The heart can be divided into two sides, the left and the right. The division is important for the heart to carry out its function effectively. Each of the sides has slightly different roles; the right side is responsible for pumping deoxygenated blood to the lungs where it can be deoxygenated. The left side of the heart is responsible for pumping the oxygenated blood to the various parts of the body. (Wesson et al, 1998)
The sequence of events in a single heartbeat is known as the cardiac output cycle. The cycle involves systole, or contraction and also diastole, or relaxation, of the atria and ventricles. The cycle has four overlapping stages. Atrial systole; Both atria contract, forcing blood into the ventricles. This stage lasts 0.1 seconds. Ventricular systole; Both ventricles contract, forcing blood through the pulmonary artery to the lungs and through the aorta to the rest of the body. This takes 0.3 seconds. Atrial diastole; the atria relax, although the ventricles are still contracted. Blood enters the atria from the large veins coming from the body. This takes about 0.7 seconds. Ventricular diastole; the ventricles relax, and become ready to fill with blood from the atria as the next cycle begins. This takes about 0.5 seconds. (Biology, Marcus Barbor, Mike Boyle, Mike Cassidy, Kathryn Senior 1997).
The heart works by producing impulses, which spread and animate the specific muscle fibres. Cardiac muscle has the unique ability to generate its own electrical impulse, called ‘autoconduction’, which allows it to contract rhythmically without neural stimulation; this is what is known as ‘myogenic’. The electrical impulse for the heart contraction begins at pacemaker this is also known as the sino-atrial node ‘S.A. node’. It is a group of specialised cardiac muscle fibres found in the back wall of the atrium. The electrical impulse generated by the S.A node spreads through both atria ventricular node ‘AV node’ which is situated in the right atrial wall near the centre of the heart. As the ‘AV node’ receives the impulse, it is conducted from the purkinje fibres, causing the ventricles to contract. ((Exercise Physiology, William Mcardle, Frank I. Katch, Victor L. Katch 1996)
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There are three methods to determine cardiac output:-
Indicator Dilution method: - This method involves venous and arterial punctures but does not require cardiac catheterization. A known quantity of an inert dye is injected into a large vein. The indicator remains in the vascular system. It then becomes mixed as the blood travels to the lungs and back to the heart before being ejected into the systemic circuit. Knowing the dilution of a known dye in an unknown quantity of blood, cardiac output can be calculated. (Exercise Physiology, William Mcardle, Frank I. Katch, Victor L. Katch 1996)
Direct Flick method: - This is achieved by threading a catheter through a vein in the arm to the reach the superior vena cava, which leads into the right side of the heart. Arterial and mixed venous blood is then obtained during the same period of measurements as the oxygen uptake. The main problem with this method is that it is invasive to the body, which can alter the cardiovascular dynamics during measurements period. (Exercise Physiology, William Mcardle, Frank I. Katch, Victor L. Katch 1996)
Indirect Flick method (CO2 Rebreathing Method):- Carbon dioxide is analysed by a rapid carbon dioxide analyser, and from this estimates can be made of mixed venous and arterial carbon dioxide concentration. This method depends on measuring the CO2
excretion rate, CO2 concentration in alveolar (end tidal) air and the CO2 concentration in equilibrium with mixed venous blood using rebreathing technique. The technique is called CO2 rebreathing, breath by breath cardiac output can be calculated with the use of the Flick principle.
Method
See Script.
Results
Group 1
Group 2
Group 3
To see more detailed results, look at cardiac output recording sheet.
Discussion
The results obtained from the cardiac output experiment, will explain how different postures affect the cardiac output.
Looking at the results above, it clearly states that the volume of bag when the subject was standing was 43.3 litres and when the subject was lying down it was 39.3 litres. This explains that when the subject was standing the volume of bag had more litres then when the subject was lying down. Also the results show that the same amount of CO2 3.40%.
Finally the cardiac output was 6.82 l min-1 when the subject was lying down and when standing up 5.16 l min-1. Results were also obtained from group 2 and 3. The cardiac output from group 2 and the 3 clearly show that when the subject was lying down, there was a greater increase in cardiac output, then when compared to standing up.
Cardiac Output varies with both size and endurance training CO2 is controlled by the need to meet the muscles demand for an increase in oxygen supply, whether it is moving from lying down position to a walk or from a walk to a run. Therefore heart rate will increase to meet the demand of oxygen needed by the muscles and maintain cardiac output. (Wilmore et al, 1993)
Cardiac output is affected by the body’s position. When the body is in an lying down position cardiac output is at the highest and most stable. However when the body is an upright position, gravity counters the return of blood to the heart which causes a drop in cardiac output. . ((Exercise Physiology, William Mcardle, Frank I. Katch, Victor L. Katch 1996)
The statement above from the authors of Mcardle, states that when the subject is lying down, the cardiac output is greater then when in an upright position. The results obtained from the experiment group 1.2 and 3; show that the cardiac output is greater when lying down. This is what would be expected to happen when the subjects were lying and standing.
To increase cardiac output the body relies on the heart, in order to increase flow of blood. In contrast, the force of gravity in the upright position acts to counter the return of blood flow to the heart, resulting in a diminished stroke volume and cardiac output. ((Exercise Physiology, William Mcardle, Frank I. Katch, Victor L. Katch 1996)