Human Circulatory System Organs

Human circulatory systems are two-dimensional: blood circulation and lymph circulation. Circulatory systems allow the cells to exchange substances with the internal environment. However, it is involved in the regulation of body temperature, the function of the immune system, the endocrine glands, and the cooperation between systems. Thus, a stable internal structure of living things is formed.

Structure, Task and Functioning of the Blood Circulation System

The tissues and organs involved in the blood circulation are the heart, blood vessels and blood. Blood, the transport fluid, is pumped by the heart to the whole body and transmitted to blood vessels, tissues and organs. In this system, which is defined as closed circulation, the movement of blood is fast. Thus, the substances needed by living beings are transferred to the cells and waste materials are removed in a short time.

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The Structure and Functioning of the Heart

The heart is located slightly to the left of the chest cavity between the two lungs. It weighs about 300 g in an adult. The human heart has four chambers as two atria at the top and two ventricles at the bottom. The walls of the auricle are thinner than the ventricle and the auricle and ventricle are divided into two with a full curtain.

There are valves between the auricles and ventricles and in the areas where the arteries exit the ventricles. There are triple valves (tricusbit) between the right atrium and the right ventricle of the heart, and double valves (bicusbit) between the left atrium and left ventricle.

These valves, which open towards the ventricles, close after the blood pass and prevent the back flow of blood. There are semilunar valves in the regions connecting the ventricles to the arteries.

These valves allow blood to flow in the direction of the artery.

The upper and lower main veins are attached to the right atrium of the heart. The pulmonary artery comes out of the right ventricle. After the pulmonary artery, which carries the dirty blood, leaves the heart, it divides into two branches to go to the right and left lungs.

Four pulmonary veins, two from each lung, bring clean blood to the left atrium of the heart. The aorta (main artery) that comes out of the left ventricle transmits clean blood to the whole body.

The heart consists of three layers from the outside to the inside. The pericardium, a double-layered membrane, is located on the outer surface of the heart. The pericardial space between the membranes is filled with a slippery fluid that facilitates the work of the heart and protects the heart against external pressures. In the middle is the heart muscle called the myocardium, which allows the heart to work rhythmically.

Arteries emerging from the aorta divide into capillaries in the myocardial layer and form coronary vessels that feed the heart. Failure to provide enough nutrients and oxygen to the heart due to the obstruction or narrowing of the coronary vessels causes a heart attack (infarction). The innermost part is the endocardium, which covers the inner surface of the heart and consists of a single layer of epithelial tissue without blood vessels.

The Working Mechanism of the Heart

The heart works when the heart muscle contracts and relaxes. The control of the contraction events in the heart is regulated by the specialized tissues in its own structure. Hence the vertebrate heart

When it is removed from the body and placed in a suitable culture medium, the contraction can continue.

The contraction of the heart begins with the stimulation of the specialized muscle tissue in the right atrium wall called the sinoatrial (SA) node by the autonomic nervous system. The impulse created by the stimulation of the sinoatrial node spreads to both auricles and the auricles contract. The impulses emitted from here reach the atrioventricular (AV) node between the atria and ventricles.

The specialized muscle fibers originating from the atrioventricular node are called bundles of sensations. Bundles of feel spread all over the ventricles and the ventricles contract when impulses are transmitted to the ventricles. Thus, the heart is contracted and relaxed. Blood is continuously transmitted from the auricles to the ventricles and from there to the arteries.

Although the control of the work of the heart is provided by the specialized tissues in its structure, some factors affect the rhythmic contraction of the heart by stimulating the sinoatrial node. For example, stimulants such as sympathetic nerves, adrenaline and thyroxine hormones, caffeine, nicotine and theine, high fever, increased carbon dioxide concentration in the blood cause the heart to work faster. The parasympathetic nerve, the vagus nerve and acetylcholine hormone, slows down the accelerated heart rhythm and returns it to normal.

During the operation of the heart, the contraction of the auricles and ventricles is called systole, and the relaxation is called diastole. The contraction creates a driving force for the movement of blood. The contraction and relaxation of the auricles and ventricles are opposite to each other. While one contracts, the other relaxes.

During the relaxation of the auricles, the right atrium is filled with polluted blood with high carbon dioxide brought by the lower and upper main veins from the body, while the left atrium is filled with clean blood with high oxygen content from the lung veins.

