The Heart As A Pump Notes

The heart as a pump

Role of the heart

-supply sufficient blood to the tissues to satisfy their O2 and nutrient requirements

-remove waste products: urea, Co2

Flow

-average flow is 5L/minute

-generating pressure –

1) Electricity: the myogenic property of the heart- SAN produces spontaneous depolarisations leading to large influx of calcium ions into the myocytes. The depolarisation is propagated throughout the heart due to gap junctions- electrical syncitium.

2)Tension: Through excitation-contraction coupling-the influx of calcium through the L type voltage gated calcium ion channels leads to calcium induced calcium release from the SR. The calcium binds to troponin C, this causes allosteric change in troponin T which relieves the inhibition of the tropomyosin complex. Cross bridge cycle- This leads to contraction and sarcomeres shorten resulting in tension in the muscular walls

3) Pressure: Tension generated creates pressure, shown by Laplace’s law

Pressure = 2 x tension/ radius

In trained athletes there is cardiac hypertrophy, according to Laplace’s law, in order to generate the same ventricular pressure the heart needs to generate greater tension as the radius has increased. This is done by contracting harder. The increased work load means the heart continues to hypertrophy- the viscious cycle leads to sudden death

Larger heart needs more tension to produce threshold pressure

4) The pressure generated results in blood flow. This is shown by Ohm’s Law where Flow=Pressure/resistance

Valves: One directional blood flow

-pressure operated resistors- prevent the backflux of blood

-when pressure on the left side of the valve is greater than the right side of the valve, the valve is open- allows blood flow-when pressure on the left side of the valve is smaller than right side, valve is closed and there is no flow

-the chordate tendinae and papillary muscle support and prevent the valves flapping inside out

-In the ventricle when both valves are closed (valve separating the atria and ventricle and the valve separating the ventricles and outflow tract) the ventricles are isovolumetric. This occurs when pressure is sufficient to close the inlet valve but insufficient to open the outlet valve.

-during the isovolumetric phase, the volume is constant but the pressure increases as the ventricles continue to contract.

-disease valves: if the valves are incomptement there is reflux and this reduces efficiency, if the valve is stenosed and can’t fully open then higher pressure is needed for the same flow

-stenosis: excessive calcification

-regurgitation: prolapsed, poorly supported or weak valve leaflets

-heart valve disease loss of effective pumping in the heart leading to reduced cardiac output- fatigue, breathlessness, angina (left-side valve disease)- oedema (pulmonary oedema for right side valve disease) (systemic oedema for left side

Normal heart sounds

-cardiac valve is closed- tensing of the cusps results in a brief vibration- vibration is transmitted through the tissues to the chest wall. Healthy valves only closure is audible and the opening is silent

-there are two heart sounds heard per beat- lubb-dubb

-vibrations can be heart with a stethoscope

-1^st heart sound- closure of the tricuspid/mitral valves – lubb – caused when the interventricular pressure exceeds the arterial pressure. Mitral valve is best heard at the intersection between the mid-clavicular line and the fifth left intercostals space. Tricuspid space: fifth interspace at the left sternal edge

-2^nd heart sound-closure of the aortic and pulmonary valves- due to interventricular pressure falling below aortic pressure- aortic valve heard in the 2^nd interspace at the right sternal edge, pulmonary valve in the 2^nd interspace at the left sternal edge

- sometimes the second sound is audibly split into an initial aortic component and a fractionally delayed pulmonary component: terrupp

-Splitting most pronounced during inspiration: inspiration lowers intrathoracic pressure, increases right ventricular filling, prolongs the right ventricular ejection time and delays pulmonary valve closure. Inspiration expands the lung blood vessels which reduces the return of pulmonary blood to the left ventricle- reduces left ventricular stroke volume, shortens ventricular ejection time- hasten aortic valve closure

Cardiac cycle

-the whole cycle is completed in 1 second.

-ventricular diastole

-atrial systole

-ventricular systole

-ventricular diastole

1. ventricular diastole- ventricular filling

Duration: 0.5s

Inlet valves: Tricuspid, mitral valves – OPEN

Outlet valves: pulmonary, aortic – CLOSED

- 2/3^rd of the cardiac cycle – Allows lots of time to fill the ventricles

Initial rapid filling phase:-first the atria and ventricles are in diastole, pressure in the heart chambers are low- blood flows passively from the great systemic veins (superior and inferior vena cava) into the right atrium, blood returing from the lungs through the pulmonary veins enter the left atria. The open atrioventricular valves allows blood to pass passively into the ventricles. During this phase, ventricular pressure falls despite blood volume increasing- this is...

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