8.3 The Heart

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the external and internal structure of the mammalian heart.
  2. Explain the differences in wall thickness between the atria and ventricles, and between the left and right ventricles.
  3. Describe the cardiac cycle, with reference to pressure changes during systole and diastole and how these changes open and close valves.
  4. Explain the roles of the sinoatrial node (SAN), the atrioventricular node (AVN), and the Purkyne tissue in the cardiac cycle.

1. Structure of the Mammalian Heart

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External Structure

The heart is a muscular organ roughly the size of a fist. From the outside, you can identify:

  • Four large blood vessels connecting to the heart — the aorta, the pulmonary artery, the vena cava (actually two: superior and inferior), and the pulmonary veins.
  • The apex — the pointed, bottom tip of the heart.
  • Coronary vessels — small blood vessels running across the outer surface of the heart that supply the heart muscle itself with oxygen and nutrients.
  • The left and right atria form the upper, smaller portions of the heart visible from outside.

Internal Structure

Cutting the heart open reveals the following:

Four Chambers

The heart has four hollow spaces (chambers) inside it:

ChamberLocationRole
Right atriumUpper rightReceives deoxygenated blood from the body via the vena cava
Left atriumUpper leftReceives oxygenated blood from the lungs via the pulmonary veins
Right ventricleLower rightPumps deoxygenated blood to the lungs via the pulmonary artery
Left ventricleLower leftPumps oxygenated blood to the whole body via the aorta

The Septum

The median septum is a thick muscular wall running down the middle of the heart. It completely separates the left side from the right side, preventing oxygenated and deoxygenated blood from mixing.

Blood Vessels Entering the Heart

  • Superior vena cava — brings deoxygenated blood from the upper body.
  • Inferior vena cava — brings deoxygenated blood from the lower body.
  • Pulmonary veins — bring oxygenated blood from the lungs into the left atrium.

Blood Vessels Leaving the Heart

  • Pulmonary artery — carries deoxygenated blood from the right ventricle to the lungs.
  • Aorta — carries oxygenated blood from the left ventricle to the rest of the body.

The Four Valves

Valves are like one-way doors — they make sure blood only flows in one direction.

Atrioventricular (AV) valves sit between the atria and ventricles:

  • Tricuspid valve — right side; has three flaps (cusps).
  • Bicuspid valve (also called the mitral valve) — left side; has two flaps (cusps).

These valves prevent blood from flowing backwards from the ventricles into the atria.

Semilunar valves sit at the exits of the ventricles:

  • Pulmonary semilunar valve — between the right ventricle and the pulmonary artery.
  • Aortic semilunar valve — between the left ventricle and the aorta.

These valves prevent blood from flowing backwards from the arteries into the ventricles.

Chordae Tendineae and Papillary Muscles

The AV valves are attached to tough, fibrous strings called the chordae tendineae (think of them as "heartstrings"). These strings are anchored to small muscles inside the ventricle walls called papillary muscles.

  • When the ventricles contract forcefully, the papillary muscles contract and pull the chordae tendineae tight. This stops the AV valve flaps from being pushed inside-out (prolapsing) back into the atria.

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