Food is broken down in the gut, and the blood carries the results to every cell. Follow one meal through the digestive system and the circulatory system, and see how structure suits function at every step.
Every cell in your body needs fuel and oxygen, and every cell produces waste that must be removed. Yet most of your cells never touch food, and most are far away from the lungs. Two body systems solve this problem together. The digestive system breaks food down into tiny particles that the body can absorb, and the circulatory system carries those particles, along with oxygen, to every cell, and carries waste away. This guide explains how each system is built, how each one works, and how they cooperate, with simple explanations of the scientific words.
The digestive system turns large food molecules into small ones (digestion) and moves them into the blood (absorption), mainly in the small intestine. The circulatory system, made of the heart, blood vessels and blood, delivers nutrients and oxygen to cells and removes waste. The heart is a double pump: one side serves the lungs and the other serves the rest of the body. Both systems stay healthy with a balanced diet, water, exercise and avoiding smoking.
A living body is organised in levels. Cells are the smallest living units. Similar cells form a tissue (for example, muscle tissue). Different tissues form an organ (for example, the stomach). Several organs that work together on one big job form an organ system (for example, the digestive system). The organ systems together make the whole organism. You can learn more about the smallest level in our article Cells: The Building Blocks of Life.
Our two systems have a clear division of work:
| System | Main job | Main parts |
|---|---|---|
| Digestive | Break down food and absorb nutrients | Mouth, oesophagus, stomach, small and large intestines, liver, pancreas |
| Circulatory | Transport substances around the body | Heart, blood vessels, blood |
This article is for learning and general understanding. It is not medical advice. If you have health concerns, speak to a doctor or another qualified health professional.
Food is made of large molecules, such as starch, proteins and fats, that are far too big to pass into the blood. Digestion is the process of breaking these large, insoluble molecules into small, soluble ones that can be absorbed. The main part of the system is a long, muscular tube called the alimentary canal (or gut), which runs from the mouth to the anus and is several metres long in an adult. Some organs, called accessory organs, are not part of the tube but help digestion by making juices: the salivary glands, the liver, the gall bladder and the pancreas.
| Stage | What happens | Example |
|---|---|---|
| Ingestion | Taking food into the mouth | Eating a sandwich |
| Digestion | Breaking food into smaller pieces and then into small molecules | Chewing; enzymes splitting starch into sugars |
| Absorption | Moving small molecules from the gut into the blood or lymph | Glucose entering the blood in the small intestine |
| Assimilation | Cells use the absorbed molecules | Glucose used for energy; amino acids used to build proteins |
| Egestion | Removing undigested food as faeces | Fibre leaving the body |
There are two kinds of digestion. Mechanical digestion breaks food into smaller pieces without changing its chemistry, for example chewing and the churning of the stomach. Chemical digestion uses enzymes, which are special proteins that speed up chemical reactions, to split large molecules into small ones.
Teeth cut and grind food, which is mechanical digestion. The salivary glands release saliva, which moistens the food and contains the enzyme amylase, which starts to break starch down into sugars. The tongue rolls the chewed food into a ball called a bolus and pushes it to the back of the throat.
When you swallow, a small flap called the epiglottis covers the opening of the windpipe so that food goes down the food pipe, the oesophagus, and not into the lungs. The muscles in the oesophagus wall squeeze behind the food in a wave called peristalsis, which pushes food along. Peristalsis happens along the whole gut, so food can even move when you are upside down.
The stomach is a stretchy muscular bag. Its walls churn the food and mix it with gastric juice. Gastric juice contains hydrochloric acid, which is very strong, with a pH of around 2. The acid kills many harmful microorganisms in food and gives the right conditions for the enzyme pepsin, which begins to break down proteins. A layer of mucus protects the stomach wall from the acid and the enzyme. The food becomes a thick liquid called chyme, and a muscle at the exit, the sphincter, releases small amounts at a time into the small intestine.
The small intestine is a long, narrow, coiled tube, roughly six metres in an adult. It is where most digestion is completed and almost all absorption of nutrients occurs. Its first part, the duodenum, receives two juices:
The large intestine is wider but shorter, about one and a half metres. It absorbs most of the remaining water and some salts. Helpful bacteria live here, and they make some vitamins and help to break down fibre. What remains, mostly fibre, dead cells and bacteria, becomes faeces, which are stored in the rectum and leave the body through the anus. Overall, food usually takes between one and three days to pass through the whole digestive system.
