October 2026 - The Cardiovascular System Study Guide

The path of blood through the heart

This single sequence is the most heavily tested thing in the topic. Blood arrives from the body deoxygenated, is sent to the lungs, comes back oxygenated, and is then pushed out to the body again — and it passes a valve on the way out of every chamber.

1. Body
2. Vena cava
3. Right atrium
4. Tricuspid valve
5. Right ventricle
6. Pulmonary valve
7. Pulmonary artery
8. Lungs
9. Pulmonary vein
10. Left atrium
11. Mitral valve
12. Left ventricle
13. Aortic valve
14. Aorta
15. Body

Two habits make this reliable under time. First, learn it as a loop you can start anywhere rather than a list you recite from the beginning, because questions ask what comes next, not what comes first. Second, notice the alternation: chamber, valve, chamber, valve. Atria receive, ventricles pump, and a valve sits at every exit to stop backflow.

The two atrioventricular valves — tricuspid on the right, mitral on the left — separate each atrium from the ventricle below it. The two semilunar valves — pulmonary and aortic — guard the exits from the ventricles into the great arteries.

The two circuits

Everything the heart does is one of two loops, and each loop is driven by one side of the heart.

• Pulmonary circulation — the right side of the heart to the lungs and back. It carries oxygen-poor blood out and returns oxygen-rich blood.
• Systemic circulation — the left side of the heart to the rest of the body and back. It carries oxygen-rich blood out and returns oxygen-poor blood.
• Why the left ventricle is the thickest chamber — it has to generate enough pressure to push blood through the entire body, while the right ventricle only has to reach the lungs next door. Thickness follows workload.

Blood vessels

Three vessel types, distinguished by direction of flow and by wall structure.

What blood is made of

• Red blood cells (erythrocytes) — carry oxygen bound to haemoglobin, an iron-containing protein. Too little iron means too little functional haemoglobin, which is why iron-deficiency anaemia presents as fatigue and breathlessness.
• White blood cells (leukocytes) — the immune cells. They rise in number during infection.
• Platelets (thrombocytes) — cell fragments that begin clotting at a site of injury.
• Plasma — the straw-coloured liquid, mostly water, that carries the cells along with hormones, proteins, nutrients and waste.

Blood pressure and the heartbeat

A blood pressure reading is written as systolic over diastolic — for example 118/76 mmHg. Systolic, the larger number, is the pressure while the ventricles contract. Diastolic, the smaller number, is the pressure while they relax and refill. The larger number is always on top.

The beat itself is electrical. The sinoatrial (SA) node in the right atrium fires on its own and sets the rate, which is why it is called the pacemaker. The signal spreads across the atria to the atrioventricular (AV) node, which delays it just long enough for the atria to finish emptying before the ventricles contract.

Reading a question that hides the sequence

Terms to know

Systole

The contraction phase, when a chamber squeezes blood out. The top number of a blood pressure reading.

Diastole

The relaxation phase, when a chamber refills. The bottom number of a blood pressure reading.

Atrium

An upper, receiving chamber. Blood arrives here from a vein.

Ventricle

A lower, pumping chamber. Blood leaves here through an artery.

SA node

The sinoatrial node, in the right atrium. It generates the heartbeat and sets the rate — the natural pacemaker.

Haemoglobin

The iron-containing protein in red blood cells that oxygen binds to.

Septum

The muscular wall dividing the right side of the heart from the left, keeping the two circuits separate.

Pulse

The wave of pressure felt in an artery each time the left ventricle contracts.

What the HESI asks most

The same core ideas appear in different wording. If you can answer these without stopping to think, you have what this section requires.

Key points

• Blood makes two loops through one heart: right side to the lungs, left side to the body.
• Chamber, valve, chamber, valve — a valve sits at the exit of every chamber to prevent backflow.
• Arteries carry blood away from the heart, veins carry it back. The pulmonary vessels are the oxygen exceptions.
• The left ventricle is the thickest chamber because it pumps against the whole body.
• Systolic over diastolic: contraction over relaxation, larger number on top.
• Capillaries are one cell thick, which is what makes exchange with tissue possible.

Review quiz

5 questions on this topic, each with the reasoning worked through. Pick an answer to see how it went, or open the explanation straight away.

1. Question 1 of 5

   Blood leaving the right ventricle passes through the pulmonary valve. Where does it travel next?

   Answer choices for question 1

   A The aorta B The pulmonary artery C The left atrium D The vena cava
   Show the answer and explanation

   Correct answer: B. The pulmonary artery

   The pulmonary valve opens into the pulmonary artery, which carries oxygen-poor blood to the lungs. The aorta is the exit from the left ventricle, not the right; the left atrium is where blood arrives after the lungs; and the vena cava delivers blood into the right atrium at the start of the loop.

2. Question 2 of 5

   Which vessel carries oxygen-poor blood away from the heart?

   Answer choices for question 2

   A The aorta B The pulmonary vein C The pulmonary artery D The carotid artery
   Show the answer and explanation

   Correct answer: C. The pulmonary artery

   The pulmonary artery carries oxygen-poor blood from the right ventricle to the lungs. This is the trap the question is built on: artery means away from the heart, not oxygen-rich. The pulmonary vein is the mirror-image exception — a vein carrying oxygen-rich blood back to the left atrium.

3. Question 3 of 5

   A patient has a blood pressure of 132/84 mmHg. What does the 84 represent?

   Answer choices for question 3

   A Arterial pressure while the ventricles are relaxing and refilling B Arterial pressure while the ventricles are contracting C The number of heartbeats per minute D The volume of blood ejected with each beat
   Show the answer and explanation

   Correct answer: A. Arterial pressure while the ventricles are relaxing and refilling

   The lower number is diastolic pressure, measured while the ventricles relax and fill. The 132 is systolic, measured during contraction. Heart rate and stroke volume are separate measurements that a blood pressure reading does not report.

4. Question 4 of 5

   A student with low dietary iron reports constant tiredness and breathlessness on stairs. Which cardiovascular component most directly explains the symptoms?

   Answer choices for question 4

   A Platelets, because clotting slows without iron B Plasma, because its volume falls without iron C White blood cells, because they cannot fight infection without iron D Red blood cells, because haemoglobin needs iron to carry oxygen
   Show the answer and explanation

   Correct answer: D. Red blood cells, because haemoglobin needs iron to carry oxygen

   Haemoglobin is an iron-containing protein, so too little iron means red blood cells carry less oxygen and tissues run short during exertion. Platelets handle clotting, plasma volume is governed by fluid balance rather than iron, and white blood cells are immune cells — none of the three would produce this pattern of symptoms.

5. Question 5 of 5

   Which feature of capillaries makes gas and nutrient exchange with body tissue possible?

   Answer choices for question 5

   A Thick, muscular walls that withstand high pressure B Walls only one cell thick C One-way valves that prevent backflow D A larger diameter than any other vessel
   Show the answer and explanation

   Correct answer: B. Walls only one cell thick

   Exchange happens by diffusion, and diffusion needs a very short distance to cross, so a wall one cell thick is the whole point of a capillary. Thick muscular walls describe arteries, one-way valves describe veins, and capillaries are in fact the narrowest vessels in the body, not the widest.

Put this HESI topic into practice

Build a set on the cardiovascular system and use the rationale on every question to reinforce what you just reviewed.

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