Cardiovascular System

Cardiac muscle

Myocytes (cardiac muscle cells) are different from skeletal muscle fibres, as they are shorter, thicker and the branching cells have one nucleus. Each myocyte is surrounded by CT endomysium, allowing access to blood capillaries. Myocytes are joined by intercalated discs with three distinct features: plasma membranes of adjacent cells display interdigitating folds, cells joined by desmosomes and gap junctions between cells

Cardiac conduction system

myogenic: pacemaker in the heart itself and the ANS can only modify heart rate

Cardiac myocytes are autorhythmic, some lose ability to contract and specialise into generating and transmitting action potentials, making up the cardiac conduction system

Begins with sinoatrial node (serves as pacemaker), signals from SA node spread throughout atria and on to the AV node near the lower interatrial septum. The AV node is an electrical gateway to the ventricles, AV bundle leaves the AV node and divides into R/L bundle branches leaving into the interventricular septum. Next: impulse is spread to the purkinje fibres

The cardiac conduction system controls route and timing of electrical conduction. The four chambers must contract in coordinated fashion. Failure to the cardiac conduction system to transmit signals is called a heart block

Physiology of SA node:

  1. autorhythmic cells of SA node have a resting potential that starts at -60mv and then drifts spontaneously upward. The gradual depolarization is called pacemaker potential
  2. When pacemaker potential reaches -40mv, fast calcium channels open and calcium rushes in producing a rising phase of AP
  3. Next potassium channels open, potassium rushes out of the cell and cytosol becomes more negative creating a falling phase of AP
  4. Repolarization is complete, K channels close again and pacemaker potential begins anew. One depolarization of SA node sets off one heartbeat, triggers depolarization in the remainder of the cardiac conduction system

Impulse conduction of the myocardium

Firing SA nodes stimulates the two atria to contract together almost immediately

  1. Impulses travel to AV node where thinner myocytes show the impulse momentarily. this gives ventricles time to fill with blood before contracting
  1. Signal reaches AV bundle and spreads rapidly through fast purkinje fibres, causing entire ventricular myocardium to depolarise nd ventricles to contract in unison

Impulse reaches papillary muscle slightly before it spreads throughout the ventricles, allowing papillary muscles to tighten chordae tendinae on AV valves first

Electrocardiogram

Phases of cardiac cycle

electrical currents in the heart can be detected by recording electrodes on the skin. The electrocardiograph amplifies these signals and produces an ECG

P wave: depolarization of the atria

QRS: marks ventricular depolarization, atrial repolarization occurs simultaneously but signal is hidden by the strong wave of depolarization seen in ventricles

T wave generated by ventricular repolarization before diastole

  1. Heart is at rest, blood flowing into atria and AV valves are open allowing blood to passively flow through to the ventricles
  1. SA nodes fire and atrial systole results
  1. Isovolumetric contraction of ventricles occupies after ventricles depolarize. Av valves close as ventricular blood surges back against the cusps. No change in volume eventhough they are contracting
  1. Ventricular ejection: stroke volume passes into major vessels on top of the heart
  1. Isovolumetric relaxation and ventricular filling follow, complete one cardiac cycle. Pressure changes that occur coincident with these events

Diastole /daɪˈæstəliː/ is the part of the cardiac cycle during which the heart refills with blood after the emptying done during systole (contraction)

A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized (contraction in heart)