Pack 6a- SN

Heart structure

• atria (thin walled) receive blood from the veins

• blood from the vena cava flows into the right atrium, while blood from the pulmonary veins flows into the left atrium

• atria contract- blood passes through atrioventricular valves into ventricles (thicker walled)

• ventricles contract, blood passes up through the open semi-lunar valves

• right ventricle- blood passes into the pulmonary artery, and then onto the lungs

• left ventricle- blood passes into the aorta, and from there to the rest of the body

Cardiac cycle

DIASTOLE

• the heart muscle relaxes

ATRIAL SYSTOLE

• bloods fills the atria from the vena cava and pulmonary vein

• the atrial muscle contracts, increasing the pressure above that of the ventricles

• this forces the atrioventricular valves open and blood flows into the ventricles

VENTRICULAR SYSTOLE

• the ventricle muscles contract

• the volume of the ventricle decreases, increasing the pressure

• this causes the atrioventricular valves to close

• (pressure greater in ventricles than atria)

• the blood is pushed upwards, towards the arteries at the top of the heart

• the semilunar valves open and the blood flows

• (pressure greater in ventricles than artery)

• into the aorta and pulmonary artery

Valves

• atrio-ventricular valves close = pressure higher in ventricles than atria

• atrio-ventricular valves open= pressure higher in atria than ventricles

• semi-lunar valves close= pressure higher in arteries than ventricles

• semi-lunar valves open= pressure lower in arteries than ventricles

Control of the cardiac cycle (paper 2)

• a wave of electrical activity (impulses) is sent out from the sinoatrial node (SAN)

• it spreads across both atria, causing them to contract

• the electrical activity reaches the atrioventricular node (AVN) where it is delayed

• this delay allows the atria to fully empty, and the ventricles to fill with blood, before the ventricles contract

• the impulses are then transmitted down to the ventricles by the Bundle of His. this contains specialised conducting tissues called Purkyen fibres

• the impulses spread up the ventricle wall from the base via Purkyne fibres, causing the ventricles to contract from the base up

• blood is forced up into the arteries

Cardiac output

CARDIAC OUTPUT (dm³ min^-1)= stroke volume (cm³) x heart rate (bpm)

Arteries

• the wall is thick, to withstand the high pressure of blood

• the thick middle coat contains many elastic fibres

• allowing them to stretch during systole and recoil during diastole to maintain blood pressure and smoothes pressure

• the lumen is relatively small

Arterioles

ARTERIOLE WALLS

• lots of muscle tissue

• thinner than arteries

• have an inner coat of a single layer of endothelium- reduces friction

• the muscle can contract, narrowing the diameter of the lumen (vasoconstriciton)

• the muscle can relax, to widen the lumen diameter (vasodilation)

Veins

• three layers, but the middle coat contains far fewer elastic or muscle fibres (than arteries)

• walls are very thin

• have valves

• lumen is relatively large

Capillaries

• rate of blood flow is reduced allowing more time for diffusion/ exchange

• single layer of squamous endothelium cells, provding a short diffusion pathway

• tiny pores between the cells in the wall- so highly permeable

• small size and the large number in total provides a large SA:vol ratio