Circulatory system – structure, functions. How does the circulatory system work?

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The human circulatory system is the name for a closed system in which blood circulates, flowing through an extensive network of blood vessels. The heart of the circulatory system is the heart, which is responsible for pumping blood and thus maintaining its continuous circulation in the human body. The human circulatory system and the lymphatic system form the circulatory system.

Circulatory system – structure

The circulatory system supplies nutrients and oxygen to all cells in the body. It is made up of the heart and blood vessels that run throughout the body. Arteries carry blood away from the heart, veins carry it back to the heart. The blood vessel system resembles a tree: the “trunk” – the main artery (aorta) – branches into large arteries that lead to smaller and smaller vessels. The smallest arteries end in a network of tiny vessels, called the capillary network.

The circulatory system includes:

1. veins – responsible for supplying blood from tissues to the heart, the pressure in the veins is much lower than in the arteries,
2. arteries – thick, high-pressure vessels; transport blood from the heart to the rest of the body, providing oxygen and nutrients and regulating blood pressure,
3. capillary blood vessels, also called capillaries – thin vessels through which the exchange of nutrients from blood to tissues takes place,
4. heart – located in the mediastinum, surrounded by the pericardial sac. In the structure of the human heart, there are 2 atria and 2 heart chambers.

Moreover, blood itself can be included in the circulatory system. It is a liquid substance that includes:

1. plasma – responsible for maintaining the acid-base balance in the body and for the phenomenon of blood clotting,
2. erythrocytes – red blood cells transporting oxygen from the lungs to other tissues and cells of the body, and carbon transferred from the tissues to the lungs with the help of hemoglobin,
3. leukocytes – also known as white blood cells. There are several subtypes of leukocytes.

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Circulatory system – blood

Blood is a mobile component of the blood system. Blood is bright red when oxygenated and dark red / purple when deoxygenated. Blood has many different functions, including:

1. transport of oxygen and nutrients to the lungs and body tissues,
2. forming blood clots to prevent excessive blood loss
3. transport of cells and antibodies to fight infections,
4. delivering metabolic products to the kidneys and liver that filter and purify the blood,
5. body temperature regulation.

Blood is made up of a cellular component called plasma, which is a mixture of water, glucose, hormones, proteins, minerals, fats, and vitamins. The main task of plasma is to transport blood cells around the body along with nutrients, waste products, antibodies, clotting proteins, chemical messengers such as hormones, and proteins that help maintain fluid balance in the body.

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Blood system – erythrocytes

Known for their bright red color, erythrocytes (red blood cells) are the most abundant group of cells in the blood, making up about 40 to 45 percent of its volume. The red blood cell is shaped like a biconcave disc with a flattened center – in other words, both surfaces of the disc have shallow, bowl-like indentations.

The production of red blood cells is controlled by erythropoietin, a hormone mainly produced by the kidneys. Red blood cells are produced in the bone marrow and are released into the bloodstream after about seven days of maturation. Unlike many other cells, red blood cells do not have a nucleus and can easily change shape, helping them to fit into the various blood vessels in the body. However, while the absence of a nucleus makes the red blood cell more flexible, it also limits its viability as it passes through the tiniest blood vessels, damaging cell membranes and depleting its energy reserves. On average, red blood cells only survive for 120 days.

Red blood cells contain a special protein called hemoglobin that helps carry oxygen from the lungs to the rest of the body, and then returns carbon dioxide from the body to the lungs for it to be exhaled. Blood appears red because of the large number of red blood cells which receive their color from hemoglobin. The percentage by volume of whole blood that is made up of red blood cells is called hematocrit, and is a common measure of red blood cell levels.

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Blood system – leukocytes

Leukocytes (also called white blood cells) are the cellular component of blood that does not have hemoglobin, but has a nucleus and is able to move. They protect the body against infection and disease by: destroying foreign bodies and cellular debris, destroying infectious agents and cancer cells, or by producing antibodies. Leukocytes are produced by the bone marrow, and their levels are regulated by organs such as the spleen, liver and kidneys.

The following types of leukocytes are distinguished:

1. Lymphocytes: they are necessary for the production of antibodies that help the body defend itself against bacteria, viruses and other threats,
2. neutrophil: these are white blood cells that destroy bacteria and fungi. These cells have a single nucleus with many lobes. Neutrophils are the most abundant white blood cells in the circulation. They are chemically attracted to bacteria (by cytokines) and migrate through the tissue towards the sites of infection. Neutrophils are phagocytic (ie engulf and destroy target cells). Once released, their granules act as lysosomes, digesting cellular macromolecules, destroying neutrophils in the process.
3. basophils: alert the body to infection by releasing chemicals into the bloodstream, mainly to fight allergies. They are the least numerous type of white blood cells. They have a multi-lobed nucleus and their granules contain immune-boosting compounds such as histamine and heparin. Heparin thins the blood and inhibits the formation of blood clots, while histamine dilates blood vessels to increase blood flow and capillary permeability, allowing leukocytes to be transported to infected areas.
4. Eozynofile: they are responsible for destroying parasites and cancer cells and are part of the allergic response. The nucleus of these cells is biplane and U-shaped in blood smears. Eosinophils are usually found in the connective tissues of the stomach and intestines. They are also phagocytic and primarily target antigen-antibody complexes, formed when antibodies bind to antigens, signaling that they should be destroyed. Eosinophils are most active during parasitic infections and allergic reactions.
5. Monocytes: These cells are the largest in size of a white blood cell. They have a large, single testicle that varies in shape, but is most commonly kidney-shaped. Monocytes migrate from blood to tissue and develop into macrophages or dendritic cells.