Egestion is the removal of undigested food that never entered the body's cells. Excretion is the removal of waste made by the cells' own chemical reactions, such as carbon dioxide and urea. Examiners often test this difference.
Each kind of enzyme works on one kind of food molecule. The name of the enzyme usually tells you what it does.
| Enzyme (and where made) | Digests | Into |
|---|---|---|
| Amylase (salivary glands, pancreas) | Starch (a carbohydrate) | Sugars, finally glucose |
| Proteases, such as pepsin (stomach, pancreas, small intestine) | Proteins | Amino acids |
| Lipase (pancreas, small intestine) | Fats (lipids) | Fatty acids and glycerol |
Enzymes work best under particular conditions of temperature and pH. Pepsin works best in the acidic stomach, while the enzymes in the small intestine work best in the slightly alkaline conditions made by bile and pancreatic juice. To explore enzymes and food tests further, read Biological Molecules, Enzymes and Food Tests, and for the chemistry of what happens to food afterwards, see Digestion and Metabolism.
The inner wall of the small intestine is covered with millions of tiny, finger-like projections called villi (one is a villus), and their surface cells have even smaller projections called microvilli. Together they greatly increase the surface area, so that more nutrients can be absorbed more quickly. Each villus has features that make absorption efficient:
Nutrients cross the wall by diffusion (movement from a higher to a lower concentration) and by active transport (movement that uses energy, which can even move substances against the concentration difference). Water is absorbed mainly by osmosis. This is a clear example of a structure that suits its function, one of the central ideas of biology.
The circulatory system (also called the cardiovascular system) is the body's transport network. It has three parts: a pump (the heart), a set of tubes (the blood vessels) and the transport fluid (the blood). An adult has about five litres of blood, and at rest the heart can pump the whole amount around the body in about a minute. The circulatory system:
The heart is a muscular organ about the size of a fist, in the middle of the chest and tilted slightly to the left. It has four chambers. The two upper chambers, the atria (one is an atrium), receive blood. The two lower chambers, the ventricles, pump blood out. A wall called the septum keeps the left and right sides separate, so that oxygen-rich and oxygen-poor blood do not mix. Valves between the chambers and in the exits prevent the blood from flowing backwards. The left ventricle has a much thicker muscle wall than the right one, because it must push blood around the entire body, while the right ventricle only pumps blood to the nearby lungs.
A healthy adult heart beats about 60 to 100 times per minute at rest. Each beat is a squeeze (contraction) followed by a relaxation, and the heart muscle has its own blood supply through the coronary arteries.
Human blood passes through the heart twice in one complete trip, which is called double circulation. The pulmonary circulation takes blood from the heart to the lungs and back. The systemic circulation takes blood from the heart to the rest of the body and back. Double circulation allows the blood to be pushed at high pressure to the body after it picks up oxygen in the lungs.
| Step | Where the blood goes |
|---|---|
| 1 | Oxygen-poor blood from the body enters the right atrium through the vena cava |
| 2 | It passes to the right ventricle |
| 3 | The right ventricle pumps it through the pulmonary artery to the lungs |
| 4 | In the lungs it gets oxygen and loses carbon dioxide |
| 5 | Oxygen-rich blood returns by the pulmonary veins to the left atrium |
| 6 | It passes to the left ventricle |
| 7 | The left ventricle pumps it through the aorta to the whole body |
You can follow this path interactively on our page Blood Flow Through the Heart.
| Vessel | Structure | Job |
|---|---|---|
| Artery | Thick, muscular, elastic walls; narrow space inside | Carries blood away from the heart at high pressure |
| Vein | Thinner walls; wider space inside; has valves | Carries blood back to the heart at lower pressure; valves stop backflow |
| Capillary | Very tiny; walls only one cell thick | Allows exchange of oxygen, nutrients and waste between blood and cells |
Notice the link between structure and function. Arteries need strong walls to withstand high pressure, and your pulse is the stretching of an artery wall each time the heart pushes blood into it. Capillaries have thin walls so that substances can pass through them easily, and they are found close to almost every cell. A common mistake is to say that arteries always carry oxygen-rich blood. The pulmonary artery is an exception, because it carries oxygen-poor blood to the lungs, and the pulmonary veins carry oxygen-rich blood back.