When the auricles contract, the ventricles relax and fill with blood from the auricles. The contraction of the ventricles pumps blood to the arteries. Blood is pumped from the right ventricle to the pulmonary artery, from the left ventricle to the main artery called the aorta due to rhythmic contractions.

Each beat of the heart lasts 0.85 seconds. Auricles contract in 0.15 seconds, ventricles in 0.30 seconds. The heart rests in 0.40 seconds. With each contraction, 70 mL of blood is pumped from the heart into the arteries. The feeling of the rhythmical contraction and relaxation of the heart in the arteries is called a pulse. The pulse gives the number of heartbeats.

👉In a healthy adult human, the heart rate is 70 to 80 per minute.

The pressure exerted by the blood on the artery wall during the contraction and relaxation of the heart is called blood pressure or blood pressure. The pressure applied by the blood to the arteries during the contraction of the ventricles is defined as high blood pressure (systolic blood pressure), and the blood pressure at the moment of relaxation of the ventricles is defined as small blood pressure (diastolic pressure). In a healthy adult, at rest, blood pressure is 120 mm Hg and small blood pressure is 80 mm Hg.

Circulation of Blood in the Body

Human blood circulation is divided into two as small and large blood circulation.

Small blood circulation begins in the right ventricle of the heart and ends in the left atrium. The dirty blood coming to the heart is sent to the lungs to be enriched with oxygen and sent back to the heart called circulation.

In this 6-second event, the carbon dioxide-laden blood passing from the right atrium to the right ventricle is pumped into the lung artery by the contraction of the ventricle. In the alveoli of the lung surrounded by capillary blood vessels, oxygen passes into the blood, and carbon dioxide in the blood passes into the alveoli. Oxygen-enriched pulmonary veins bring blood to the left atrium of the heart. Thus, the small blood circulation between the heart and lungs ends.

Great blood circulation begins in the left ventricle of the heart and ends in the right atrium. In this 30-second event, the clean blood that passes from the left atrium to the left ventricle is pumped into the aorta by contraction of the ventricle. After leaving the heart, the aorta curves to the left, forming an arc and divides into arteries that carry blood to organs in both the upper and lower parts of the body.

Substance exchange between tissue cells and blood takes place in capillaries. Later, the blood circulation is completed by bringing the blood loaded with carbon dioxide to the right atrium of the heart through the lower and upper main veins.

Structure and Functions of Blood Vessels

There are three types of veins in the blood circulation system: arteries, veins and capillaries.

1. Arteries:

It is the veins that carry blood away from the heart and into the organs. All arteries except the pulmonary artery carry oxygen-rich blood.

Blood pressure is high in the arteries. The walls of the veins are thick and their diameters are small. Arteries consist of three layers from the outside to the inside. The outer layer is made of fibrous connective tissue and ensures that the vessel is resistant to blood pressure.

The middle layer is made up of smooth muscles. The elastic fibers in this layer facilitate the movement of blood with the flexibility it gives to the vessels. The inner layer consists of a single layer of flat epithelial tissue. This layer, called the endothelium, creates a slippery surface that allows blood to move easily.

In the movement of blood in the arteries; The blood pressure caused by the contraction of the ventricles, the movement of the smooth muscles in the arteries, gravity and the pushing of the blood coming from the back are effective.

2. Veins:

They are the vessels that bring blood from tissues and organs to the auricles of the heart. Like arteries, veins consist of three layers. However, there are few connective tissue fibers in the outer layer, and the amount of elastic fibers in the middle layer is very low. Blood pressure is low in the veins. The walls of these vessels are thinner and larger in diameter than the arteries. Therefore, it contains more blood. The flow rate of blood is slower than that of the arteries.

Gravity plays a role in the flow of blood from the upper parts of the body to the heart. In the veins that bring blood to the heart in the lower parts of the body, there are valves that open in one direction and prevent the back flow of blood due to gravity. In addition, the skeletal muscles around the vessels, the increasing volume of the chest cavity during breathing, the suction pressure created by the relaxation of the auricles are effective in the movement of the blood in the veins.

3. Capillaries:

Capillaries, called capillaries, are found between arteries and veins. Capillaries that provide substance exchange between blood and tissue fluid consist of a single layer of flat epithelial tissue (endothelium). Capillaries that wrap the whole body like a net create a large surface.

Tissue fluid is found between body cells and capillaries. Apart from blood cells and large blood proteins, substances that are dissolved in the plasma can pass through the pores in the endothelium of the capillaries. Substances dissolved in blood pass from capillaries to tissue fluid, while substances in tissue fluid pass into capillaries.

