The Circulatory System Introduction
Blood - A fluid consisting of plasma in which many substances and nutrients (such as glucose) are dissolved. Blood is also composed of white blood cells and red blood cells (erythrocytes) and contains platelets for clotting. Blood carries other gasses, urea and hormones around the body and travels through the blood vessels.
Heart - the organ that pumps oxygenated blood to the cells of the body and pumps deoxygenated blood out of the body (to the lungs).
Heart - the organ that pumps oxygenated blood to the cells of the body and pumps deoxygenated blood out of the body (to the lungs).
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Blood Vessels
There are three main types of blood vessel: arteries, veins and capillaries. Arteries take blood from the heart to organs of the body while Veins take blood from the organs to the heart. Capillaries are between the two as they carry blood through organs so it is near each cell of the organ. Arteries Blood is pumped out from the ventricles into the arteries at high pressure so a high pressure is exerted on the artery lining. This pressure is necessary for the transportation of blood around the body through arteries so the lining consists of elastic tissue to allow it to stretch and recoil to maintain the blood pressure. The flow of blood is maintained by thick muscle lining which is one of the identifying features of arteries. Arteries can also be identified by their small lumen (central cavity where blood passes through). When blood is pumped out of the heart it is through the biggest artery: the aorta which splits into separate arteries. Veins Blood travelling through veins has lower pressure which allows for a thinner lining, as well as less elastic tissue and muscle and a larger lumen. The veins take blood from organs to the heart and (most) enter through the biggest vein: the vena cava (which is split into superior (from the head) and inferior (from the rest of the body)). The blood is then taken into the atria. Veins have semilunar valves (half-moon shaped) to keep blood flowing in the right direction. When naming arteries and veins (other than the aorta and the vena cava), the following words are used: pulmonary - to do with the lungs. The pulmonary artery and the umbilical artery are the only arteries to carry deoxygenated blood while the corresponding veins are the only veins to carry oxygenated blood. coronary - to do with the heart. The coronary artery and vein take blood to and from the cells of the heart so they can respire. hepatic - to do with the liver renal - to do with the kidneys |
Capillaries
Capillaries are very small to fit between cells in the organs. They bring blood close to every cell in the organ and are only one cell thick to allow for a shorter diffusion pathway. As capillaries are so thin, blood cells just fit and are close to the capillary lining to further aid diffusion. Capillaries have arterioles and venules on either side of them. Arterioles take blood from the arteries to the capillaries and venules take blood from the capillaries to the veins. Arterioles can control how much blood goes to an organ due to a combination of muscle fibres and nerve endings in their walls. 
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Blood
Blood is a tissue with many different components...
Plasma
Approximately 55% of blood is plasma (the substance) and plasma is approximately 95% water. Water has chemical properties (such as polarity) that make it a really good solvent (fluid that dissolves solutes) which is essential for plasma as it is where nutrients such as glucose are dissolved. Carbon dioxide produced as a waste product of aerobic respiration also dissolves into the plasma. Plasma carries blood cells, hormones and urea around the body in the blood.It also distributes heat around the body.
Red Blood Cells
Red blood cells (erythrocytes) carry oxygen around the body for aerobic respiration. They contain a lot of haemoglobin in their cytoplasm which is an iron-containing protein that reacts temporarily and reversibly with oxygen: $$ haemoglobin + oxygen \leftrightharpoons oxyhaemoglobin $$
Oxygenated blood is blood with oxygen while deoxygenated blood is blood without oxygen.
Red blood cells have no nucleus so they can contain more haemoglobin and have a biconcave structure which aids diffusion. The biconcave structure increases the surface area of the blood and, combined with the lack of a nucleus, allows the blood cells to be flexible for when travelling through capillaries. It means, however, that blood cells only live for a few months (average of 120 days) as their membranes are worn away and they lack many organelles like the nucleus. Thin membranes also help with diffusion. RBCs are made in the bone marrow of some larger bones like the femur.
Plasma
Approximately 55% of blood is plasma (the substance) and plasma is approximately 95% water. Water has chemical properties (such as polarity) that make it a really good solvent (fluid that dissolves solutes) which is essential for plasma as it is where nutrients such as glucose are dissolved. Carbon dioxide produced as a waste product of aerobic respiration also dissolves into the plasma. Plasma carries blood cells, hormones and urea around the body in the blood.It also distributes heat around the body.
Red Blood Cells
Red blood cells (erythrocytes) carry oxygen around the body for aerobic respiration. They contain a lot of haemoglobin in their cytoplasm which is an iron-containing protein that reacts temporarily and reversibly with oxygen: $$ haemoglobin + oxygen \leftrightharpoons oxyhaemoglobin $$
Oxygenated blood is blood with oxygen while deoxygenated blood is blood without oxygen.
