide lumen, and valves to prevent backflow (blood is helped along by skeletal-muscle squeezing). Capillaries are the exchange vessels: walls just one cell thick and huge total surface area, allowing rapid diffusion of O₂, CO₂, nutrients and wastes between blood and tissues. Structure follows function throughout.
Q3 Why is the human double circulation more effective than a single circulation?
In a double circulation, blood passes through the heart twice per complete circuit: once to the lungs (pulmonary) and once to the body (systemic). This lets blood be re-pressurised after the lungs, so it travels to body tissues at high pressure and fast flow, delivering oxygen quickly. A single circulation (as in fish) loses pressure across the gills, slowing delivery. Double circulation supports the high metabolic rate of warm-blooded mammals by keeping oxygen delivery fast and efficient.
Q4 Contrast nervous and hormonal (endocrine) control.
Nervous control uses electrical impulses along neurons: it is very fast, short-lived, and targeted to specific cells (precise). Hormonal control uses chemical messengers carried in the blood: it is slower to act but longer-lasting and widespread, affecting many target cells with the right receptors. Nervous control suits split-second responses (pulling from a flame); hormonal control suits sustained, whole-body processes (growth, metabolism, the menstrual cycle). The two are integrated, notably at the hypothalamus–pituitary link.
Q5 A person with type 1 diabetes cannot produce insulin. Predict what happens to their blood glucose after a sugary meal and why it is dangerous.
Without insulin, the liver and muscles are not signalled to take up glucose and store it as glycogen, so after a sugary meal blood glucose rises very high and stays high (hyperglycaemia). This is dangerous: excess glucose is lost in urine, dragging water with it (thirst, dehydration), cells are starved of usable fuel and may burn fat, producing acidic ketones that lower blood pH (ketoacidosis), and chronic high glucose damages blood vessels and nerves. Treatment supplies the missing hormone by insulin injection to restore the negative-feedback loop.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Heart rate is 70 bpm and stroke volume is 70 mL. Find cardiac output.
- Cardiac output = heart rate x stroke volume
- 70 x 70 = 4900 mL/min
Example 2You touch a hot pan and pull back before feeling pain. Trace the path.
- Receptor in skin detects heat
- Sensory -> relay in spinal cord -> motor neuron
- Muscle (effector) contracts
Now you try
Work each one out first, then tap to reveal the worked answer.