Cardiac control (7.8, 7.9)
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Cardiac muscle is myogenic, meaning that it contracts on its own without nervous stimulation. The heartbeat is initiated by electrical activity within the heart muscle itself.
Small changes in the electrical charge (polarity) of cardiac muscle cells spread as a wave of electrical excitation from cell to cell, causing them to contract in a coordinated sequence that produces the rhythmic heartbeat.
The coordination of a heartbeat is a multistep process:
- The Sinoatrial node (SAN) initiates an electrical impulse, causing a wave of depolarisation over the atria (atrial systole).
- The atrioventricular node (AVN) receives the impulse and delays its spread to allow the ventricles to fill with blood.
- The impulse travels down the bundle of His and reaches the apex of the ventricles.
- Purkyne fibres conduct the impulse triggering ventricular contraction (ventricular systole) from the apex upwards, forcing blood into the pulmonary artery and aorta.
An electrocardiogram (ECG) trace records the heart’s electrical activity.
Electrodes are placed on the skin to detect electrical signals generated by the heart.
ECGs help diagnose heart disorders by analysing the shape and timing of the waves.
Regular ECG monitoring helps track heart health and assess the effectiveness of treatments for cardiac problems.
There are several features associated with an ECG wave of a normal heartbeat:
- P wave – a small peak representing atrial depolarisation, leading to atrial systole.
- PR interval – the section between the P wave and QRS complex, representing the delay at the AVN as the impulse is held before passing on to the ventricles.
- QRS complex – a large, sharp peak representing ventricular depolarisation, leading to ventricular contraction (systole). It is the largest feature because the ventricles have greater muscle mass.
- T wave – a broad, gentle peak representing ventricular repolarisation, as the ventricles recover and relax (diastole).
ECG’s can be used to diagnose normal and abnormal heart activity. A normal ECG shows a regular pattern of waves at a resting rate of 60–100
- Tachycardia – resting heart rate above 100 The ECG shows normal wave patterns but they are closer together, with shorter intervals between beats.
- Bradycardia – resting heart rate below 60 The ECG shows normal wave patterns but they are further apart, with longer intervals between beats.
- Ectopic heartbeat (arrhythmia) – an extra or early beat caused by an electrical impulse originating somewhere other than the SAN. The ECG shows an additional or irregular wave disrupting the normal rhythm.
- Atrial fibrillation – the atria contract rapidly and irregularly. The ECG shows no first wave and an irregular pattern subsequently.
- Ventricular fibrillation – the ventricles contract rapidly and uncoordinatedly, so the heart cannot pump blood effectively. The ECG shows chaotic, irregular waves with no distinguishable structure. This is life-threatening and requires immediate defibrillation.
The cardiac control centre in the medulla oblongata of the brain and the autonomic nervous system control heart rate:
When heart rate needs to increase (e.g., during exercise), receptors detect increases in lactate levels, or body temperature. The cardiac control centre sends impulses through the sympathetic nerve (accelerator) to the Sinoatrial Node (SAN), increasing the heart rate.
When heart rate needs to decrease (e.g., at rest, when and lactate levels fall), the cardiac control centre sends impulses along the vagus nerve (parasympathetic; decelerator) to the SAN, slowing the heart rate.
During the fight or flight response, the adrenal glands release adrenaline into the blood.
Adrenaline travels to the heart and binds to receptors on the SAN, increasing the heart rate.
This ensures more blood is delivered to the muscles, providing the extra oxygen and glucose needed for increased respiration during periods of stress or physical activity.
Cardiac output is the volume of blood pumped per minute.
Where:
Stroke volume = the volume of blood pumped from the left ventricle in a single beat
Heart rate = the number of beats per minute
Units of cardiac output are volume per unit time (e.g. ).
Chemoreceptors detect changes in the of arterial blood, which reflects dissolved levels (more means lower ).
The chemoreceptors send impulses to the ventilation centre in the medulla oblongata, which responds by increasing the rate and depth of breathing to remove excess from the blood via the lungs. When levels fall and rises, the ventilation centre reduces breathing rate and depth accordingly.
During exercise, the body’s demand for oxygen increases and more is produced by respiring tissues.
Both ventilation rate and cardiac output increase in response, ensuring rapid delivery of oxygen to active tissues and efficient removal of
Increased cardiac output (heart rate × stroke volume) pumps more oxygenated blood per minute, while increased ventilation (breathing rate × tidal volume) ensures more gas exchange occurs at the lungs.