Red blood cells have no nucleus so they can contain more haemoglobin and have a biconcave structure which aids diffusion. The biconcave structure increases the surface area of the blood and, combined with the lack of a nucleus, allows the blood cells to be flexible for when travelling through capillaries. It means, however, that blood cells only live for a few months (average of 120 days) as their membranes are worn away and they lack many organelles like the nucleus. Thin membranes also help with diffusion. RBCs are made in the bone marrow of some larger bones like the femur.
Platelets
Platelets are cell fragments used for clotting blood to prevent excessive blood loss when wounded. When you are cut, the blood vessels constrict to reduce the speed of blood flow and thus blood loss. The tear in the blood vessel causes collagen fibres to react with the platelets, causing the now glue-like platelets to gather at the site of the wound. The air that the platelets and tissue are exposed to cause them to produce a chemical that causes a soluble plasma protein (fibrinogen) to change into insoluble fibres (fibrin) which hold the tear together and eventually aids in the reparation of the blood vessel. Before the vessel is repaired, blood cells get trapped as opposed to spilling out forming a blood clot which tends to develop into a scab.
Phagocytes and Lymphocytes
There are many types of white blood cells including phagocytes and lymphocytes whose main function is to protect the body from pathogens.
About 70% of wbcs are phagocytes which ingest pathogens and microorganisms via phagocytosis ('cell-eating'). Phagocytes are large cells with a lobed nucleus that ingest and destroy cells by changing their shape (which they can due better due to the nucleus' shape). When a phagocyte detects antigens on a cell that shouldn't be in the body, it will change its shape to produce projections of their cytoplasm (pseudopodia) which surround the microorganism and ingest it into a 'vacuole'. Then, digestive enzymes are secreted into the vacuole to break down the microorganism, killing it.
Platelets are cell fragments used for clotting blood to prevent excessive blood loss when wounded. When you are cut, the blood vessels constrict to reduce the speed of blood flow and thus blood loss. The tear in the blood vessel causes collagen fibres to react with the platelets, causing the now glue-like platelets to gather at the site of the wound. The air that the platelets and tissue are exposed to cause them to produce a chemical that causes a soluble plasma protein (fibrinogen) to change into insoluble fibres (fibrin) which hold the tear together and eventually aids in the reparation of the blood vessel. Before the vessel is repaired, blood cells get trapped as opposed to spilling out forming a blood clot which tends to develop into a scab.
Phagocytes and Lymphocytes
There are many types of white blood cells including phagocytes and lymphocytes whose main function is to protect the body from pathogens.
About 70% of wbcs are phagocytes which ingest pathogens and microorganisms via phagocytosis ('cell-eating'). Phagocytes are large cells with a lobed nucleus that ingest and destroy cells by changing their shape (which they can due better due to the nucleus' shape). When a phagocyte detects antigens on a cell that shouldn't be in the body, it will change its shape to produce projections of their cytoplasm (pseudopodia) which surround the microorganism and ingest it into a 'vacuole'. Then, digestive enzymes are secreted into the vacuole to break down the microorganism, killing it.
About 25% of white blood cells are lymphocytes which create chemicals called antibodies - soluble proteins in plasma that destroy pathogens. Lymphocytes are around the same size as red blood cells and have large spherical nuclei. Antibodies work by attaching themselves to antigens - chemicals on cell surfaces that effectively identify cells. The antibodies which are produced by the lympocytes (some of which are on lymphocyte membranes) help to destroy pathogens by:
- causing pathogens to stick together - making them easier for phagocytes to destroy
- helping phagocytes identify pathogens (acting as a marker)
- causing bacterial cells to burst open
- neutralising toxins from pathogens
The Heart
Humans have a double circulatory system which pumps blood to the lungs then back to the heart and round the body. It is more efficient than a single circulatory system like that found in a fish. The part where blood is pumped to and from the lungs to be oxygenated is called pulmonary circulation while the part that pumps oxygenated blood around the body is called systematic circulation.
