The Heart and Cardiac Activity (OCR A-Level Biology A): Revision Notes

The Heart and Cardiac Activity

Introduction to the mammalian heart

The heart functions as a muscular pump that drives blood circulation throughout the body. Cardiac muscle contracts rhythmically, with the rate of contraction adjustable according to physiological demands. Despite being filled with blood, the heart wall is so thick that it requires its own dedicated blood supply — the coronary circulation — to deliver oxygen and nutrients to the cardiac tissue itself.

External structure

The aorta emerges from the heart and divides into three main branches. It transports oxygenated blood under high pressure to body tissues. The left ventricle generates this pressure as it contracts and pumps blood into the aorta.

The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for gas exchange. The coronary artery branches across the heart's surface, supplying the cardiac muscle with oxygenated blood. Blood returns from the heart muscle via the coronary vein. Fatty deposits may accumulate on the heart surface, sometimes obscuring these coronary vessels.

Internal structure of the heart

The mammalian heart contains four chambers: two atria (upper chambers) and two ventricles (lower chambers). The heart is divided into right and left halves by the septum, a muscular wall that prevents mixing of oxygenated and deoxygenated blood.

Blood flow pathway

Wall thickness and function

The thickness of each chamber's muscular wall directly relates to its function:

• Left ventricle: Contains the thickest muscular wall because it must generate sufficient pressure to pump blood throughout the entire systemic circulation
• Right ventricle: Has a thinner wall as it only needs to pump blood through the pulmonary circuit to the nearby lungs. The pulmonary circulation offers less resistance because the lungs are spongy, air-filled organs with fewer arterioles and a shorter distance to travel
• Atria: Possess the thinnest walls since they only need to move blood into the adjacent ventricles

Valve systems

Two sets of valves maintain unidirectional blood flow through the heart:

Atrioventricular valves sit between the atria and ventricles. When the ventricles contract, these valves are held in place by valve tendons (chordae tendineae) that attach to the ventricular walls. These tendons prevent the valve flaps from inverting under the considerable pressure generated during ventricular contraction.

Semilunar valves are located at the base of the major arteries leaving the heart (the pulmonary and aortic valves). They prevent backflow of blood into the ventricles after contraction.

The cardiac cycle

The cardiac cycle describes the sequence of events during one complete heartbeat. The cycle consists of alternating periods of contraction and relaxation. Systole refers to the contraction phase, while diastole describes the relaxation phase.

Pressure changes during the cardiac cycle

Muscular contraction compresses the blood within the heart chambers, increasing blood pressure. By monitoring pressure changes in different heart regions, we can track the cardiac cycle events.

The following sequence describes one cardiac cycle for the left side of the heart (similar events occur on the right):

1. Atrial contraction The atrium contracts, raising pressure slightly due to its relatively thin muscular wall. Blood flows into the ventricle, causing a small pressure increase there.

2. Atrioventricular valve closure As the ventricle begins contracting, ventricular pressure exceeds atrial pressure. This pressure difference forces the atrioventricular valve shut. The atrium relaxes.

3. Ventricular contraction The thick ventricular wall generates substantial pressure as it contracts. The closed atrioventricular valve bulges slightly into the atrium, causing a small rise in atrial pressure.

4. Semilunar valve opening When ventricular pressure surpasses aortic pressure, the semilunar valves open. Blood flows rapidly into the aorta, increasing aortic pressure.

5. Ventricular relaxation begins As the ventricle relaxes, pressure within it decreases.

6. Semilunar valve closure When ventricular pressure drops below aortic pressure, the semilunar valves close, preventing backflow.

7. Atrioventricular valve opening Blood has been accumulating in the atrium, gradually raising atrial pressure. When ventricular pressure falls below atrial pressure, the atrioventricular valve opens and blood flows into the ventricle.

8. Aortic elastic recoil When blood enters the aorta, the elastic arterial walls stretch. As blood flow slows, elastic tissue passively recoils, maintaining blood pressure and flow even when both heart chambers are relaxed. This is passive elastic recoil, not active contraction — the aorta does not "pump" blood.

Coordination of the heartbeat

Myogenic control

The heart is myogenic, meaning cardiac muscle can initiate its own contractions without requiring external nervous stimulation. The contraction stimulus originates entirely within the heart tissue itself. However, the autonomic nervous system can modify heart rate to match physiological demands. This system controls involuntary functions such as heart rate, blood vessel diameter, and digestion.

The autonomic nervous system has two divisions:

• Parasympathetic nerves: Decrease heart rate
• Sympathetic nerves: Increase heart rate

The cardiac conduction pathway

Cardiac contraction follows a precise sequence coordinated by specialised conducting tissue:

Sino-atrial (SA) node Located in the upper wall of the right atrium, the SA node acts as the heart's natural pacemaker. This specialised tissue generates electrical impulses that trigger a wave of contraction across both atria.

Atrio-ventricular (AV) node Non-conductive tissue separates the atria and ventricles, preventing direct spread of electrical impulses. The AV node provides the only electrical connection between atria and ventricles. It delays the impulse briefly, allowing the atria to complete their contraction before ventricular contraction begins.

Bundle of His After the delay, the AV node transmits the impulse down specialised muscle fibres called the Bundle of His. These fibres conduct the signal rapidly to the bottom of the ventricles.

Purkinje fibres From the base of the ventricles, the impulse travels upward through another set of specialised fibres — the Purkinje fibres (also spelled Purkyne fibres). As the impulse moves up the ventricular walls, it triggers contraction from bottom to top. This ensures blood is efficiently forced upward and out through the arteries at the top of the heart.

Detecting heart activity

Electrocardiography

Since electrical activity in cardiac muscle drives the heartbeat, this electrical signal can be detected and recorded. Electrocardiography (ECG) involves placing electrodes on the skin to detect electrical signals and produce an electrocardiogram (also abbreviated to ECG).

Normal ECG pattern

A healthy heart produces a characteristic ECG waveform:

The ECG trace shows several distinctive waves:

• P wave: Results from depolarisation of the atria, causing atrial contraction
• QRS complex: Represents ventricular depolarisation and contraction. The larger amplitude reflects the greater muscle mass of the ventricles
• T wave: Caused by ventricular repolarisation, resulting in ventricular relaxation
• U wave: Origin uncertain

Heart problems and ECGs

ECG analysis can identify several cardiac rhythm abnormalities:

Tachycardia The heart beats excessively rapidly, with a resting heart rate exceeding $100 bpm (beats per minute). On an ECG, the peaks appear too close together.

Bradycardia The heart beats too slowly, with a rate below $60 bpm. ECG peaks are too widely spaced.

Ectopic heartbeat The heart contracts prematurely, followed by a compensatory pause. This common condition usually requires no treatment.

Fibrillation The heartbeat becomes irregular and loses its coordinated rhythm. Severe fibrillation represents a life-threatening emergency that can result in death.

Summary