wk5: cardiac physiology

These flashcards cover key concepts related to the heart's conduction system and cardiac physiology, aiding in understanding of how the heart functions.

To pump blood around the body, the heart alternates between

Diastole = relaxation (allows heart to fill) and systole = contraction (allows heart to pump blood), both stimulated by electrical signals.

Pacemaker Cells

Specialised cardiac muscles (~1%) that generate their own electrical signals and can initiate a heartbeat ~80-100bpm

functions of cardiac muscles via electrical signals

self-excitable and autorhythmic, external inputs are not required but do modulate heart activity

Sinoatrial Node (SA Node)

Located on the wall of the right atria below the superior vena cava. SA node initiates impulses that travel to the left atria via the interatrial bundle (stimulates atrial contraction)

Atrioventricular Node (AV Node)

Located junction between the atria and ventricles that slightly delays the action potential to ensure proper blood flow → signal travels along AV bundle → separates into left and right bundle branches

Interatrial Bundle (Bahcman's Bundle)

Group of fibers that conduct the action potential between the two atria.

AV Bundle (Bundle of His)

A pathway that conducts action potentials from the AV node to the ventricles through the right and left bundle branches.

Purkinje Fibers

Small conducting fibres that extend from the bundle branches to the ventricular muscle (apex of heart), extending upwards → stimulate ventricle contraction

Heart conduction pathway

Sinoatrial (SA) node (pacemaker) → via interatrial bundle and fibers → LA, RA and AV node → AV bundle → left and right bundle branches → Purkinje fibers → LV and RV muscle and contraction

full cardiac cycle

atrial systole → early ventricular systole → late ventricular systole → early ventricular diastole → late ventricular diastole

cardiac cycle phase 1 (atrial systole)

veins bring blood to fill atrium → atria contacts → intra-atrial pressure increases → AV valves open → atrial contraction pushes blood into ventricles

cardiac cycle phase 2 (early ventricular systole)

atria relaced (diastole) → ventricles begin contraction (systole) → pressure increased and pushes blood against AV valves (valves close) → isovolumetric contraction occurs (no valves open)

cardiac cycle phase 3 (late ventricular systole)

increase in pressure in ventricles forces semilunar valves open → blood pushed out of ventricles → blood moves into arteries (aorta and pulmonary artery)

cardiac cycle phase 4 (early ventricular diastole)

ventricles relax → pressure decreases on aorta → blood filling arteries put pressure on semilunar valves (valves close) → ventricular pressure still greater than atrial (AV valves remain closed) → isovolumetric relation

cardiac cycle phase 5 (late ventricular diastole)

atria and ventricles diastole → blood flows from high pressure to low (systemic vena cava and pulmonary veins into atrium) → atria fills with blood → ventricular pressure drops below atria → AV valves open → passive filling of ventricles ~70%)

Definition of cardiac output

The volume of blood leaving the heart per minute (ejected from one ventricle), influenced by heart rate and stroke volume. (Q = HR X SV).

Heart rate (resting ~60-100bpm)

number of cardiac cycles (heart beats) per minute measured in bpm (regulated by nervous and endocrine systems)

stroke volume (~70mL per beat)

average volume of blood pumped out of the heart per contraction, influenced by heart size, contractility, preload (↑ volume = ↑ stretch = ↑ SV), and after load (↑ pressure = ↑ resistance = ↓ SV).

Intrinsic pathways of cardiovascular system (autoregulation)

↑ or ↓ blood flow (vasodilation and vasoconstriction of vessels) to tissues to protect capillaries (detected by metabolic and myogenic changes)

Intrinsic autoregulation (metabolic changes)

stimulus: metabolic activity (chemical changes) → receptor: sensors in endothelial cells of blood vessels detect changes → effector: endothelial cells release vasodilators → outcome: increased blood flow (homeostasis restored)

Intrinsic autoregulation (myogenic changes)

stimulus: pressure on arterial walls → receptor: mechanoreceptors in smooth muscle cells detect change → effector: smooth muscle cells relax/contract → outcome: vasocontriction or vasodilation (homeostasis restored)

Extrinsic pathway of cardiovascular system (neural regulation) definition

Neural (short-term) regulation of vascular function is controlled by the cardiovascular centre in the medulla oblongata (brainstem). This centre responds to changes in blood pressure (baroreceptors) and in chemistry (chemoreceptors).

Cardiovascular centre contains 3 regions

Cardioacceleratory centre (stimulates HR and SV), cardioinhibitory centre (slows HR and SV), and vasomotor centre (controls blood vessel smooth muscle).

Chemoreceptors

located peripheral (aortic body and carotid bodies)

Baroreceptor (mechanoreceptor)

located aortic arch sinus and carotid artery sinus

Extrinsic cardiovascular system communication (baroreceptors)

↑ or ↓ in aortic sinus/carotid sinus stretch → baroreceptors detect change → cardioinhibitory or cardioacceleratory centre stimulated → vasomotor centre inhibited or stimulated → decrease or increase in sympathetic stimulation (homeostasis restored)

Extrinsic cardiovascular endocrine regulation

↑ or ↓ in cardiac activity resulting from (short-term adrenaline or noradrenaline) or (long-term hormones from kidneys, erythropoietin or renin)

Renin-angiotensin system (RAS)

↓ in blood pressure or volume → renin release from kidneys → conversion of angiotensinogen to angiotensin I → conversion to angiotensin I to II via angiotensin-converting enzyme release from lungs → vasoconstriction and increased blood volume