The heart is a muscular pump that keeps blood moving through the lungs and the rest of the body. Its four chambers, valves, and major vessels work together to deliver oxygen and nutrients while removing carbon dioxide and wastes. Understanding the heart helps explain pulse, blood pressure, exercise response, and many common medical conditions.
The cardiac cycle is the repeating pattern of filling and pumping that produces each heartbeat.
Blood moves through the heart in one direction because valves open and close in response to pressure differences. The right side sends oxygen-poor blood to the lungs, while the left side sends oxygen-rich blood to the body. During diastole, the chambers relax and fill with blood, and during systole, the ventricles contract to eject blood.
Electrical signals from the sinoatrial node coordinate these contractions so the heartbeat is timed and efficient.
Key Facts
- The heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle.
- Blood path: body -> venae cavae -> right atrium -> right ventricle -> pulmonary arteries -> lungs -> pulmonary veins -> left atrium -> left ventricle -> aorta -> body.
- Diastole is the relaxation and filling phase of the cardiac cycle.
- Systole is the contraction phase that pumps blood out of the ventricles.
- Cardiac output = heart rate x stroke volume.
- Average adult resting heart rate is about 60 to 100 beats per minute.
Vocabulary
- Atrium
- An atrium is an upper heart chamber that receives blood returning to the heart.
- Ventricle
- A ventricle is a lower heart chamber that pumps blood out of the heart.
- Valve
- A valve is a flap-like structure that prevents blood from flowing backward.
- Cardiac cycle
- The cardiac cycle is one complete sequence of heart relaxation, filling, contraction, and blood ejection.
- Sinoatrial node
- The sinoatrial node is a group of cells in the right atrium that starts the electrical signal for each heartbeat.
Common Mistakes to Avoid
- Confusing arteries with oxygen-rich blood, because arteries are defined by carrying blood away from the heart, not by oxygen level. Pulmonary arteries carry oxygen-poor blood to the lungs.
- Putting the left and right sides of the heart from the viewer's perspective, because anatomical left and right refer to the person's body. In a front-facing diagram, the heart's right side appears on the viewer's left.
- Thinking valves actively pull blood forward, because valves do not pump. They open and close because pressure changes push blood in the correct direction.
- Mixing up systole and diastole, because both happen during every heartbeat. Systole is contraction and ejection, while diastole is relaxation and filling.
Practice Questions
- 1 A student has a resting heart rate of 72 beats per minute and a stroke volume of 70 mL per beat. Calculate cardiac output in mL per minute and L per minute.
- 2 During one minute, the left ventricle pumps 5.6 L of blood. If the heart rate is 80 beats per minute, what is the stroke volume in mL per beat?
- 3 Explain why damage to the mitral valve could cause blood to leak backward into the left atrium during ventricular systole.