The digestive system absorbs nutrients and the respiratory system takes in oxygen, but neither is useful until those substances reach cells all over the body. The circulatory system is the delivery network: a pump (the heart), a network of tubes (the blood vessels) and a transport fluid (the blood). It carries oxygen, glucose, amino acids, hormones and heat to cells, and carries carbon dioxide and other wastes away. Humans have a double circulation: blood passes through the heart twice on each full trip, once in the pulmonary circuit (heart to lungs and back) and once in the systemic circuit (heart to the rest of the body and back).
The heart: a double pump
The heart is a fist-sized organ made of cardiac muscle, a muscle type that contracts rhythmically without tiring. A wall called the septum divides it into a right side and a left side, and each side has two chambers: a thin-walled atrium on top that receives blood, and a thick-walled ventricle below that pumps blood out. The right side handles oxygen-poor blood and pumps it only to the nearby lungs. The left side handles oxygen-rich blood and must push it through the entire body, so the left ventricle has the thickest, most muscular wall. Diagrams show the heart as if you are facing the person, so the heart's left side appears on the right of the page.
- Oxygen-poor blood from the body enters the right atrium through the superior and inferior vena cavae.
- It passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary valve into the pulmonary arteries, which lead to the lungs.
- In the lung capillaries, blood picks up oxygen and releases carbon dioxide.
- Oxygen-rich blood returns through the pulmonary veins to the left atrium.
- It passes through the bicuspid (mitral) valve into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta, which branches to deliver blood to the whole body.
| Valve | Location | Prevents backflow from |
|---|---|---|
| Tricuspid (right atrioventricular) valve | Between right atrium and right ventricle | Right ventricle into right atrium |
| Bicuspid or mitral (left atrioventricular) valve | Between left atrium and left ventricle | Left ventricle into left atrium |
| Pulmonary semilunar valve | Between right ventricle and pulmonary artery | Pulmonary artery into right ventricle |
| Aortic semilunar valve | Between left ventricle and aorta | Aorta into left ventricle |
Each heartbeat starts with an electrical signal from the sinoatrial (SA) node, a small patch of tissue in the wall of the right atrium that acts as the heart's natural pacemaker. The signal makes both atria contract, then passes to the atrioventricular (AV) node, which delays it briefly so the ventricles can fill before they contract. The nervous system and hormones such as adrenaline speed up or slow down this rhythm. Through a stethoscope, a heartbeat sounds like "lub-dub": "lub" is the atrioventricular valves closing as the ventricles contract, and "dub" is the semilunar valves closing as the ventricles relax. Heart muscle does not absorb oxygen from the blood inside its chambers; it has its own supply through the coronary arteries, which branch off the aorta and run over the heart's surface.
Blood vessels
| Feature | Arteries | Capillaries | Veins |
|---|---|---|---|
| Direction of flow | Away from the heart | Connect arterioles to venules | Toward the heart |
| Wall | Thick, with elastic tissue and smooth muscle | One cell thick | Thinner than arteries, less muscle |
| Blood pressure | High, and pulses with each beat | Low | Lowest |
| Valves | None (except where they leave the heart) | None | One-way valves prevent backflow |
| Main job | Carry blood quickly under pressure; elastic walls stretch and recoil | Exchange of gases, nutrients and wastes with body cells | Return blood; squeezing by nearby skeletal muscles helps push it along |
What blood is made of
| Component | Share of blood | Description and function |
|---|---|---|
| Plasma | About 55% | Straw-coloured liquid, mostly water. Carries dissolved nutrients, hormones, proteins, salts, wastes and most carbon dioxide (as bicarbonate). |
| Red blood cells (erythrocytes) | Most of the remaining 45% | Disc-shaped cells packed with hemoglobin that carry oxygen. Mature human red blood cells have no nucleus, leaving more room for hemoglobin. Made in red bone marrow; they last about 120 days. |
| White blood cells (leukocytes) | Less than 1% (together with platelets) | Larger cells with nuclei that defend against infection. Some engulf microbes; others help make antibodies. |
| Platelets (thrombocytes) | Less than 1% (together with white blood cells) | Small cell fragments that gather at a damaged vessel and start the clotting process that seals the wound. |
Blood pressure and heart rate
Blood pressure is the force of blood pushing against the walls of the arteries. It rises and falls with every heartbeat, so it is written as two numbers. The top number, systolic pressure, is the peak pressure when the ventricles contract. The bottom number, diastolic pressure, is the lowest pressure when the ventricles relax and refill. Blood pressure is measured with a sphygmomanometer (an inflatable cuff with a pressure gauge), often with a stethoscope, in millimetres of mercury (mmHg). A typical value for a healthy adult at rest is around 120/80 mmHg, read as "120 over 80". Heart rate is the number of beats per minute; for healthy adults at rest it is typically about 60 to 100 beats per minute, and very fit people often have lower resting rates because each beat pumps more blood. You can feel each beat as a pulse where an artery runs close to the skin, such as at the wrist or neck.