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Heart Anatomy and the Cardiac Cycle
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The heart is a muscular pump that keeps blood moving through the lungs and the rest of the body. Understanding its anatomy helps explain how oxygen is delivered to tissues and how blood pressure is maintained. For pre med and early medical students, the heart is a core example of how structure supports function. Its chambers, valves, and vessels work together in a precise sequence during every heartbeat.
The cardiac cycle describes the repeating pattern of filling and ejection that occurs as the atria and ventricles contract and relax. Pressure changes inside the chambers open and close valves, which keeps blood moving in one direction. Electrical activity from the conduction system coordinates this mechanical action so that timing stays efficient. Knowing the normal flow pattern and cycle phases makes it easier to understand murmurs, heart failure, arrhythmias, and valve disease.
Key Facts
- The heart has 4 chambers: right atrium, right ventricle, left atrium, and left ventricle.
- Normal blood flow: venae cavae -> right atrium -> tricuspid valve -> right ventricle -> pulmonary valve -> pulmonary arteries -> lungs -> pulmonary veins -> left atrium -> mitral valve -> left ventricle -> aortic valve -> aorta.
- Cardiac output = heart rate x stroke volume.
- Stroke volume = end diastolic volume - end systolic volume.
- Ejection fraction = stroke volume / end diastolic volume x 100%.
- Typical resting values: heart rate 60 to 100 beats/min, stroke volume about 70 mL/beat, cardiac output about 5 L/min, ejection fraction about 55% to 70%.
Vocabulary
- Diastole
- Diastole is the phase of the cardiac cycle when the ventricles relax and fill with blood.
- Systole
- Systole is the phase of the cardiac cycle when the ventricles contract and eject blood into the arteries.
- Stroke volume
- Stroke volume is the amount of blood pumped out by one ventricle in a single heartbeat.
- Ejection fraction
- Ejection fraction is the percentage of ventricular filling volume that is pumped out during systole.
- Coronary circulation
- Coronary circulation is the blood supply that delivers oxygen and nutrients to the heart muscle itself.
Common Mistakes to Avoid
- Confusing oxygenated and deoxygenated pathways, which leads to mixing up the right and left sides of the heart. The right heart sends deoxygenated blood to the lungs, while the left heart sends oxygenated blood to the body.
- Thinking valves open because muscles pull them open, which is wrong because valves open and close mainly due to pressure differences across them. Papillary muscles and chordae tendineae prevent valve prolapse rather than actively opening the valves.
- Assuming systole means the whole heart contracts at once, which misses the timing of the cardiac cycle. Atrial systole occurs before ventricular systole to help complete ventricular filling.
- Memorizing chamber names without tracking blood flow direction, which makes it hard to solve physiology questions. Always follow the blood step by step through chambers, valves, and great vessels.
Practice Questions
- 1 A patient has a heart rate of 72 beats/min and a stroke volume of 75 mL/beat. Calculate the cardiac output in L/min.
- 2 A left ventricle has an end diastolic volume of 130 mL and an end systolic volume of 50 mL. Calculate the stroke volume and ejection fraction.
- 3 Explain why the left ventricular wall is thicker than the right ventricular wall, and relate your answer to the pressure each side must generate.