The flow rate of blood in the capillaries is very low. The low flow rate of blood facilitates the exchange of substances between blood and tissue fluid.

Blood pressure depends on the type and location of the vein. Blood pressure; Increase in heart rate, increase in blood volume and constriction of blood vessels increase. Blood pressure is in the highest aorta. As the blood moves through the arteries, blood pressure decreases. It reaches lower levels in capillaries and lowest in veins.

Substance Exchange Between Blood and Cells

Blood pressure and osmotic pressure are effective in substance exchange between blood and cells. Osmotic pressure is caused by the density of albumin and globulin proteins in the blood plasma. Since these proteins have large molecules, they cannot go out of the vein and the osmotic pressure of the blood is kept constant throughout the vein.

The blood pressure caused by the contraction of the heart decreases from the end of the artery to the end of the vein. Since the blood pressure at the arterial end of the capillaries is higher than the osmotic pressure, the water and dissolved substances in the blood plasma pass into the tissue fluid by diffusion.

The decrease in blood pressure towards the vein end of the capillaries causes the osmotic pressure to fall below the stable osmotic pressure along the vein. The fact that the osmotic pressure is higher than the blood pressure allows the water and dissolved substances in the tissue fluid to pass back into the blood. However, the white blood cells, some proteins and some of the water that leave the blood cannot enter the blood, but return to the bloodstream through the lymphatic system.

Structure and Duties of Blood

The blood circulating in our veins is a specialized connective tissue consisting of a fluid called plasma and different types of cells. The main task of blood is the transport process. It carries the oxygen it takes from the lungs and the nutrient monomers it takes from the digestive system to the cells, and the metabolic wastes formed in the cells to the excretory organs such as the kidney, lung and skin. Blood also takes hormones produced by the glands to target cells. However, blood also plays a role in regulating the body's water, acid and base balance and keeping the body temperature constant.

Our blood also plays an important role in the body's defense. This situation is provided by the white blood cells and antibodies in the blood. In addition, in case of injury, the bleeding is stopped by coagulation, and the entry of microbes through the wound opening is prevented and the loss of blood, which is a valuable substance, is prevented.

💗55% of blood consists of plasma and 45% of blood cells. If a portion of our blood is taken and an anticoagulant is dropped into it, and then centrifuged in a centrifuge, an intense red precipitate is seen at the bottom of the tube. The dense red precipitate is blood cells, the light yellow liquid on it is plasma.

Although approximately 90% of blood plasma, which constitutes 55% of blood, is water, dissolved salts are important elements of blood. However, our blood also contains a wide variety of substances such as plasma proteins (albumin, globulin, prothrombin, fibrinogen, antibodies), nutrients transported from one part of the body to another, metabolic residues, respiratory gases and hormones.

👉Serum is the name given to plasma separated from coagulation factors.

Cellular part that makes up 45% of blood; It consists of oxygen-carrying red blood cells (erythrocytes), white blood cells (leukocytes) involved in defense, and blood platelets (platelets) involved in coagulation. Let's take a closer look at the cells of the blood tissue.

a. Red blood cells (Erythrocytes):

It is the most abundant blood cells. There are about 25 trillion red blood cells in 5 L of blood in the whole body. They contain hemoglobin, which gives blood its red color. Hemoglobin is involved in the transport of oxygen and carbon dioxide. The main function of red blood cells is to carry O2, and their structure is also suitable for this function. Human red blood cells are shaped like small discs with centers thinner than their edges. This shape increases the O2 diffusion through the plasma membranes by increasing the surface area.

Mature red blood cells of mammals do not contain nuclei and organelles. This feature leaves more space for the hemoglobin pigment found in red blood cells. Hemoglobin is a pigment containing O2 carrier iron element. Since there are no mitochondrial organelles in red blood cells, ATP is obtained anaerobically. O2 transport would be less efficient if the red blood cells were aerobic and used some of the oxygen they carry.

Despite its small size, a red blood cell contains approximately 250 million hemoglobin molecules. Since each hemoglobin can bind four molecules of O2, a red blood cell can carry about one billion O2 molecules.

The number of red blood cells can vary from person to person, due to various factors. For example, 1 mm3 of blood contains 5 million red blood cells in men and 4 million in women. However, the higher the sea level, the higher the number of red blood cells. As the amount of oxygen in the atmosphere decreases at high altitudes, the number of red blood cells increases.

