Physiology: reading graphs, loops and curves, the few numbers worth knowing and how to reason when you forget one

Physiology is the subject where students say "I understood it when I read it" and then lose marks anyway. A graph appears in a question. A curve is shifted to the right. A loop has a changed shape. A table gives you a few values and asks what happens next. You know you've seen this, and you can't quite remember which way it goes.

There are two skills that fix most of this. The first is reading a graph as a statement about cause and effect, instead of as a picture to memorise. The second is keeping a short list of anchor numbers, and knowing how to reason when you forget one. Neither needs a photographic memory.

The facts below are standard textbook material, but please check any specific value against the book you use, since different books round differently.

Read every graph in four moves

When a graph appears, do the same four things, in the same order.

1. Read the axes. What is on the x-axis, what is on the y-axis, and what are the units? A surprising number of mistakes come from skipping this. A curve of pressure against volume is a different object from a curve of volume against time.

2. Find the normal. Where does the "usual" line or point sit? Everything else is a change from this.

3. Name the direction of change. Shifted left or right, steeper or flatter, higher or lower. Say it in words before you think about why.

4. Ask what caused it, and what it means. A shift is a sign that something has changed in the system. Your job is to say which factor and what the consequence is.

That sequence works for almost every graph in physiology. Let's use it on a few of the commonest.

The oxygen-haemoglobin dissociation curve

Axes: the partial pressure of oxygen on the x-axis and the percentage saturation of haemoglobin on the y-axis. Normal shape: sigmoid, because haemoglobin binds oxygen cooperatively. Once one oxygen is bound, the others bind more easily. That's why the curve is steep in the middle and flat at the top.

The flat top matters clinically. Over a wide range of high oxygen pressures, saturation barely changes, so a modest fall in alveolar oxygen doesn't drop the blood's oxygen content much. The steep part is where tissues sit: small falls in pressure release a lot of oxygen.

Now the shifts. A right shift means haemoglobin has less affinity for oxygen, so it gives up oxygen more readily to tissues. The standard causes are raised carbon dioxide, a fall in pH (the Bohr effect), raised temperature and increased 2,3-bisphosphoglycerate. Think of exercising muscle: warm, acidic, full of carbon dioxide. It needs oxygen, and the curve moves to deliver it. A left shift is the opposite: higher affinity, so oxygen is held more tightly. Fetal haemoglobin, carbon monoxide and low temperature are typical examples.

Notice that you don't need to memorise two separate lists. If you remember "right shift is what exercising muscle looks like", you can derive the list. Cold, alkaline and low in 2,3-BPG is the reverse.

A useful anchor number is the P50, the oxygen pressure at which haemoglobin is half saturated, which is in the high twenties of mmHg in adults. A right shift raises the P50, a left shift lowers it. If you forget the number but remember the direction, you'll still get most questions right.

Pressure-volume loops of the left ventricle

Axes: ventricular volume on the x-axis, ventricular pressure on the y-axis. The loop is a closed shape, and each corner is an event.

Walk round it once as a story. Filling begins at the lower left as the mitral valve opens, and volume rises with little pressure change. When contraction begins, pressure rises with no change in volume, because both valves are shut: this is isovolumetric contraction. When ventricular pressure exceeds aortic pressure, the aortic valve opens and blood is ejected, volume falling while pressure stays high. The valve shuts, and pressure falls with no change in volume: isovolumetric relaxation. Then the mitral valve opens again.

Two facts come directly from the geometry. The width of the loop is the stroke volume, the difference between end-diastolic and end-systolic volume. The area of the loop is the work done by the ventricle in one beat.

Then the "what if" questions become visual. Increase preload (more filling) and the loop widens to the right, because end-diastolic volume rises, so stroke volume rises, which is the Frank-Starling mechanism. Increase afterload (higher aortic pressure) and the loop gets taller, and the stroke volume tends to fall if contractility doesn't change. Increase contractility and the end-systolic point moves up and to the left, so the ventricle empties more completely. Read each change as a movement of one corner and you can answer without recalling a table.

Flow-volume loops and spirometry

Axes: flow on the y-axis, volume on the x-axis. The expiratory limb goes up to a peak, then falls as lung volume falls.

The picture that helps most is the contrast. In obstructive disease such as asthma or COPD, the airways are narrowed, so flow is low relative to the volume, and the expiratory limb looks scooped out, "like a spoon". The FEV1/FVC ratio is reduced, and the lungs may be hyperinflated. In restrictive disease the lungs can't expand, so the loop is a smaller, narrow version of the normal shape, volumes are reduced, and the FEV1/FVC ratio is normal or even high, because both numbers fall together.

If you remember only that the obstructive problem is "getting air out" (flow suffers, ratio falls) and the restrictive problem is "getting air in" (volume suffers, ratio is preserved), you've got the core.

Cardiac action potentials and the "which phase" question

Examiners like to show a trace and ask which phase corresponds to what. The trick is to attach each phase to one ion movement.