Blood is a tissue made of a liquid and several kinds of cells.
| Part | Main features | Job |
|---|---|---|
| Plasma | A pale yellow liquid, a little over half of the blood, mostly water | Carries dissolved nutrients, carbon dioxide, urea, hormones and heat |
| Red blood cells | Disc-shaped with a dip in the middle; no nucleus; contain haemoglobin | Carry oxygen; haemoglobin is the red pigment that binds oxygen |
| White blood cells | Larger than red cells; have a nucleus | Fight infection by engulfing microorganisms or making antibodies |
| Platelets | Small cell fragments | Help blood to clot and seal wounds |
A red blood cell has no nucleus, which leaves more room for haemoglobin, and its shape gives a large surface area for taking in oxygen. Red blood cells live for about four months, and the body continually makes new ones in the bone marrow. For more on how blood types are identified, see How to Identify Blood Groups.
Blood pressure is the force of the blood on the walls of the arteries. It is written as two numbers, such as 120/80, measured in "mmHg" (millimetres of mercury). The first number, the systolic pressure, is the pressure when the heart contracts. The second, the diastolic pressure, is the pressure when the heart relaxes between beats. A reading of around 120/80 is a common example of a healthy adult reading, but the correct range for any person is a matter for a health professional. Blood pressure that stays too high puts extra strain on the heart and blood vessels, and it often has no obvious symptoms, so regular checks are important.
The best way to see the partnership is to follow one meal. Imagine that you eat a piece of bread.
The two systems are therefore partners. If the digestive system fails to absorb nutrients, the blood has nothing to carry, and if the circulatory system fails to deliver, the nutrients are wasted. Water is also part of the story: the body needs it for blood, digestive juices and the removal of waste, and you can read about this in Hydration: The Role of Water in the Body.
Health services give very similar advice for both systems, because what is good for the digestive system is usually good for the heart as well.
Diet and lifestyle affect long-term health, and our article Diet-Related Diseases explains the links with heart disease and diabetes. If you ever have sudden chest pain, severe difficulty in breathing, fainting or other alarming symptoms, seek emergency medical help immediately and do not wait.
| Misconception | Correct idea |
|---|---|
| Most nutrients are absorbed in the stomach | Most absorption happens in the small intestine |
| Bile is an enzyme | Bile has no enzymes; it emulsifies fats and neutralises acid |
| Food passes through the windpipe | The epiglottis covers the windpipe when you swallow |
| Egestion and excretion are the same | Egestion removes undigested food; excretion removes waste made by cells |
| Veins carry blue blood | Blood is always red; oxygen-poor blood is a darker red, and veins only look blue through the skin |
| Arteries always carry oxygen-rich blood | The pulmonary artery carries oxygen-poor blood to the lungs |
| The heart is on the left of the chest | It is in the middle, tilted slightly to the left |
| The large intestine is where food is digested | It mainly absorbs water and forms faeces |
| Term | Meaning |
|---|---|
| Digestion | Breaking large food molecules into small, soluble ones |
| Enzyme | A protein that speeds up a chemical reaction |
| Peristalsis | Waves of muscle contraction that push food along the gut |
| Absorption | Movement of digested nutrients into the blood or lymph |
| Villi | Tiny finger-like projections that increase the surface area of the small intestine |
| Egestion | Removal of undigested food from the body |
| Double circulation | Blood passes through the heart twice in each complete circuit |
| Atrium and ventricle | The upper receiving chamber and the lower pumping chamber of the heart |
| Haemoglobin | The red pigment in red blood cells that carries oxygen |
| Blood pressure | The force of blood on the walls of the arteries |
The next time you eat a meal, you can picture a continuous, well-organised journey: teeth, enzymes and muscles break the food down, the small intestine passes the nutrients into the blood, and the heart sends them to every cell in your body within moments. It is a partnership of two systems that works every second of your life without any effort from you. Understanding it makes it easier to care for it, and to answer exam questions with confidence.
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