The heart
The heart has four chambers: two upper atria that receive blood and two lower ventricles that pump it out. The right side sends oxygen-poor blood to the lungs (the pulmonary circuit), and the left side sends oxygen-rich blood to the rest of the body (the systemic circuit). The left ventricle has the thickest wall because the systemic circuit offers greater resistance.
Path of blood: vena cavae, right atrium, tricuspid valve, right ventricle, pulmonary valve, pulmonary arteries, lungs, pulmonary veins, left atrium, bicuspid (mitral) valve, left ventricle, aortic valve, aorta. Valves keep blood moving one way. Closure of the AV valves makes the first heart sound ("lub"); closure of the semilunar valves makes the second ("dub").
Electrical conduction
The sinoatrial (SA) node is the natural pacemaker. Its impulse spreads across the atria, reaches the atrioventricular (AV) node where it is briefly delayed, then passes down the AV bundle and Purkinje fibres so the ventricles contract from the bottom upward.
Vessels
Arteries carry blood away from the heart and have thick, elastic walls. Veins return blood and have valves. Capillaries, with walls one cell thick, are the sites of exchange. Pulmonary arteries carry oxygen-poor blood and pulmonary veins carry oxygen-rich blood.
Key calculations
- Stroke volume (SV) = end-diastolic volume - end-systolic volume.
- Cardiac output (CO) = heart rate x stroke volume.
- Ejection fraction = SV / end-diastolic volume.
- Mean arterial pressure (MAP) = diastolic pressure + one third of the pulse pressure.
Worked example
Take an invented resting person with a heart rate of 75 beats per minute, an end-diastolic volume of 140 mL and an end-systolic volume of 60 mL.
- SV = 140 - 60 = 80 mL per beat.
- CO = 75 x 80 = 6000 mL/min, which is 6.0 L/min.
- Ejection fraction = 80 / 140 = about 57 percent.
- Each beat lasts 60 / 75 = 0.8 seconds.
Now take a blood pressure of 126/78 mmHg. Pulse pressure = 126 - 78 = 48. MAP = 78 + 48/3 = 78 + 16 = 94 mmHg. MAP is not the simple average of the two numbers (which would be 102), because the heart spends more of each cycle in diastole than in systole. Together these figures show that cardiac output rises when either heart rate or stroke volume rises.