Mammalian Heart Structure and Cardiac Cycle

The mammalian heart is a remarkable pump: around the size of a clenched fist, it beats approximately 100,000 times per day without rest, shifting about 7,000 dm³ of blood. It consists of four chambers that work in synchrony to maintain two parallel circulations — pulmonary and systemic — at different pressures. This lesson examines the external and internal anatomy of the heart, the roles of its valves and blood vessels, and explores the cardiac cycle: the sequence of events in a single heartbeat. Content matches OCR A-Level Biology A specification 3.1.2 (e)–(g).

Key Definitions:

• Cardiac cycle — the sequence of events making up one complete heartbeat, lasting about 0.8 seconds at rest in humans.
• Systole — phase during which a chamber of the heart contracts.
• Diastole — phase during which a chamber relaxes.
• Stroke volume — the volume of blood ejected from the left ventricle during one contraction (~70 cm³ at rest).
• Cardiac output — stroke volume × heart rate; typically ~5 dm³ min⁻¹ at rest.

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External Anatomy

The heart lies between the lungs in the mediastinum, slightly tilted to the left. It is enclosed in a tough, fibrous sac — the pericardium — which is separated from the outer surface of the heart (the epicardium) by a thin film of pericardial fluid that lubricates movement. The heart's own muscular wall is the myocardium, composed of cardiac muscle (myocytes). Its inner lining is the endocardium, a smooth squamous layer continuous with the endothelium of the blood vessels.

The myocardium itself requires its own blood supply; this is provided by the coronary arteries, which branch off the aorta immediately above the aortic valve. Blockage of a coronary artery causes myocardial infarction — heart attack — because the cardiac muscle downstream is starved of oxygen.

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Internal Anatomy and Blood Flow

The mammalian heart has four chambers:

• Right atrium — receives deoxygenated blood from the body via the superior and inferior venae cavae.
• Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery.
• Left atrium — receives oxygenated blood from the lungs via the pulmonary veins (four of them).
• Left ventricle — pumps oxygenated blood to the whole body via the aorta.

The left ventricle has a much thicker muscular wall than the right ventricle, because it must generate enough pressure to push blood through the entire systemic circulation. The right ventricle only needs to push blood a short distance through the pulmonary circulation and at much lower pressure — high pressure would damage the delicate pulmonary capillaries.

The left and right sides are completely separated by the septum, preventing mixing of oxygenated and deoxygenated blood (this is essential for an efficient double circulation).

flowchart TB
    VC[Venae cavae] --> RA[Right atrium]
    RA -->|Tricuspid valve| RV[Right ventricle]
    RV -->|Pulmonary valve| PA[Pulmonary artery]
    PA --> LUNGS[Lungs]
    LUNGS --> PV[Pulmonary veins]
    PV --> LA[Left atrium]
    LA -->|Bicuspid / mitral valve| LV[Left ventricle]
    LV -->|Aortic valve| AO[Aorta]
    AO --> BODY[Body tissues]
    BODY --> VC

The Valves

Valves prevent backflow, ensuring one-way movement of blood:

• Atrioventricular valves (AV valves) — between atria and ventricles.
  • Right: tricuspid (three cusps).
  • Left: bicuspid / mitral (two cusps).
• Semilunar valves — at the bases of the pulmonary artery and aorta, each with three pocket-shaped cusps.
• Chordae tendineae — tough tendon-like cords that tether the AV valve cusps to papillary muscles on the ventricle walls, preventing the valves from being everted into the atria during ventricular systole.

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The Cardiac Cycle

At a resting heart rate of 75 beats per minute, each cardiac cycle lasts 60/75 ≈ 0.8 s. It is conventionally divided into three phases:

1. Atrial Systole (~0.1 s)

• Both atria contract simultaneously.
• Pressure in the atria rises above that in the ventricles.
• The AV valves are already open, and the contracting atria push the remaining ~20% of blood into the ventricles.
• The ventricles are now filled completely.
• The semilunar valves are closed.