Cardiac Physiology

What are the positives of exercise?

• promotes general metabolic wellness

• improves mental health

• builds and preserves musculoskeletal function

• increases lifespan

What counts as physical activity?

Moderate activity - activity that leads to increase in breathing rate

Vigorous activity - activity leading to being ‘out of breath or sweating’

1 min vigorous activity = 2 min moderate activity

What does acute exercise do?

activates the sympathetic nervous system and is therefore a stressor (healthy stress)

What is the process of acute exercise?

1. SNS (adrenaline)

2. Metabolic and CV homeostasis

3. Heat/fluid regulation

What is chronic exercise?

Overtraining.

causes adaptation in key peripheral organs involved in the regulation of energy homeostasis and associated whole-body metabolic effects and systemic health effects.

What is eccentric?

increase in cardiac mass where RWT is maintained resulting in coordinated increase in chamber size and wall thickness

What is concentric?

increase in cardiac mass accompanied by a disproportionate increase in wall thickness relative to chamber size.

What is concentric cardiac growth?

correlated with worse clinical outcomes when coupled to cardiac disease.

How does exercise affect heart rate regulation?

The heart has intrinsic rhythmicity, but extrinsic factors like nerves and blood chemicals rapidly adjust HR. Anticipation of exercise and exercise itself can raise HR up to ~220 bpm.

How is CHD diagnosed?

• electrocardiogram

• stress test

• nuclear scanning

• coronary angiography

How is coronary angiography conducted?

• catheter inserted in leg and guided up the aorta

• catheter tip stops at left coronary artery

• contrast agent injected into arteries

• X ray imagining shows stenosis in left coronary artery

What is the stress response via the sympatheic adrenomedullary axis?

The adrenal medulla releases adrenaline, a fast-acting hormone that increases heart rate, blood pressure, and breathing rate.

This rapid, short-lived "fight or flight" response prepares the body to handle immediate threats, lasting seconds to minutes.

What is the stress response via the hypothalamic pituitary adrenal-cortical (HPA) axis?

1. Hypothalamus releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to secrete adrenocorticotropin hormone (ACTH).

2. ACTH then stimulates the adrenal glands to produce and release cortisol.

3. Cortisol increases glucose release, suppresses immune system, and regulates the stress response itself by inhibiting the release of CRH and ACTH (negative feedback loop).

What is alarm, resistance, exhaustion?

• During alarm phase, there is physiological arousal and sympathetic nervous system activation.

• The resistance phase recruits the hypothalamic-pituitary-adrenal
  (HPA) axis.

• Exhaustion phase is associated with reduced metabolic and immune functioning.

What is the effect of stress on the heart?

• increased HR

• increased BP

• Release of catecholamines (adrenaline, cortisol) from the adrenal glands

• Increased O2 demand on the body (temporarily higher metabolic rate)

• Lower threshold for abnormal heart rhythms including ventricular

  tachycardia, ventricular fibrillation, and atrial fibrillation. Electrical

  instability in the heart makes it easier for these abnormal heart

  rhythms to occur.

• Spasm of coronary (heart) blood vessels, leading to ischemia (inadequate blood flow to the heart)

What is myocardial infarction?

• complete obstruction of blood flow to coronary artery

• death of an area of tissue because of lack of blood supply

What is cardiac arrest?

• cessation of effective heartbeat

• could stop completely or ventricular fibrillation

How are cardiac action potentials created?

• Phase 0: Rapid depolarization from Na⁺ influx via voltage-gated Na⁺ channels.

• Phase 1: Early repolarization due to transient outward K⁺ current (I_to).

• Phase 2 (Plateau): Balance between inward Ca²⁺ (I_CaL) and outward K⁺ (delayed rectifier I_K) currents.

• Phase 3: Rapid repolarization as Ca²⁺ channels inactivate and K⁺ efflux increases.

• Phase 4: Resting potential maintained by inward rectifier K⁺ current (I_K1).

What happens during ventricular contraction in the cardiac cycle?

It includes the systolic interval, when blood is pumped out of the heart

What is Starling’s Law of the Heart?

It means that the heart pumps out more blood when more blood returns to it (venous return matches cardiac output).

What determines arterial blood pressure?

Cardiac output × Systemic vascular resistance.

How is hypertension diagnosed?

when arterial blood pressure is above the normal range.

Which wave in an ECG represents ventricular depolarisation?

The QRS complex.

What happens during the isovolumic relaxation phase of the heart?

It follows the T wave and all heart valves are closed

What causes the first heart sound?

Closure of the mitral valve.

How is arterial blood pressure sensed?

By baroreceptors.

What determines cardiac output?

Heart rate × Stroke volume.

What is true about the baroreceptor reflex?

It’s a negative feedback loop that helps regulate blood pressure.

Why does heart rate increase during exercise?

Due to increased sympathetic activity and adrenaline.

What happens immediately when you stand up?

Heart rate increases and blood pressure drops briefly.

Which ECG component shows atrial depolarisation?

The P-wave.

What is diastolic blood pressure?

The minimum pressure recorded in the arteries.

What is the correct sequence of electrical conduction in the heart?

SA node → Atria → AV node → His Bundle → Purkinje fibres → Ventricles.

What does the P wave represent on an ECG?

Atrial depolarization – the electrical signal spreading through the atria, causing them to contract.

What is the PR interval and what does it indicate?

is the time from the start of the P wave to the start of the QRS complex.
It represents the delay at the AV node, allowing the ventricles time to fill before they contract.

What does the QRS complex represent on an ECG?

the rapid electrical activation of the ventricles, leading to their contraction.
(Note: Atrial repolarization occurs at this time too but is masked by the QRS.)

What is the ST segment?

the flat line between the QRS complex and the T wave.
It represents the period when ventricles are depolarized.

What does the T wave represent on an ECG?

Ventricular repolarization – the process of the ventricles resetting electrically for the next cycle.

What does the QT interval represent?

measures the time from the start of the QRS complex to the end of the T wave.
It reflects the total time for ventricular depolarization and repolarization.