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Body systems working together
How the digestive, respiratory, circulatory and excretory systems depend on each other to supply cells and remove wastes, how the nervous and endocrine systems coordinate them during exercise, aerobic and anaerobic respiration in muscles, fitness, and anaemia.
At rest, your heart pumps about 5 litres of blood a minute. In a hard sprint it can pump 20 litres or more, and in elite endurance athletes it has been measured at over 35. Your muscles demand it, and four or five body systems change at once to deliver it.
Every cell needs the same things
Each of your cells carries out cellular respiration all the time, using glucose and oxygen to release energy and producing carbon dioxide and water. Cells also make other wastes, such as urea from breaking down proteins. In Year 8 you studied the digestive, circulatory, respiratory and excretory systems one at a time. In reality, none of them can work alone.
| System | What it does for cells | Depends on |
|---|---|---|
| Digestive | Breaks food into small molecules (glucose, amino acids, fatty acids) and absorbs them, with water, into the blood | Circulatory system to carry nutrients away; nervous system to control gut movement |
| Respiratory | Takes oxygen into the blood and removes carbon dioxide at the alveoli | Circulatory system to transport the gases; brain stem to control breathing |
| Circulatory | Carries oxygen, nutrients, hormones, heat and wastes between every cell and the other systems | All the others: it is the delivery network |
| Excretory | Kidneys filter urea, excess salts and water out of the blood as urine | Circulatory system to bring the blood; ADH to control water loss |
| Nervous and endocrine | Detect changes and coordinate the other systems | Need oxygen and glucose like every other tissue; the brain is especially hungry |
Even the systems you rarely think of join in. The skeletal and muscular systems need blood for energy, and the skin is part of temperature control. If one system fails, the others soon suffer. Kidney failure lets wastes build up in the blood until they poison the brain and heart; a blocked airway starves every cell of oxygen within minutes.
What changes during exercise
When you start to run, your leg muscles suddenly need much more energy, so they need more glucose and oxygen, and they make more carbon dioxide and heat. Several systems respond together.
- Detecting the change. Chemoreceptors in large blood vessels and the brain stem detect the rise in carbon dioxide in the blood. Stretch receptors in muscles and joints detect the movement. Even before you start, your brain anticipates the effort and adrenaline is released.
- Breathing. The brain stem sends impulses to the diaphragm and rib muscles, so you breathe faster and deeper. At rest you might take 12 breaths a minute of about 0.5 L each; in hard exercise 40 or more breaths of over 2 L.
- Heart. Sympathetic nerves and adrenaline make the heart beat faster and more strongly, so each beat pumps more blood.
- Where the blood goes. Arterioles supplying the muscles widen, while those supplying the gut and kidneys narrow. At rest, muscles get about a fifth of your blood; in hard exercise they can get around 80%.
- Cooling. The extra heat from respiration triggers vasodilation in the skin and sweating (4E).
- Fuel. Adrenaline and glucagon make the liver release stored glucose into the blood.
air breathed per minute = breaths per minute × volume of each breath
At rest, Jayden takes 12 breaths per minute of 0.5 L each. While jogging he takes 30 breaths per minute of 1.8 L each. How many times more air is he breathing?
When oxygen runs short
In a sprint, even with all these changes, oxygen can't reach the muscles fast enough. The muscle cells switch to anaerobic respiration, which releases energy from glucose without oxygen but makes lactic acid (lactate) and releases far less energy per glucose molecule. Muscles get tired and you have to slow down. After you stop, you keep breathing hard and your heart stays fast for several minutes. The extra oxygen you take in during recovery is used to break down the lactic acid and restore the muscles' energy stores. (You met anaerobic respiration in 3D.)
- Go from resting to walking to jogging. Record the heart rate and air breathed at each level once they settle. Which increases by the bigger factor?
- Switch to sprinting and watch the lactic acid bar. Why does it rise in a sprint but not while jogging?
- Look at "Where the blood goes" during a sprint. Which organs lose blood flow, and why does that make sense?
- Stop suddenly after a sprint. Why don't heart rate and breathing drop straight back to resting values?
- Turn on low iron (anaemia) and jog again. Why does the body switch to anaerobic respiration sooner?
Fitness and health
Regular exercise changes how these systems work together. The heart muscle gets stronger and pumps more blood with each beat, so a fit person's resting heart rate is lower; many endurance athletes rest at 40–50 beats per minute. Muscles grow more capillaries and more mitochondria, so they can respire aerobically at higher effort before lactic acid builds up. Recovery is also faster.
Anaemia is a shortage of haemoglobin, the iron-containing protein in red blood cells that carries oxygen. The most common cause is not getting enough iron, and teenage girls are at higher risk because iron is lost in menstrual blood. People with anaemia get tired and breathless easily because less oxygen reaches their cells. Red meat, legumes, leafy greens and iron-fortified cereals are good sources.
Test yourself
Which system carries glucose from the small intestine to the leg muscles?
The blood is the delivery network.
During exercise, what is mainly detected to make you breathe faster?
Chemoreceptors detect the rise in CO₂.
During a sprint, blood flow to which organ decreases?
Blood is diverted away from the gut to the muscles.
A runner takes 40 breaths per minute of 2 L each. How much air does she breathe per minute?
40 × 2 = 80 L per minute.
Name the substances passed between the systems.
Put the steps in order, from starting to run to more oxygen reaching the muscles.
Without enough oxygen, muscles respire and produce . Oxygen is carried in red blood cells by , which contains iron. A shortage of it is called . Fit people usually have a resting heart rate.
Explain how the respiratory, circulatory and nervous systems work together to supply a swimmer's muscles with the extra oxygen they need during a race.
As the swimmer's muscles work harder, they respire faster and release more carbon dioxide into the blood. Chemoreceptors detect the rise and send impulses to the brain stem, which sends impulses to the diaphragm and rib muscles so she breathes faster and more deeply. More air reaches the alveoli, so more oxygen diffuses into the blood. At the same time, sympathetic nerves and adrenaline make her heart beat faster and more strongly, so more blood is pumped each minute. Arterioles to her muscles widen and those to her gut narrow, so a larger share of the blood goes to the muscles. Together these changes deliver much more oxygen to the muscle cells, where it is used in aerobic respiration to release energy.