Cardiovascular systems

• Stroke volume is the volume of blood pumped out of the left ventricle with each contraction. A resting value for stroke volume is approximately 70ml.
• Starlings law explains how SV increases during exercise. An increased venous return leads to greater diastolic fill, stretching the cardiac muscle creating more powerful contraction force increasing ejection and therefore SV.
• Heart rate is the number of times the heart beats per minute. Maximum heart rate is calculated as 220-age. Average resting heart rate is between 60 and 100bpm.
• Bradycardia is a slow heart rate below 60 bpm. It's caused by regular exercise leading to hypertrophy of the heart resulting in increased SV and maximum cardiac output.
• Cardiac output is the volume of blood ejected from the left ventricle per minute. Cardiac output increases with exercise.
• There are 2 phases of the cardiac cycle, cardiac diastole where cardiac muscles relax and cardiac systole which is the contraction of the cardiac muscles.
• During atrial systole the atria walls contract. This forces blood through the AV valves into the ventricles.
• Ventricular systole is when the ventricle walls contract. Pressure of the blood opens the semilunar valves and blood is ejected into the pulmonary artery to the lungs, aorta and body. The AV valve then shuts.
• During diastole blood refills the heart chambers, first the atria then the ventricles passively fill.
• The heart is myogenic so creates it's own electrical impulse in the SA node. This impulse then passes through the AV node, down the bundle of his, branching into 2 bundles called the purkinje fibres which cause the ventricles to contract.
• Neural factors regulate heart rate during exercise and are controlled by the CCC. They're either sympathetically controlled (stimulating the heart to beat faster sending a signal from the CCC via the accelerator nerve) or parasympathetically (returning the heart to it's resting levels sending a signal from the CCC vis the vagus nerve).
• The CCC is stimulated by chemoreceptors detecting an increase in CO2 in the blood. The CCC sends an impulse through the sympathetic system down the accelerator nerve causing the SA node to increase HR.
• The CCC is stimulated by baroreceptors detecting an increased stretch in the blood vessel walls. The CCC sends an impulse through the parasympathetic system down the vagus nerve causing the SA node to decrease HR.
• The CCC is stimulated by proprioceptors detecting an increase in motor activity. The CCC sends an impulse through the sympathetic system down the accelerator nerve causing the SA node to increase HR.
• Adrenaline is a hormone that affects HR. It's released during exercise by cardiac and sympathetic nerves. It stimulates the SA node resulting in an increase in cardiac output.
• Changes in temperature is an intrinsic factor affecting blood viscosity and speed of nerve impulse transmission. Venous return also increases stretching the cardiac muscle stimulating the SA node increasing stroke volume.
• Arteries transport oxygenated blood to the muscles. They have a small lumen, thick muscular walls and an outer elastic layer to cope with carrying blood at a high pressure.
• Veins transport deoxygenated blood back to the heart. They have thin muscle and elastic tissue layers, valves and a large lumen. This allows them to carry blood at a low pressure.
• Capillaries are the site of gaseous exchange. Their walls are one cell thick providing a short diffusion pathway, have a large surface area and a narrow diameter to slow blood flow down.
• Venous return increases during exercise due to active mechanisms. For example valves, skeletal muscle pump, respiratory pump, smooth muscle, gravity and low intensity exercise during recovery maintaining muscle and respiraatory pumps.
• The skeletal muscle pump squeezes blood back towards the heart as when muscles contract ad relax they press on nearby veins. The respiratory pump assists blood return back to the heart as when respiratory muscles contract and relax they push on nearby veins.
• Redistribution of blood during exercise is needed because more blood needs to go to muscles to increase O2 supply and remove waste products such as CO2 and lactic acid and more blood needs to be directed to the heart because it's a muscle.
• Redistribution of blood is controlled by the vascular shunt mechanism. During exercise vasodilation will occur in arterioles to muscles to increase blood flow and therefore O2 supply. Vasoconstriction will occur in arterioles to non-essential muscles decreasing blood flow.
• Pre-capillary sphincters are tiny rings of muscles located at capillary openings that aid blood redistribution during exercise. They relax around the muscle during exercise to increase blood flow and saturate the muscles with oxygen.