The Heart

Cardiovascular system

A sealed system that transports blood to respiring tissues and returns it to the heart.

main functions of the cardiovascular system

rapid nutrient transport, waste removal, hormonal control, temperature regulation and host immune defence

Myocardium

Thick muscle walls of the heart made up of cardiac muscle cells.

Endocardium

Inner surface of the walls of the heart in contact with the blood.

Epicardium

Inner lining of the pericardium and continuous with the covering of the heart itself.

Pericardium

Fluid-filled membranous sac that contains the heart.

Systole

Contractile phase of the cardiac cycle.

Diastole

Phase of relaxation and filling with blood in the cardiac cycle.

Gap junctions

Connections between cardiac muscle cells that allow the spread of electrical excitation between cells so the cardiac muscle can function as a whole tissue.

Conducting system

Specialised cells within the myocardium that are essential for heart excitation.

Depolarisation

Process in which the myocyte plasma membrane becomes less polarised

What happens as a result of depolarisation?

Sodium ion channels open → voltage change. Voltage gated calcium ion channels open. Calcium enters cell and is also released from the sarcoplasmic reticulum → rise in cytosolic calcium → contraction.

Sinoatrial node

Generates action potentials which travel the walls of the heart causing both atria to contract.

Atrioventricular node

Delays impulses from the sinoatrial node, allowing the atria to fully contract and empty their contents into the ventricles

Bundle of His and Purkinje fibers

Conduct impulses from the atrioventricular node to the ventricles, causing them to contract from the apex up

Intrinsic conduction system

Made up of fast and slow depolarising pacemaker cells.

Why do sinoatrial node cells have a less polar resting potential?

They’re ‘leaky’ so ions can enter spontaneously to the point there’s an action potential.

Absolute refractory period

Period of time during which myocytes cannot be stimulated to contract again. Around 250 ms.

Cardiac arrhythmias

Irregular heartbeats caused by random signals from the sinoatrial node.

Chronotropic control

Control of heart rate mediated by nodal tissues and the central nervous system.

Stroke volume

Volume of blood pumped per contraction. Normally 70 to 140ml - approx 60% of the end diastolic volume.

End diastolic volume

Volume of blood in the ventricle prior to contraction.

Frank-Starling law of the heart

Relationship between stroke volume and end-diastolic volume.

Inotropy

Contractile function of the heart.

Sympathetic control

Involves noradrenaline and β1 receptors. cAMP activates protein kinase → increased calcium concentration.

Arterial blood pressure =

cardiac output x total peripheral resistance

Total peripheral resistance

Dependent on blood viscosity and arteriolar radius.

Arteriolar radius

Constricted by sympathetic nerves and dilated by sympathetic cholinergic nerves and other factors.

Immediate regulation of arterial blood pressure

Controlled by ANS and chemoreceptor reflexes.

Long-term regulation of arterial blood pressure

Controlled by hormones vasopressin, angiotensin II, aldosterone, and atrial natriuretic peptide.

Drugs for high blood pressure

β1 antagonists, selective α1 antagonists, and ACE inhibitors.

What enhances actin-myosin interactions?

kinases and binding of calcium to troponin