In a ventricular muscle cell, phase 0 is the rapid upstroke from sodium entry. Phase 1 is a brief early repolarisation. Phase 2, the plateau, is where calcium entry balances potassium exit, and it's the reason cardiac muscle contracts for a long time and can't be tetanised. Phase 3 is repolarisation as potassium leaves. Phase 4 is the resting potential.

In the sinoatrial node there is no stable resting potential. The cell drifts upward in phase 4, from the "funny" current and calcium channels, until it reaches threshold. The slope of that drift sets the heart rate. Sympathetic stimulation steepens it, and vagal stimulation flattens it and makes the cell more negative. That is the whole mechanism behind how autonomic input changes heart rate, from one picture.

The few numbers worth carrying

You don't need hundreds of numbers. You need a short list of anchors, remembered as rough values, and then reasoning for everything around them. The ranges below are typical textbook figures for a healthy adult. Always check your own textbook, and accept that different sources give slightly different ranges.

• Body water: about 60% of body weight in an adult male, roughly two thirds inside cells and one third outside. Plasma is a fraction of the extracellular part.
• Cardiac output: about 5 litres per minute at rest, equal to heart rate times stroke volume.
• Mean arterial pressure: diastolic pressure plus one third of the pulse pressure.
• Arterial pH: about 7.35 to 7.45.
• Plasma sodium and potassium: sodium in the mid-100s of mmol/L, potassium a little under 5 at the top of its range. The key relationship: sodium is mainly outside cells, potassium mainly inside.
• Glomerular filtration rate: around 120 mL per minute in a healthy young adult, a figure that falls with age.
• Normal arterial blood gases: PaCO2 in the high 30s to low 40s mmHg, and PaO2 well above 80 mmHg on room air.
• Resting membrane potential of a neuron: roughly -70 mV, close to the potassium equilibrium potential because the resting membrane is most permeable to potassium.

That's about a dozen anchors. Put them on one page and review that page often. Everything else you're likely to be asked can be reasoned from these.

When you forget the number: how to reason

This is the skill that separates a frightening exam from a manageable one. When the exact figure escapes you, don't panic. Try these in order.

Use direction, not value. Many questions only need to know whether something goes up or down. If you can say "raised CO2 shifts the curve right", you don't need the P50.

Use relationships. If cardiac output is heart rate times stroke volume, and you remember one and the product, you can find the other. Mean arterial pressure comes from the diastolic and pulse pressure. Clearance is urine concentration times urine flow divided by plasma concentration. Formulas are often more reliable than numbers.

Use proportions. The options in a question often differ by ten-fold or more. You may not remember a value to two figures, but you'll know whether it is closer to one or to a hundred.

Use the extremes. Ask what would happen at zero and at a very large value. If a quantity can't plausibly be negative or larger than a clear upper limit, you can often eliminate options.

Use the compensation rule. In acid-base and many other topics, the body's response goes in a predictable direction and never overshoots. A primary rise in CO2 is compensated by retaining bicarbonate. If the options include a response in the wrong direction, they're wrong.

Use the neighbouring facts. If you forget the normal range for a hormone, think about what you know of its effect. A guess that fits the biology is better than a blank.

How to practise graphs

Graphs are learned by drawing them, not by looking at them.

Redraw from memory. Take a blank page, draw the axes, label them with units, draw the normal curve, then draw the shifted curve and write the causes beside it. Do this for the oxygen curve, the pressure-volume loop, the flow-volume loops and the action potentials.

Pair every graph with a sentence. "A right shift means lower affinity, so oxygen unloads more easily." If you can say the sentence, you can answer almost any question on that graph.

Write the "what if" yourself. For each graph, write three changes and what happens to the picture. Then check against your textbook.

Use questions. After studying a topic, try a handful of practice questions and pay attention to the ones that show a figure. Those are the ones where the four-move method helps most.

Mix the topics. Put graphs from different systems in the same session, so you practise recognising what kind of problem you're looking at.

A note on what is tested

People like to say that physiology is "mostly graphs" or that certain numbers are "always asked". Treat that kind of claim with caution. What the paper contains, how it is weighted and how it is marked are set out in official notices from NBEMS and NMC, and those notices may change. Prepare for the understanding, which carries over whatever the format, and check the official sources for the format itself.

The short version

• Read every graph in four moves: axes, normal, direction of change, cause and meaning.
• Learn the shape of a curve as a story. A right shift is what an exercising muscle looks like.
• Walk around a pressure-volume loop once, corner by corner; width is stroke volume, area is work.
• Keep a one-page list of about a dozen anchor numbers and reason about the rest.
• When you forget a number, use direction, formulas, proportions, extremes and compensation rules.
• Learn graphs by redrawing them, with a sentence for each.

What to do this week

1. Write your anchor page. List the dozen or so numbers you want to keep, and check each against your textbook. Stick it where you'll see it.
2. Redraw four graphs from memory (oxygen dissociation, ventricular pressure-volume loop, flow-volume loops, cardiac action potential), and write one sentence under each.
3. For each graph, invent three "what if" changes and say what happens to the shape. Check against your book.
4. Do one short set of physiology questions that include figures, and for each miss, write which of the four moves you skipped.
5. Review the anchor page and the sentences on three different days, not all in one sitting.