The cardiovascular system includes all the blood vessels within the body, the heart and the lungs.
The engine, pump or driving force behind all of this is the heart. It measures about 300g, and is about the size of a fist. The heart is an incredibly powerful pump that not only has to move blood around the body at high pressure, but also has to pump blood to the lungs under low pressure. It is this “double pressure” system that makes the cardiovascular system so effective.
It is a four-chamber pump, with the right side receiving deoxygenated blood from the body at low pressure and pumping it to the lungs (the pulmonary circulation) and the left side receiving oxygenated blood from the lungs and pumping it at high pressure around the body (the systemic circulation).
The atria make up the upper portion of the heart whilst the ventricles make up the lower portion. Oxygen poor blood returning to the heart collects in the right atria before moving forward into the right ventricle. As this ventricle contracts, blood is forced up under low pressure into the lungs via the pulmonary artery where waste products of metabolism such as carbon dioxide are released. Fresh oxygen is taken up by haemoglobin in the blood and transported back down to the left atria via the pulmonary veins. This oxygen rich blood is then moved into the left ventricle ready for ejection into the aorta for transportation around the entire body.
The heart is therefore the pump responsible for maintaining adequate circulation of oxygenated blood around the vascular network of the body.
The Conducting System & ECG
Although the heart is an amazing piece of biological engineering, it needs a constant source of stimulation for it to work in a synchronous manner. Within the heart, there are groups of specialized cells which are responsible for conducting tiny electrical impulses to various areas of the heart in order to stimulate the pumping mechanism. The heart’s natural pacemaker is called the Sinoatrial node, and is located in the right atria. This node sends out an impulse which travels to another group of cells called the Atrioventricular (AV) node.
The AV node lies at the bottom of the right atria. The impulse then leaves the AV node, travels down the nerve fibres into the right and left ventricle where they spread rapidly across the ventricles causing the muscle fibres to contract. This contraction is responsible for ejection of blood into the aorta and around the body. These impulses can be monitored on an ECG. Depending on the waveforms produced, cardiologists can diagnose different abnormalities and arthymmias of the heart.
Within the cardiovascular system, there are many different levels of pressure exerted by the blood. The most commonly measured blood pressure is that of Systolic (ejection of blood into the aorta) and Diastole (when the heart is resting and receiving blood). For most people who do not have heart disease, this is generally around 100-140/60-80mmHg, with males having a slightly higher pressure. Blood pressure reduces as blood flows to the different areas of the body. When blood leaves the heart, it is under its highest pressure (systole approx 120mmHg). As it flows out of the arteries and into arterioles and capillaries, its pressure is very much lower (approx 25mmHg). This pressure reduces further when measured at the veins.
Systolic pressure is determined by the amount if blood being forced into the aorta and arteries with each ventricular contraction (heartbeat). If the volume of blood is increased, the systolic pressure is increased. Systolic pressure can also be affected buy the elasticity of the arteries. If the walls of the artery become stiff, a condition called arteriosclerosis, they are unable to compensate for the sudden increase in blood flow which occurs on ventricular contraction. Thus, systolic pressure increases, leading to hypertension.
Diastolic pressure provides valuable information regarding peripheral resistance. This is the resistance to blood flow through the vascular network of arteries and veins. If the body need to increase blood pressure e.g. during fainting, one of the ways it achieves this is by constricting the arterioles; a process known as arteriolar vasoconstriction. This constriction of the vessel impedes blood flow from the arterial network, thus increasing blood pressure. The diastolic pressure is also dependent on a number of other factors, such as systolic blood pressure, the thickness or viscosity of the blood, and presence or absence of arteriosclerosis.