Cardiac Physiology

Composition of Blood

• Nearly half its volume is composed of cells.

  • The most numerous cells are erythrocytes (red blood cells)

  • The remainder of the cells are leukocytes (white blood cells)

  • Present are platelets, which are not actually cells but rather cell fragments that play an important role in blood clotting. • The liquid portion of the blood, called plasma, is made up of water containing dissolved proteins, electrolytes, and other solutes.

Parallel arrangement of organs in a systematic circuit

• Each organ is fed by a separate artery, each receives fully oxygenated blood.

• Blood reaches the organs via parallel paths, blood flow to the organs can be independently regulated. Thus, blood flow can be adjusted to match the constantly changing metabolic needs of organs.

Anatomy of the heart

• Atria

• Ventricles

• AV valves

• Semilunar valves

Blood flow through the heart

• Right to lungs

• Left to body

Cardiac Conduction

• Contraction of the heart is systole and blood is ejected from the heart. First atria then ventricles contract.

• Relaxation of the heart is diastole allowing the atrium and ventricles to fill with blood

• Collections of specialised pacemaker cells (nodes) drive the heart beat. (sinoatrial node to AV node to AV bundle to right and left branches (bundle of His.) to purkinje fibres)

  

Specialised Cardiac Muscle Fibre Cells

• Purkinje fibres and the bundle of HIS

• Autorhythmic cells generate little contractile force but coordinate and provide rhythm to the heartbeat (pacemaker cells and conduction fibres)

  

Cardiac Muscle Cells

• Contractile cells

• Branched

• Connected by intercalated discs that have gap junctions that allow muscle AP’s to travel through cardiac muscle.

• Larger diameter to conduct AP’s faster

• Stable resting potential of 90mV and only depolarise when stimulated

• At threshold (-70mV)

  • Na channels open causing depolarisation

  • Slow Ca channels open causing a slow and steady influx at -40mV

  • Near the peak Na channels close and K channels open

  • Small decrease in membrane potential called early repolarization

  • Ca and K balance causing plateau (contracts longer) and muscle contraction halfway through.

  • Ca is transported out and back to SR

  • Sodium potassium pump restores ionic balance with a longer absolute refractory period to prevent summation and tetnus

What triggers the heartbeat?

• Signals originate from the muscle itself - myogenic

• Impulse traveling from the AV node to the bundle of His.

Pacemaker Cells

• Initiate contractions spontaneously generating AP’s

• Usually generate contractions in 2 specific reigions

  • SA node - 80 AP’s per min - 80 beats per min

  • AV node

• The SA and AV node spontanously generate AP’s at different rates - the SA node (pacemaker) is faster driving depolarisation of the AV node

• SA controls heart rate, if damaged other parts may take its role

• No true resting potential

  • Voltage starts at -60mV and spontaneously moves up until the threshold

  • This is due to funny channels that open when the membrane voltage is less than -40mV and allow slow influx of sodium causing pacemaker potential

  • At threshold calcium channels open depoalring further.

  • Spreads to conduction system and contractile myocytes

Cellular Mechanisms

• Resting potential is the potential for K

• NS can make the AP go faster or slower but cannot generate them

• Pacemaker and contractile myocytes have different forms of AP’s

The Cardiac Cycle: Ventricular filling

• Mid to late diastole

• Blood returns to the heart via systemic and pulmonary veins and enters the relaxed atria.

• From there it passes through the AV valves into the ventricles under its own pressure

• The return of blood from the veins to the heart - venous return - occurs because the pressure in the veins is greater than that in the atria.

• During this time, the pulmonary and semilunar valves are closed because ventricular pressure is lower than the aorta and pulmonary artery pressure.

• In late diastole the atria contract driving more blood into the ventricles and the atria relax as systole begins.

The Cardiac Cycle: Isovolumic Contraction

• Ventricles contract raising pressure as the blood stays in

• When the ventricular pressue exceeds the pressure in the atria (early systole) AV valves close and semilunar valves remain closed (ventricular pressure isn’t high enough yet)

• Ends when ventricular pressure is large enough to force open the semilunar valves so blood can leave the ventricles.

The Cardiac Cycle: Ventricular Ejection

• Blood is ejected into the aorta and pulmonary arteries through the semilunar valves

• Ventricular volume decreases falling below aortic pressure.

• This causes the semilunar valves to close (marks being of diastole as blood is no longer being ejected)

The Cardiac Cycle: Isovolumetric Relaxation

• Ventricular myocardium relaxes

• Some blood is present in the ventricles under pressure as it takes a long time for the ventricles to relax.

• Ventricular pressure is too low for the semilunar valves to remain open and too high for the AV valves to open.

• Volume of blood is constant and valves are closed.

Wiggers Diagram

Mean Arterial Pressure