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🫀 Science · Human Anatomy and Physiology · Lesson 7 of 12 · 6 min

Cardiovascular system: heart, vessels and blood

🫀 The TEAS loves one question above all in this system: trace a drop of blood through the heart. Learn the loop once and you can answer it every time.

💡 Key idea

Body → right atrium → tricuspid → right ventricle → pulmonary valve → lungs → left atrium → mitral → left ventricle → aortic valve → aorta.

🧠 The idea

The heart has four chambers. The right ATRIUM receives oxygen-poor blood from the body through the venae cavae; it passes the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary arteries to the lungs. Oxygen-rich blood returns by the pulmonary veins to the left atrium, crosses the mitral (bicuspid) valve into the left VENTRICLE, and leaves through the aortic valve into the aorta. The CONDUCTION system starts at the SA node (the pacemaker), spreads to the AV node, then the bundle of His, bundle branches and Purkinje fibers. Arteries carry blood away from the heart, veins carry it toward the heart, and capillaries are the thin exchange vessels between them. Blood is plasma plus red blood cells (carry oxygen on hemoglobin), white blood cells (defense) and platelets (clotting).

✏️ Worked example

A question asks which vessel carries oxygen-rich blood to the left atrium. Step 1: blood leaving the lungs has just picked up oxygen. Step 2: vessels returning to the heart are veins, whatever their oxygen content. Step 3: so the answer is the pulmonary veins, the only veins in the adult that carry oxygen-rich blood. The pulmonary arteries are the reverse exception: arteries carrying oxygen-poor blood.

📌 Takeaways

  • Right side pumps to the lungs; left side pumps to the body, so the left ventricle wall is thickest.
  • Artery = away from the heart, vein = toward it, regardless of oxygen level.
  • Electrical path: SA node → AV node → bundle of His → bundle branches → Purkinje fibers.

✅ Check yourself

Question 1 of 3

Which valve does blood pass through as it moves from the left atrium to the left ventricle?

  1. Tricuspid valve
  2. Pulmonary valve
  3. Mitral valve
  4. Aortic valve
Show the answer and rationale

Answer: C

The mitral (bicuspid) valve sits between the left atrium and left ventricle. The tricuspid valve is the matching valve on the right side, and the pulmonary and aortic valves guard the exits of the ventricles.

Question 2 of 3

Which structure normally starts each heartbeat?

  1. Bundle of His
  2. Purkinje fibers
  3. Atrioventricular node
  4. Sinoatrial node
Show the answer and rationale

Answer: D

The sinoatrial (SA) node in the right atrium fires first and sets the rate, which is why it is called the pacemaker. The AV node delays the signal, and the bundle of His and Purkinje fibers carry it through the ventricles afterward.

Question 3 of 3

Which blood component is mainly responsible for forming a clot at a cut?

  1. Neutrophils
  2. Plasma albumin
  3. Erythrocytes
  4. Platelets
Show the answer and rationale

Answer: D

Platelets (thrombocytes) stick to the injured vessel and help build the clot. Erythrocytes carry oxygen, neutrophils fight bacteria, and albumin mainly holds fluid in the vessels by osmotic pressure.

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