See also: Monocytes – role, research, norms, interpretation of results. How to lower the level of monocytes?

Circulatory system – capillaries

Capillaries, the smallest and most numerous of blood vessels, form the connection between the vessels that carry blood away from the heart (arteries) and those that return blood to the heart (veins). The primary function of capillaries is to exchange materials between blood and tissue cells.

Capillaries provide tissues with nutrients and oxygen, and remove by-products from cellular reactions, such as carbon dioxide and water. Except for the lungs, where the opposite is true, the capillaries deliver oxygenated blood to the organs, carrying blood, and drain deoxygenated blood, blood with oxygen removed.

Their walls are very thin to allow substances to diffuse or pass through them easily and quickly. Capillaries are much thinner than arteries and veins because their walls are made up of only a single layer of endothelial cells, the flat cells that line all blood vessels.

Capillaries are selectively permeable, which means they let some substances pass and others not. Their permeability is what allows them to do their job, and their permeability varies depending on the organ or tissue they’re in.

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Circulatory system – heart

The heart is an organ that helps deliver blood and oxygen to all parts of the body. It is divided by a partition (or partition) into two halves. The halves, in turn, are divided into four compartments. The heart is located in the chest cavity and is surrounded by a fluid-filled bag called the pericardium. This amazing muscle produces electrical impulses that cause the heart to contract, pumping blood throughout the body.

There are four chambers in the heart that work together as a two-way pump. The left side of the heart pumps blood into the body through the arteries, while the right side of the heart collects blood through the veins.

The upper chambers of the heart are called the left atrium and the right atrium. The lower chambers of the heart are the left and right ventricles, which have thicker walls:

1. the right atrium receives blood from the veins that has already been circulating in the body and pumps it into the right ventricle,
2. the right ventricle transfers blood to the pulmonary artery, which sends it to the lungs for oxygen intake
3. the left atrium picks up oxygen-rich blood from the lungs and pumps it into the left ventricle 
4. the left ventricle pumps oxygen-rich blood into the body through a large network of arteries. The contractions of the left ventricle, the strongest of the four ventricles, create blood pressure in the body.

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A series of four valves opens and closes to allow blood to flow through the heart:

1. the tricuspid valve, which controls blood flow from the right atrium to the right ventricle
2. a pulmonary valve that regulates blood flow from the right ventricle to the pulmonary arteries
3. a mitral valve that opens to allow blood to flow from the left atrium to the left ventricle
4. an aortic valve, which allows oxygen-rich blood to flow from the left ventricle into the aorta, the largest artery in the body.

The heart itself needs oxygen-rich blood from the coronary arteries that run along its surface to function. The heart also contains a complex system of nerves that carry the electrical impulses that make the heart beat.

A two-layer bag called the pericardium surrounds the heart, with an inner layer attached to the heart muscle and an outer layer attached by ligaments to the spine, diaphragm, and other parts of the body. The fluid between the two layers of the pericardium allows it to move to the rhythm of the heartbeat.

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Blood system – arteries

Arteries make up the main part of the blood system, and the other main parts are the veins and the heart. Arteries form tube-like structures that transport fluids (i.e. blood to the circulatory system and lymph to the lymphatic system) to and from every organ in the body. Primarily, the arteries manage the transport of oxygen, nutrients, and hormones through the body.

Oxygen binds to hemoglobin and is carried through the arteries to areas where there is a lack of oxygen. By changing the affinity for oxygen, it is then discharged to specific areas through the capillaries. While not being a fixed structure, arteries adapt to signals from the central nervous system as they also respond to external stimuli such as pressure, temperature, and substances. Vascular nerves are responsible for the innervation of the arteries, allowing them to switch to stimuli. When catecholamines are released into the bloodstream, nerves send signals to the arteries to either narrow or expand, leading to changes in pressure.

The arteries are made up of smooth muscles that allow the constriction and expansion of the parasympathetic nervous system. Arteries differ from veins in that they most often carry oxygenated blood from the heart to the rest of the body. This is not always the case – when the pulmonary artery carries unoxidized blood from the heart to the lungs to complete the gas exchange in the alveoli.

Arteries play a key role in maintaining the body’s homeostasis. As we age, health problems start to appear as arteries stiffened or thickened. Additionally, the arteries start to clog with a thickening of the plaque. When problems arise in the structure of the arteries, it begins to put a greater strain on the heart which develops congestive heart failure and which is often fatal. Arteries are essential for maintaining a healthy cardiovascular system and thus a healthy lifestyle.

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Circulatory system – veins

Veins are a type of blood vessel that carry deoxygenated blood from the body to the heart. Human veins are one of three types of blood vessels that transport blood throughout the body, the other two being arteries and capillaries. The structure and function of the veins are designed to return blood to the heart from distant parts of the body at relatively low pressure.

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