Cardiovascular system

What the exam asks

Expect a pressure-volume loop, a pressure trace, an ECG, or a table of flows and resistances. The commonest items are: name the valve event at a labelled corner; read stroke volume off a loop; predict how the loop changes when the stem raises preload, afterload or contractility; and apply flow equals pressure over resistance to a branch of the circulation. The trap that catches most candidates is reading a taller loop as a better beat. Raise afterload and the ventricle does generate more pressure, but it opens the valve later and shuts it earlier, so the loop is taller and narrower: more pressure, less blood. Width is what you were asked about, not height. The second trap is calling capillaries the site of greatest resistance because they are the narrowest vessels. Individually they are, but there are so many in parallel that the network's resistance is low, and it is the arterioles upstream that hold the pressure.

The cardiovascular topic is a pump, a set of pipes, and a controller, and the exam tests all three as reasoning about pressure. Almost nothing here needs memorising, because the stem hands you the numbers. What you carry in is a small set of rules about what pressure does.

The first rule is that valves have no motors. Every valve in the heart opens and shuts because the pressure on one side crossed the pressure on the other, and it does so passively. That single fact converts a pressure-volume loop from a picture to be memorised into a diagram to be read. Volume cannot change while all four valves are shut, so the two vertical sides of the loop are the isovolumetric phases, and each corner is a valve event you can name from the pressures alone.

The second rule is that flow is a pressure difference divided by a resistance, and resistance depends savagely on radius. A small change in the calibre of a vessel is a large change in the flow through it, which is why arterioles, the vessels with the muscle to change their radius, are where the body controls both where blood goes and what the pressure upstream of them is.

The third rule is that the controller is negative feedback with a stretch sensor. Baroreceptors report arterial stretch, and every response they trigger acts to undo the change that stimulated them. Given a fall in pressure from any cause, the direction of every downstream variable follows without any recall at all.

What to hold

• Heart valves are passive. They open and close only because the pressure gradient across them reversed, which means every valve event can be located from a pressure trace.
• On a pressure-volume loop the two vertical segments are the isovolumetric phases, because volume cannot change while all four valves are closed. The loop's width is stroke volume, EDV minus ESV, and the area inside it is the work the ventricle did that beat.
• Preload is the volume the ventricle is filled to before it contracts. More filling stretches the muscle and it contracts harder, so stroke volume rises: this is Frank-Starling, and it is the cardiac length-tension curve.
• Afterload is the pressure the ventricle must exceed to open the aortic valve and keep ejecting. Raise it alone and ejection starts later and stops sooner, so end-systolic volume rises and stroke volume falls.
• Contractility changes force at a given filling, so it moves stroke volume without needing a change in preload or afterload.
• The SA node sets the rhythm because it depolarises to threshold faster than any other pacemaker. If it fails, a slower latent pacemaker takes over rather than the heart stopping.
• The AV node delays conduction, which lets the atria finish emptying before the ventricles squeeze. Without the delay, the chambers would fight each other.
• The ECG records electrical events, which precede the mechanical ones they trigger. The QRS is ventricular depolarisation, not ventricular contraction.
• Flow equals pressure difference over resistance, and resistance scales with the fourth power of radius, so halving a vessel's radius multiplies its resistance by sixteen.
• Arterioles are the main resistance vessels because they have the smooth muscle to change radius, and they are where the largest pressure drop in the circulation occurs.
• Blood moves slowest in the capillaries because total cross-sectional area is greatest there, not because each capillary is narrow.
• Baroreceptors sense stretch in the carotid sinus and aortic arch and drive a negative feedback loop that buffers short-term pressure changes. They reset over days, so they cannot set long-term pressure.