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Nutrients keep us alive, and every cell in our body needs a constant supply of food and oxygen to function. In a simple single-celled organism, this can be easily achieved through diffusion. However, humans are complex organisms made up of trillions of cells. So how do we ensure that every cell, no matter how far from the surface, receives the substances it needs?
This is where the circulatory system comes in. Acting as the body’s transport network, it delivers oxygen and nutrients to cells while removing waste products such as carbon dioxide. Like a team of delivery couriers, the circulatory system ensures that materials are efficiently transported from one part of the body to another.
In this article, we will explore the key components of the circulatory system, the cardiac cycle, the exchange of materials between capillaries and tissue fluid, the components of blood, the ABO blood group system, and coronary heart disease.
The human circulatory system is a double circulatory system, meaning blood passes through the heart twice during one complete circuit around the body. The first passage sends blood to and from the lungs for gas exchange, while the second supplies the rest of the body with oxygen and nutrients and removes waste products.
The main components of the circulatory system are the heart, blood vessels and blood. In a double circulatory system, blood flow is divided into two circuits, the pulmonary circuit and the systemic circuit.
The pulmonary circuit carries blood between the heart and the lungs, allowing carbon dioxide to be exchanged for oxygen. The systemic circuit transports blood between the heart and the rest of the body, delivering oxygen and nutrients to tissues and carrying away waste materials.
The key players in the human circulatory system consist of:
The heart consists of four chambers, separated by a septum. These chambers ensure clear separation of oxygenated and deoxygenated blood. Blood flow through the heart is regulated by four one-way valves, which ensure that blood only flows in one direction.
The four chambers consist of two upper atria (right and left, singular: atrium) that receive blood into the heart, and two ventricles (right and left, singular: ventricle) that pump blood out of the heart. As the ventricles pump blood over longer distances compared to the atria, they have thicker, more muscular walls.
To prevent backflow of blood, the atria and ventricles are separated by atrioventricular valves. The valve separating the right atrium and ventricle is known as the tricuspid (it has three flaps!) valve, while the valve separating the left atrium and ventricle is known as the bicuspid (it has two flaps) valve. The bicuspid valve is also known as the mitral valve as it resembles a bishop’s hat when viewed at a certain angle.
The right atrium receives deoxygenated blood returning from the body through the venae cavae and passes it to the right ventricle through the tricuspid valve. The right ventricle then pumps this blood to the lungs through the pulmonary artery, where carbon dioxide is removed and oxygen is absorbed.
The left atrium receives oxygenated blood from the lungs and transfers it to the left ventricle through the bicuspid valve. The left ventricle then pumps oxygen-rich blood to the rest of the body through the aorta. Since it has to pump the hardest, the left ventricle has the thickest and strongest muscle walls. This blood also carries waste products, which are later removed by organs such as the kidneys.
Semilunar valves are located between the ventricles and arteries, and prevent the backflow of blood. These valves are shaped like half-moons, which is why they are termed “semilunar”. The valve between the left ventricle and the aorta is known as the aortic valve, while the valve between the right ventricle and the pulmonary artery is known as the pulmonary valve.
Apart from the chambers and valves, other key parts of the heart include the pericardium, which is a protective sac surrounding the heart, the coronary arteries, which supply the heart muscle with blood and the chordae tendineae and papillary muscles, fibrous cords and muscles that anchor the atrioventricular valves and prevent backflow.
The cardiac cycle is the sequence of events that occur within one heartbeat. This cycle involves coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles in order to pump blood throughout our bodies. The rhythmic filling and emptying of heart chambers, along with the opening and closing of atrioventricular valves, produce the characteristic “lub-dub” sound heard during each heartbeat.
During this step, the atria contract (systole) while ventricles remain relaxed (diastole). This pushes blood from the atria into the ventricles through the atriaventricular valves. At the end of this step, the ventricles are filled with blood and the valves close, resulting in the first “lub” sound.
The ventricles contract (systole) while atria remain relaxed (diastole). As the ventricles contract, blood is pushed out of the ventricles into the arteries leaving the heart through the semilunar valves. After the ventricles have been emptied, the semilunar valves close resulting in the second “dub” sound.
The blood vessels are the pipes that carry blood across our body. The three main types of blood vessels are the artery, vein and capillary.
Contrary to popular belief, arteries do not always carry oxygenated blood and veins do not always carry deoxygenated blood. Instead, arteries carry blood away from the heart (think A for away), while veins carry blood back towards the heart. Although detailed muscle structure is not required, both arteries and veins contain layers of muscle that can contract to constrict or relax to dilate the vessels, helping to regulate blood flow.
Meanwhile, capillaries are tiny, delicate blood vessels that link arteries and veins. Their thin walls allow oxygen, nutrients, and waste products to be exchanged efficiently between the blood and body cells.
Arteries carry blood away from the heart. While this means arteries usually carry oxygenated blood to the rest of the body, the pulmonary artery is an exception as it carries deoxygenated blood away from the right ventricle towards the lungs. The largest artery is the aorta, which carries oxygenated blood from the left ventricle to all parts of the body.
As arteries move blood at high pressure, their walls are thick, muscular and elastic to withstand and smooth out pressure changes. Blood flows rapidly through the arteries.
