The Cardiovascular and Lymphatic Systems
The cardiovascular and lymphatic systems domain tests blood flow, cardiac excitation and the cardiac cycle, blood pressure regulation, capillary exchange, and lymphatic return. Expect questions that connect vessel structure with function and use simple hemodynamic calculations.
Core rules and concepts
- The heart’s conduction sequence is SA node, AV node, bundle of His, bundle branches and Purkinje fibers. The SA node sets the resting rate; the AV node delays conduction so atria can empty before ventricles contract.
- Cardiac output equals heart rate times stroke volume. Stroke volume depends on preload, contractility and afterload.
- Arteries carry blood away from the heart and withstand pressure; veins carry blood toward the heart and have valves. Capillaries have thin walls for exchange.
- Blood pressure is highest in arteries and falls along the circuit. Systolic pressure reflects ventricular ejection; diastolic pressure reflects runoff. Mean arterial pressure is roughly diastolic pressure plus one-third of pulse pressure.
- Capillary exchange occurs by diffusion, filtration and reabsorption. Net filtration depends on hydrostatic and oncotic pressure gradients.
- Lymphatic vessels return excess interstitial fluid and proteins to the blood, transport absorbed fats as chyle, and support immune surveillance through lymph nodes.
Worked example
A person has a heart rate of 75 beats per minute and a stroke volume of 70 mL. Cardiac output is 75 × 70 = 5,250 mL/min, or about 5.25 L/min. If stroke volume falls to 50 mL but heart rate rises to 100 beats per minute, cardiac output becomes 5,000 mL/min. The calculation shows that a faster heart rate cannot fully compensate for a large stroke-volume loss.
Traps that cost marks
Do not say veins have thicker walls than arteries. Do not confuse the SA node with the AV node’s delay function. Do not treat lymphatic vessels as a closed circuit; they are one-way vessels returning fluid to the venous system. Remember that oncotic pressure pulls water into capillaries, while hydrostatic pressure pushes water out.
Worked example
If capillary hydrostatic pressure rises and oncotic pressure is unchanged, net filtration increases and edema can result.