Control of Heart Activity and Disorders of Circulation

Who sets the heartbeat, and who adjusts its speed?

The human heart can initiate its own electrical activity because specialised nodal cardiac muscle is autoexcitable. Its normal rhythm begins at the SA node, not at the spinal cord or a continuously transmitted command from the brain. This makes the human heart myogenic. However, automaticity does not mean the heart is unaffected by the nervous system. Neural and hormonal signals alter how frequently impulses occur and how strongly the heart pumps.

Neural control from the medulla

Cardiovascular centres in the medulla oblongata of the brainstem adjust cardiac activity through the autonomic nervous system. Sympathetic input generally accelerates SA-node firing, increases the force of ventricular contractions, and can increase cardiac output. This helps the circulation respond to situations such as vigorous exercise, when tissues require a greater delivery of oxygenated blood.

Parasympathetic signals, carried importantly through the vagus nerves, generally decrease heart rate and slow AV nodal conduction. Their dominant resting effect is on rhythm and conduction, whereas sympathetic activity has a strong influence on both rhythm and ventricular contractility. These are physiological controls, not two separate pacemakers. The heart still contains the cells generating its own electrical impulses.

Hormones from the adrenal medulla, especially adrenaline and noradrenaline, can also increase heart rate and the force of contraction through receptor-mediated effects on cardiac cells. This hormonal pathway complements rapid neural regulation. Because cardiac output equals heart rate multiplied by stroke volume, either a change in rhythm or in the amount ejected per beat can alter total blood flow per minute.

What does a blood-pressure measurement show?

Blood pressure represents the force exerted by blood against vessel walls. A typical reading is given as systolic over diastolic pressure, such as 120/80 mm Hg. The first value reflects arterial pressure during ventricular systole, and the second reflects pressure during ventricular relaxation. These values do not measure the volume of blood in an artery. A reading must be interpreted in the context of repeated measurements, technique and individual circumstances.

The chapter uses repeated readings of 140/90 mm Hg or higher as an illustrative threshold for hypertension. Modern clinical thresholds and treatment decisions may differ by guideline and patient context; a single home reading does not establish a diagnosis. Sustained high arterial pressure increases the workload on the heart and is associated with damage to blood vessels and organs including brain and kidneys.

Coronary artery disease and angina

Coronary arteries supply the myocardium. In coronary artery disease, atherosclerotic plaques containing lipids, inflammatory and fibrous tissue narrow these vessels or sometimes contribute to abrupt blockage. Calcium can also accumulate within plaques. Narrowing may reduce blood flow sufficiently that heart muscle does not receive enough oxygen, especially when its workload rises.

Angina pectoris describes chest discomfort resulting from transient myocardial ischaemia: insufficient blood flow relative to the muscle's oxygen needs. Angina is a symptom or clinical syndrome, not simply another name for any chest pain and not identical to heart failure. New, severe or persistent chest pain requires urgent medical assessment because conditions other than stable angina, including heart attack, can be dangerous.

Heart failure is not the same as a heart attack or cardiac arrest

Heart failure means the heart cannot pump, or sometimes fill, sufficiently well to meet the body's needs without abnormal pressures. Blood and fluid may back up in the lungs or elsewhere, producing breathlessness, fatigue or swelling. The term congestive heart failure refers to fluid congestion as an important manifestation, not to the heart having stopped entirely.

A heart attack, or myocardial infarction, occurs when part of the myocardium is injured or dies because its blood supply has been severely reduced or interrupted, often by a coronary artery blockage. Cardiac arrest means effective circulation stops because the heart no longer pumps blood adequately, commonly due to a dangerous rhythm disturbance. A heart attack can trigger cardiac arrest or contribute to later heart failure, but the three labels describe different events.

Connect disease mechanisms to normal physiology

The normal coronary supply must deliver oxygen to the continuously active cardiac muscle. When coronary arteries narrow, that supply can become inadequate and lead to angina or injury. The heart's muscular pumping depends on ventricular contraction and a coordinated conduction system; damage to these can impair cardiac output. Elevated blood pressure adds resistance to systemic ejection and, over time, may burden the heart and blood vessels.

Understanding healthy circulation helps distinguish the disorders causally rather than memorising similar-sounding names. High pressure describes a measurement and chronic vascular load, coronary disease describes impaired vessels, angina describes ischaemic discomfort, and failure describes inadequate overall heart performance.

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