The heart is a muscular pump that keeps blood moving through the body every minute of life. It sends oxygen-poor blood to the lungs and oxygen-rich blood to the organs, tissues, and brain. This circulation delivers oxygen and nutrients while carrying away carbon dioxide and wastes.
Understanding how the heart pumps helps explain blood pressure, pulse, exercise response, and many common medical conditions.
The pumping cycle depends on four chambers, four valves, and an electrical timing system. Blood moves from atria to ventricles, then out through major arteries because pressure changes open and close valves in one direction. The right side of the heart pumps blood to the lungs, while the left side pumps blood to the entire body.
Each heartbeat is a coordinated cycle of filling, contraction, and relaxation called the cardiac cycle.
Understanding How the Heart Pumps Blood
Each beat starts with an electrical signal, not with a conscious command from the brain. A small group of cells in the right atrium, called the sinoatrial node, usually sets the pace. The signal spreads across both atria, causing them to squeeze and top up the ventricles.
It then pauses briefly at the atrioventricular node. This short delay matters because the ventricles need time to fill before they contract. Next, the signal travels through specialised fibres in the heart walls.
Both ventricles then contract from the lower part upward, pushing blood efficiently into the large vessels. An electrocardiogram, or ECG, records this electrical activity from the skin. It can help doctors detect timing problems called arrhythmias.
Heart valves do not pull themselves open or shut. They respond to pressure differences. When pressure behind a valve becomes greater than pressure ahead of it, the valve opens.
When pressure reverses, the valve closes and prevents backflow. The familiar heart sounds come mainly from valve closure. The first sound occurs when the valves between the atria and ventricles close at the start of ventricular contraction.
The second occurs when the valves leading out of the ventricles close as those chambers relax. A damaged or narrowed valve can make blood flow turbulent. This may produce a murmur, which is an extra sound heard with a stethoscope.
The heart muscle itself needs a constant blood supply through the coronary arteries. A blockage in one of these arteries can injure heart muscle and cause a heart attack.
The amount pumped in one minute depends on how often the heart beats and how much leaves a ventricle with each beat. This is described by the statement cardiac output equals heart rate times stroke volume. During exercise, muscles use more oxygen and produce more carbon dioxide.
Nerves and hormones make the heart beat faster and more forcefully. More blood returns from active muscles too, which can increase stroke volume. Blood pressure rises during the pumping phase because arteries are being filled under force.
It falls during relaxation, but it does not reach zero because elastic artery walls recoil and keep blood moving. Narrower blood vessels create more resistance, so the heart must work harder to maintain flow.
Students often picture the heart as a simple machine that pumps at one fixed strength. In reality, it constantly adjusts to sleep, standing up, stress, fever, exercise, and illness. A pulse felt at the wrist is a pressure wave in an artery, not the blood itself rushing under the fingers.
Its rate can be counted to estimate heart rate, though some irregular beats may not produce a strong wrist pulse. When learning this topic, track three linked ideas carefully.
Follow the electrical signal, follow the changing pressures, and follow the one way movement through valves. These ideas explain why a rhythm problem, a valve problem, or stiff blood vessels can affect the whole circulation in different ways.
Key Facts
- The right atrium receives oxygen-poor blood from the body through the superior and inferior vena cava.
- The right ventricle pumps oxygen-poor blood to the lungs through the pulmonary artery.
- The left atrium receives oxygen-rich blood from the lungs through the pulmonary veins.
- The left ventricle pumps oxygen-rich blood to the body through the aorta.
- Cardiac output = heart rate x stroke volume.
- Blood pressure is commonly written as systolic pressure over diastolic pressure, such as 120/80 mmHg.
Vocabulary
- Atrium
- An atrium is one of the two upper heart chambers that receives blood returning to the heart.
- Ventricle
- A ventricle is one of the two lower heart chambers that pumps blood out of the heart.
- Valve
- A valve is a flap-like structure that keeps blood flowing in one direction through the heart.
- Systole
- Systole is the phase of the heartbeat when the ventricles contract and push blood into the arteries.
- Diastole
- Diastole is the phase of the heartbeat when the heart muscle relaxes and the chambers fill with blood.
Common Mistakes to Avoid
- Thinking arteries always carry oxygen-rich blood. This is wrong because pulmonary arteries carry oxygen-poor blood from the heart to the lungs.
- Confusing the right and left sides of the heart in a diagram. This is wrong because diagrams often show the patient’s right side on the viewer’s left.
- Saying valves actively pump blood. This is wrong because valves open and close due to pressure differences, while the heart muscle provides the pumping force.
- Treating the heart as one simple pump. This is wrong because the right and left sides are separate pumps connected in series through the lungs and body.
Practice Questions
- 1 A student has a heart rate of 75 beats per minute and a stroke volume of 70 mL per beat. Calculate the cardiac output in mL per minute and L per minute.
- 2 During exercise, a person’s heart rate rises to 150 beats per minute and stroke volume rises to 100 mL per beat. What is the cardiac output in L per minute?
- 3 Explain why the wall of the left ventricle is thicker than the wall of the right ventricle, even though both chambers pump the same amount of blood per heartbeat.