Veins carry blood from the rest of the body back towards the heart. Blood pressure in the veins is much lower than in the arteries, so the walls of the vein are thinner, less muscular and less elastic. Veins contain valves that prevent the backflow of blood, ensuring a one-directional flow towards the heart. Contraction of muscles such as in the legs also help to squeeze blood back up towards the heart.
Usually, the veins carry deoxygenated blood back to the heart, such as the venae cavae. However, the pulmonary vein carries oxygenated blood from the heart towards the lungs.
Capillaries are the smallest blood vessels. The walls of capillaries are extremely thin and comprise a layer of epithelial cells and a basement membrane, allowing diffusion and exchange of substances between the blood and cells within the body’s organs.
Blood leaves the heart through the arteries, which branch into smaller arterioles, eventually branching into capillaries where substance exchange takes place between blood and the body’s cells.
As the arteries branch into smaller vessels, the surface area of the vessels increase to facilitate diffusion and substance exchange. The speed of blood flow and blood pressure also decreases as blood flows from arteries into arterioles and capillaries.
After this exchange has taken place, capillaries join to form venules, which then merge into larger veins that return blood to the heart. The surface area decreases and the speed of blood flow increases slightly in the veins, while blood pressure continues to fall.
Blood is a tissue fluid made up of four main components, plasma, red blood cells, white blood cells and platelets. It circulates through blood vessels and is pumped by the heart to transport nutrients and oxygen to body tissues while removing carbon dioxide and other waste products.
Plasma is the yellowish liquid that makes up over half of the liquid volume in blood. It contains mainly water and substances such as glucose, salts, proteins, fats, hormones and excretory products like urea. Plasma acts as the transport medium for these substances throughout the body.
Red blood cells are circular, biconcave cells with no nucleus. The biconcave shape increases surface area for gas exchange, while the lack of nucleus provides more space for hemoglobin. Hemoglobin readily binds to oxygen, allowing red blood cells to transport oxygen efficiently around the body. These cells are also flexible, enabling them to squeeze through the narrowest capillaries to deliver oxygen.
There are two main types of white blood cells, phagocytes and lymphocytes. Both types are involved in immune responses, which protects us against infections. Phagocytes perform phagocytosis, engulfing and destroying foreign particles like bacteria. Lymphocytes on the other hand, are more specialized and can recognize specific pathogens, producing antibodies to neutralize them.
Like red blood cells, platelets do not have nuclei and are fragments of cell cytoplasm. Platelets plan an essential role in blood clotting by sealing breaks in blood vessels and stopping bleeding. Platelets release an enzyme that converts soluble fibrinogen to fibrin, which forms an insoluble mesh of fibres. This fibrin mesh traps other blood cells, creating a clot that seals the wound and prevents further blood loss.
The ABO blood group system classifies blood cells based on the type of antigen present on the red blood cell surface. Type A blood carries the A antigen, type B carries the B antigen, type AB carries both A and B antigens while type O carries neither.
As part of the body’s natural immune system, lymphocytes produce antibodies that recognise and neutralise foreign antigens. Hence, an individual with type A blood produces anti-B antibodies that recognise type B antigens, while type B produces anti-A antibodies.
As type AB blood contains both A and B antigens, the body does not produce any antibodies against either antigens. In contrast, type O blood contains no antigens but produces both anti-A and anti-B antibodies.
When antibodies encounter an antigen they specifically recognize, they bind to them and neutralize them, causing blood cells to clump together. This is why the ABO system is crucial for determining compatible blood types during transfusion. The general rule is that the donor’s blood must not contain antigens that would react with the recipient’s antibodies.
Based on these antigen-antibody combinations, individuals with type AB blood are known as universal recipients because they have neither anti-A nor anti-B antibodies. In contrast, individuals with type O blood are considered universal donors, as their red blood cells lack A and B antigens and can be safely given to any blood type.
You may have seen dramatic scenes in film where a character clutches their chest and collapses in pain. Heart attacks aren’t just for dramatic effect, they are very serious and can be life-threatening. A heart attack occurs when blood flow to part of the heart muscle is blocked, causing cells in that area to die, leading to damage of the heart tissue.
The most common cause of heart attacks is coronary heart disease, in which the coronary arteries become narrowed or blocked by a buildup of fatty deposits called plaque. The coronary arteries supply the heart muscle itself with oxygen-rich blood, allowing it to keep beating continuously. If one of these arteries becomes severely narrowed or blocked, the blood supply to the heart is greatly reduced, which can trigger a heart attack.
Not everyone is built equal, and some individuals may be more susceptible to coronary heart disease. Some risk factors include genetic factors, age, smoking, an unhealthy diet and a sedentary lifestyle. While we may not be able to choose our genetic makeup or control our age, we can make a choice to live more healthily and reduce our risk of coronary heart disease.
In this article, we explored the circulatory system, the body’s delivery network that ensures every cell receives the nutrients and oxygen it needs. The organs of the circulatory system, especially the heart, are therefore crucial for our survival. We also discussed the dangers of coronary heart disease, a major cause of circulatory system failure. So remember to stay healthy and take good care of your heart, as it works nonstop to keep you alive.
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Prepared by: Michelle
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