Pressure changes and stroke volume

Show and tell Show us your models that explain how the heart works! I am looking for: Labelling of the heart and a description of the function Chambers, valves, major blood vessels, SAN/AVN, Bundle of His, Purjynke fibres, both septums, apex and base, Direction of blood flow and oxygenated/deoxygenated Pressure levels in the chambers How the valves work to stop backflow

Mammals have a closed circulatory system that allows pressure to be maintained and regulated

Definitions Stroke volume is the amount of blood ejected by the left ventricle in one contraction. Cardiac output is the volume of blood pumped by the heart per minute. It is calculated using this formula: Cardiac output = Stroke volume x heart rate The heart rate is simply the number of heart beats per minute.

Pressure changes in the cardiac cycle As a chamber fills with blood, the pressure is going to rise. When a chamber contracts, the pressure also rises. Changes in pressure affect whether a valve is open or closed because fluids always move from areas of high pressure to areas of low pressure. As the blood passes into the atria, the AV valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles. The ventricular pressure rises as the ventricles fill with blood. This closes the AV valves. Contraction of the ventricles means that the ventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at). The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle.

Left atrium Contract Left ventricle Relax Aortic pressure rises when the ventricles contract as blood is forced into the aorta As the blood passes into the atria, the AV valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles. The ventricular pressure rises as the ventricles fill with blood. This closes the AV valves. Contraction of the ventricles means that the ventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at). The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle. It then gradually falls but never below ~12 kPa because of the elasticity of its wall which creates a recoil action – this is necessary if blood is to be continuously supplied to the tissue The recoil produces a temporary rise in pressure at the start of the relaxation phase

blood as the atria contract Left atrium Contract Left ventricle Relax Ventricular pressure is low at first but gradually increases as the ventricles fill with blood as the atria contract As the blood passes into the atria, the AV valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles. The ventricular pressure rises as the ventricles fill with blood. This closes the AV valves. Contraction of the ventricles means that the ventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at). The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle. The left atrioventricular valves close and pressure rises dramatically as the thick muscular Walls of the ventricle contract As pressure rises above that of the aorta, blood is forced into the aorta past the semilunar valves Pressure falls as the ventricles empty and the walls relax

closes and its walls relax Left atrium Contract Left ventricle Relax Atrial pressure is always relatively low because the thin walls of the atrium cannot create much force As the blood passes into the atria, the AV valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles. The ventricular pressure rises as the ventricles fill with blood. This closes the AV valves. Contraction of the ventricles means that the ventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at). The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle. It is highest when they are contracting, but drops when the left atrioventricular valve closes and its walls relax The atria then fill with blood, which leads to a gradual build-up of pressure until a slight drop when the left atrioventricular valve opens and some blood moves into the ventricle

the semilunar valve opens Left atrium Left ventricle Ventricular volume rises as the atria contract and the ventricles fill with blood As the blood passes into the atria, the AV valves are open so most will fall immediately into the ventricle. There is a gradual rise in pressure in the atria until the end of atrial systole when the blood has moved into the ventricles. The ventricular pressure rises as the ventricles fill with blood. This closes the AV valves. Contraction of the ventricles means that the ventricular pressure is higher than the pressure in the artery which forces the blood out of the ventricle and into the aorta or pulmonary artery (depending on which side of the heart you’re looking at). The increase in pressure of the artery causes the closing of the semilunar valves preventing the back flow of blood into the ventricle. The volume then drops suddenly as blood is forced out into the aorta when the semilunar valve opens Volume increases again as the ventricles fill with blood

Discuss the following: Why is there a difference in pressure between the left and right ventricles? What features of the heart allow it to withstand this high pressure on the left side?

The Cardiac Cycle Stage 1 – Atrial Diastole Stage 2 - Ventricular Diastole Stage 3 – Atrial Systole Stage 4 – Ventricular Systole Relax & Refill (0.5 Secs) Contraction (0.3 Secs)

Cardiac Cycle Stage of Cardiac Cycle Description Action of Valves Atrial diastole (relaxation) Atria fill with blood Atrioventricular valves closed. Semi-lunar valves open Ventricular diastole (relaxation) Rising pressure in atria causes the AV valves to open and ventricles to fill AV valves open. Semi-lunar valves closed Atrial systole (contraction) Atria contract, forcing blood into ventricles AV valves open Ventricular systole (contraction) Ventricles contract, increasing pressure in the ventricles and forcing blood into the aorta and pulmonary artery AV valves are forced to close

Performance of the Heart Dependent on 2 variables: STROKE VOLUME (SV) & HEART RATE (HR) Stroke Volume is determined by: Venous Return Elasticity of cardiac fibres Contractuality of cardiac tissue

Performance of Heart - SV Venous Return: Volume of blood returning to right atrium. Greater VR = Greater SV Elasticity of Cardiac Tissue: Degree of stretch of cardiac tissue just before contraction. Greater contraction = Greater force of contraction (STARLING’S LAW) Contractuality of Cardiac Tissue: Increased contractuality = Greater force of contraction = Increase in SV. This is a result of the INJECTION FRACTION (% of blood pumped out of left ventricle per contraction) At Rest = 55% (45% of blood remains in heart) Exercise = Up to 85%

The Effect of Posture on Stroke Volume When a person is lying down, the large veins in the chest are plump with blood. And because these veins are stretched, the pressure in them is higher than when they contain less blood. Consequently, when lying down, the central venous pressure is relatively high, the end-diastolic volume is relatively high and thus the stroke volume is comparatively high. But this changes when we stand. The pressure in the large veins in the legs increases greatly. This causes the distensible, voluminous veins to expand, and blood pools in the leg veins. This reduces the blood in the central veins, and the central venous pressure drops. Because these central veins are very compliant structures, pressure cannot increase again in them until blood flows back into the thorax. So why might cardiac out put remain the same when sitting and standing?

Performance of Heart - HR Is the number of complete cardiac cycles and therefore the number of times the left ventricle ejects blood into the aorta per minute. Av resting HR = 72 bpm Elite athlete = below 60 bpm (Bradycardia) OLYMPIC ROWING CHAMPION, STEVE REDGRAVE HAS A RESTING HEART RATE OF BETWEEN 40-45 BPM

Cardiac Output Is the relationship between SV & HR Cardiac output (Q) = stroke volume (SR) x heart rate (HR) Measured in litres per minute (1 litre = dm3) Cardiac output = SV x HR Definition The volume of blood ejected from heart per minute The volume of blood ejected from heart per beat The number of cardiac cycles per minute Untrained subject 5 litres per min 70 ml 72 bpm Trained subject 5 litres per minute 85 ml 60 bpm

Heart Rate Response to Exercise Heart rate typically increases with intensity of exercise This continues until reach max HR (220 – Age) During SUB-MAXIMAL exercise (eg. 1500m swim) HR reaches a plateau (steady state) This represents point at which O2 demand is being met by O2 supply. HR also rises due to anticipation just before exercise commences After exercise ends, HR takes a while to return to resting rate – to rid body of waste products

Heart Rate Response to Exercise Key Terms: Starling’s Law – mechanism by which an increase in venous return leads to stronger ventricular contraction and increase in SV Ejection Fraction - % of blood actually pumped out of left ventricle per contraction Bradycardia – reduction in resting HR below 60 bpm as a result of endurance training Sub-Maximal Exercise – exercise that is low intensity and well within capabilities of performer Anticipatory Rise – the pre exercise response of the heart to the release of andrenalin (rise in HR) Matthew Pinsent James Cracknell Resting HR 45 40 Just prior to start 55 50 500m 160 155 1000m 1500m 1750m 190 185 2000m (Finish)