The four major functions of the cardiovascular system are:
1. To transport nutrients, gases and waste products around the body
2. To protect the body from infection and blood loss
3. To help the body maintain a constant body temperature (‘thermoregulation’)
4. To help maintain fluid balance within the body
The cardiovascular system acts as an internal road network, linking all parts of the body via a system of highways (arteries and veins), main roads (arterioles and venules) and streets, avenues and lanes (capillaries).
This network allows a non-stop courier system (the blood) to deliver and expel nutrients, gases, waste products and messages throughout the body.
Nutrients such as glucose from digested carbohydrate are delivered from the digestive tract to the muscles and organs that require them for energy.
Hormones (chemical messengers) from endocrine glands are transported by the cardiovascular system to their target organs, and waste products are transported to the lungs or urinary system to be expelled from the body.
The cardiovascular system works in conjunction with the respiratory system to deliver oxygen to the tissues of the body and remove carbon dioxide. In order to do this effectively the cardiovascular system is divided into two circuits, known as the pulmonary circuit and the systemic circuit.
The pulmonary circuit is made up of the heart, lungs, pulmonary veins and pulmonary arteries. This circuit pumps deoxygenated (blue) blood from the heart to the lungs where it becomes oxygenated (red) and returns to the heart.
The systemic circuit is made up of the heart and all the remaining arteries, arterioles, capillaries, venules, and veins in the body.
This circuit pumps oxygenated (red) blood from the heart to all the tissues, muscles and organs in the body, to provide them with the nutrients and gases they need in order to function.
After the oxygen has been delivered the systemic circuit picks up the carbon dioxide and returns this in the now deoxygenated (blue) blood, to the lungs, where it enters the pulmonary circuit to become oxygenated again.
2. Protection from infection and blood loss
Blood contains three types of cells as listed below and shown in the adjacent image.
1. Red blood cells
2. White blood cells
Red blood cells are responsible for transporting oxygen around the body to the tissues and organs that need it.
As oxygen enters the blood stream through the alveoli of the lungs it binds to a special protein in the red blood cells called haemoglobin. This can be seen in the adjacent image.
The job of white blood cells is to detect foreign bodies or infections and envelop and kill them, as seen in the below image.
When they detect and kill an infection they create antibodies for that particular infection which enables the immune system to act more quickly against foreign bodies or infections it has come into contact with previously.
Platelets are cells which are responsible for clotting the blood, they stick to foreign particles or objects such as the edges of a cut.
Platelets connect to fibrinogen (a protein which is released in the site of the cut) producing a clump that blocks the hole in the broken blood vessel. On an external wound this would become a scab.
If the body has a low level of platelets then clotting may not occur and bleeding can continue.
Excessive blood loss can be fatal – this is why people with a condition known as haemophilia (low levels or absence of platelets) need medication otherwise even minor cuts can become fatal as bleeding continues without a scab being formed.
Alternatively, if platelet levels are excessively high then clotting within blood vessels can occur, leading to a stroke and or heart attack. This is why people with a history of cardiac problems are often prescribed medication to keep their blood thin to minimise the risk of clotting within their blood vessels.
3. Maintenance of constant body temperature (thermoregulation)
The core temperature range for a healthy adult is considered to be between 36.1°C and 37.8°C, with 37°C regarded as the average ‘normal’ temperature.
If the core temperature drops below this range it is known as hypothermia and if it rises above this range it is known as hyperthermia.
As temperatures move further into hypo or hyperthermia they become life threatening. Because of this the body works continuously to maintain its core temperature within the healthy range.
This process of temperature regulation in known as thermoregulation and the cardiovascular system plays an integral part.
Temperature changes within the body are detected by sensory receptors called thermoreceptors, which in turn relay information about these changes to the hypothalamus in the brain.
When a deviation in temperature is recorded the hypothalamus reacts by initiating certain mechanisms in order to regain a safe temperature range. There are four sites where these adjustments in temperature can occur, they are:
a. Sweat glands: These glands are instructed to secrete sweat onto the surface of the skin when either the blood or skin temperature is detected to be above a normal safe temperature. This allows heat to be lost through evaporation and cools the skin so blood that has been sent to the skin can in turn be cooled.
b. Smooth muscle around arterioles: Increases in temperature result in the smooth muscle in the walls of arterioles being stimulated to relax causing vasodilation (increase in diameter of the vessel).
This in turn increases the volume of blood flow to the skin, allowing cooling to occur. We see this is in the adjacent diagram where blood that is normally concentrated around the core organs is shunted to the skin to cool when the body is under heat stress.
If however the thermoreceptors detect a cooling of the blood or skin then the hypothalamus reacts by sending a message to the smooth muscle of the arteriole walls causing the arterioles to vasoconstrict (reduce their diameter), thus reducing the blood flow to the skin and therefore helping to maintain core body temperature.
c. Skeletal muscle: When a drop in blood temperature is recorded the hypothalamus can also react by causing skeletal muscles to start shivering. Shivering is actually lots of very fast, small muscular contractions which produce heat to help warm the blood
d. Endocrine glands: The hypothalamus may trigger the release of hormones such as thyroxin, adrenalin and noradrenalin in response to drops in blood temperature. These hormones all contribute to increasing the bodies metabolic rate (rate at which the body burns fuel) and therefore increasing the production of heat.
4. Maintaining fluid balance within the body
The cardiovascular system works in conjunction with other body systems (nervous and endocrine) to balance the body’s fluid levels. Fluid balance is essential in order to ensure sufficient and efficient movement of electrolytes, nutrients and gases through the body’s cells.
When the fluid levels in the body do not balance a state of dehydration or hyperhydration can occur, both of which impede normal body function and if left unchecked can become dangerous or even fatal.
Dehydration is the excessive loss of body fluid, usually accompanied by an excessive loss of electrolytes.
The symptoms of dehydration include; headaches, cramps, dizziness, fainting and raised blood pressure (blood becomes thicker as its volume decreases requiring more force to pump it around the body).
Hyperhydration on the other hand results from an excessive intake of water which pushes the normal balance of electrolytes outside of their safe limits. This can occur through long bouts of intensive exercise where electrolytes are not replenished and excessive amounts of water are consumed.
This can result in the recently consumed fluid rushing into the body’s cells, causing tissues to swell. If this swelling occurs in the brain it can put excessive pressure on the brain stem that may result in seizures, brain damage, coma or even death.
Dehydration or a loss of body fluid (through sweat, urination, bleeding etc) results in an increase in ‘blood tonicity’ (the concentration of substances within the blood) and a decrease in blood volume. Where as hyperhydration or a gain in body fluid (intake of water) usually results in a reduction of blood tonicity and an increase in blood volume.
Any change in blood tonicity and volume is detected by the kidneys and osmoreceptors in the hypothalamus.
Osmoreceptors are specialist receptors that detect changes in the dilution of the blood. Essentially they detect if we are hydrated (diluted blood) or dehydrated (less diluted blood).
In response hormones are released and transported by the cardiovascular system (through the blood) to act on target tissues such as the kidneys to increase or decrease urine production. Another way the cardiovascular system maintains fluid balance is by either dilating (widening) or constricting (tightening) blood vessels to increase or decrease the amount of fluid that can be lost through sweat.