The Heart is essential to circulation and has many parts:
The Heart is essential to circulation and has many parts:
link to source which goes into extra detail about the heart
* The mitral valve is also called the bicuspid valve
*The tricuspid and bicuspid valves are often referred to as the atrioventricular valves
to remember which valve is which, the 'ri' in tricospid is the begining of right
*The aortic valve and the pulmonic (pulmonary) valve are often referred to as the semilunar valves
*The wall between the right side and left side of the heart is called the septum and prevents the mixing of oxygenated and deoxygenated blood
* The mitral valve is also called the bicuspid valve
*The tricuspid and bicuspid valves are often referred to as the atrioventricular valves
to remember which valve is which, the 'ri' in tricospid is the begining of right
*The aortic valve and the pulmonic (pulmonary) valve are often referred to as the semilunar valves
*The wall between the right side and left side of the heart is called the septum and prevents the mixing of oxygenated and deoxygenated blood
The Cardiac Cycle
The series of contractions and relaxations that move blood around the heart and thus around the body is referred to as the cardiac cycle:
CHD and CVD
Coronary Heart Disease (CHD) is the result of a build up of fatty substances (like cholesterol) that causes blockages in the coronary artery which can impede the blood supply to parts of the heart. This could prevent the cardiac muscle cells from respiring and prevent them contracting, resulting in heart attacks. It can be caused by (risk factors):
*high blood pressure causes atherosclerosis - damage to arteries that result in white blood cells moving in though they produce fat in the process
Risk factor - something that increases the risk of an event occurring
CVD - cardiovascular disease - the same issue (build up of fat in blood vessels) but somewhere else in the body (anywhere)
Heart Rate
The average human heart rate is around 70bpm (beats per minute) though this varies. Generally, the more athletic you are, the lower your heart rate when resting. Your heart rate changes according to the amount of energy you need/are using. When we exercise, our muscles need more energy so our heart rate increases...
The series of contractions and relaxations that move blood around the heart and thus around the body is referred to as the cardiac cycle:
- Blood enters the heart via the atria. The atria walls then contract increasing the blood pressure in the atria and forcing open the atrioventricular valves and moving blood into the respective ventricles (right-to-right, left-to-left) and out of the atria.
- When the ventricles fill with blood, they contract which, again, increases the blood pressure. This closes the bicuspid and tricuspid valves and pumps blood out through the semilunar valves.
- The right ventricle pumps deoxygenated blood (from the right atrium) through the pulmonic valve and the pulmonary artery into the lungs. The left ventricle pumps oxygenated blood (from the left atrium) through the aortic valve and the aorta to the body's cells. (As the left ventricle has to pump blood along a further distance, it needs a stronger contraction so has more cardiac muscle and a visibly thicker wall. )
- As blood leaves the ventricles, the higher pressure in the pulmonary artery and the aorta will result in the semilunar valves closing too, ready for the cycle to restart: Oxygenated blood comes in from the lungs through the pulmonary vein into the left atrium and the deoxygenated blood from the body's cells enters the right atrium via the vena cava.
CHD and CVD
Coronary Heart Disease (CHD) is the result of a build up of fatty substances (like cholesterol) that causes blockages in the coronary artery which can impede the blood supply to parts of the heart. This could prevent the cardiac muscle cells from respiring and prevent them contracting, resulting in heart attacks. It can be caused by (risk factors):
- heredity (genes)
- high blood pressure*
- a diet high in saturated fat/diet choices
- smoking
- stress
- a sedentary life style
- age
*high blood pressure causes atherosclerosis - damage to arteries that result in white blood cells moving in though they produce fat in the process
Risk factor - something that increases the risk of an event occurring
CVD - cardiovascular disease - the same issue (build up of fat in blood vessels) but somewhere else in the body (anywhere)
Heart Rate
The average human heart rate is around 70bpm (beats per minute) though this varies. Generally, the more athletic you are, the lower your heart rate when resting. Your heart rate changes according to the amount of energy you need/are using. When we exercise, our muscles need more energy so our heart rate increases...
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Your muscles need more energy for muscle contraction during exercise so your muscle cells need to respire more. This requires more oxygen and glucose which is transported in the blood. As more blood is needed, stroke volume (the volume of blood pumped per beat) increases. More respiration occurs in your muscle cells and so more carbon dioxide is produced as a result. The carbon dioxide diffuses into the blood and dissolves in the plasma, forming carbonic acid. When the blood reaches sensors in the aorta and the carotid artery (leading to the head), they detect the decrease in pH and the increase in carbon dioxide. This then sends impulses along the sensory nerve to the cardiac centre in the medulla (in the brain). The medulla then sends impulses along the accelerator nerve to the pacemaker causing it to send more electric shocks round the heart, increasing the heart rate.
When the carbon dioxide levels decrease, fewer impulses are sent to the cardiac centre so nerve impulses are sent down the decelerator nerve to the pacemaker to reduce the amount of electric shocks. The accelerator and decelerator nerves also increase and decrease blood pressure. |
link to source - picture taken from Edexcel iGCSE student Biology textbook
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Other scenarios that affect heart rate are:
Increase |
Decrease |
stimulants |
sedatives/depressants |
adrenaline (fight/flight hormone), cortisol (stress hormone) |
sleep |
Hormones are carried in the blood and can affect the heart rate as well.