Red blood cells are 3-5 in the fetus. While it is produced in the liver and spleen between months of age, it is produced in the red bone marrow from the 5th month of pregnancy until the end of life. Red blood cells do not have active movement ability, so their transport from one place to another is provided by blood flow. The lifespan of red blood cells is approximately 120 days, and the red blood cells that complete their life span break down in the liver and spleen.

b. White Cells (Leukocytes):

They are cells that defend the body against microorganisms. The number of white blood cells increases when germs enter the body. The lifespan of white blood cells usually varies between 4 hours and 4 days. Some white blood cells turn into memory cells that can recognize foreign substances entering the body and can survive longer. All blood cells can be made by stem cells found in the bone marrow.

Lymphocytes do not phagocytosis. Those that mature in the bone marrow, which is the place of production, are called B lymphocytes, and those that mature in the thymus gland are called T lymphocytes. B lymphocytes fight microbes by producing antibodies. T lymphocytes provide cellular immunity.

c. Blood platelets (Platelets):

It is formed by breaking off the large cells of the bone marrow. They have no nuclei, live up to eight days, then break down in the liver and spleen. They are used in blood coagulation.

Blood clotting:

In the bleeding that occurs due to the injury of one place, some reaction process takes place in our blood in order to prevent blood flow from the vein. As a result of these reactions, the blood coagulates and the blood loss is prevented.

Occasional small cuts in daily life do not endanger our lives, these cuts and scrapes are repaired by clogging by the substances in the blood. However, blood clotting begins as a result of the contact of the damaged vessel wall with blood in larger cuts.

Blood flakes (thrombocyte), prothrombin and fibrinogen proteins take part in the coagulation process. Prothrombin and fibrinogen proteins

It is produced as inactive in the liver and given to the blood.

The coagulation process begins as a result of vascular injury. Damaged vessel shrinks and platelets adhere to connective tissue fibers in the damaged vessel wall, forming a platelet plug. If the vessel damage is minor, only the platelet plug stops blood loss completely. However, if the damage is large, additional fibrin strands must be formed.

Fibrinogen, which is constantly inactive in the blood, is the most important substance in the coagulation process. The active form of fibrinogen is fibrin. Fibrin, which is in the form of protein strands, collects the cells of the blood and collapses and forms the clot. When the inner wall of a vessel is damaged, the connective tissue in the vessel wall comes into contact with blood. Platelets adhere to collagen fibers of connective tissue. These platelets also make nearby platelets sticky with the adhesive substance they secrete. Platelets form a plug to immediately prevent blood loss.

Coagulation elements called thromboplastin, which are secreted from entangled platelets and damaged cells, mix with coagulation elements such as calcium ions (Ca ++) and vitamin K in the plasma. This mixture enables the inactive plasma protein called prothrombin to be converted to its active form, thrombin. Thrombin is an enzyme that catalyzes the conversion of fibrinogen to fibrin. With the fibrin fibers forming a braid, the damaged place is closed.

Tissue Incompatibility in Blood Tissue Transplantation

In order for tissue transplants to be successful, the antigens in the transferred tissue must be compatible with the tissue in the organism to which they are transferred. The immune system reacts when incompatible tissues are transplanted. The body creates antibodies against antigens in the transplanted tissue. Antibodies inactivate the antigens and tissue transplantation fails.

Today, the most successful transplant is blood tissue transplants between blood groups. Blood types are determined by antigens on the surface of red blood cells. These are A, B and Rh antigens. In order for blood transfusions to be successful, the antigens of the donor and recipient must be compatible. As an example of tissue incompatibility, antigen-antibody interaction in blood incompatibility that occurs due to Rh antigen between mother and fetus can be given. Blood incompatibility can occur when the mother is Rh– and the fetus is Rh +.

Disruptions in the placenta or minor bleeding due to pregnancy may cause fetal blood to mix with the mother's blood. When the mother encounters the Rh + antigen in the fetus, she creates antibodies against this antigen she does not recognize. Antibodies formed by the mother's immune system reach the fetus via the placenta and umbilical cord.

It causes the red blood cells of the fetus to break down. Accordingly, anemia and heart failure occur. Jaundice occurs in the fetus when bilirubin, which is formed as a result of the breakdown of red blood cells, enters the blood. Premature and stillbirths can be seen in blood incompatibility.

In general, as the antibodies in the mother do not reach the level that will harm the fetus in the first pregnancy, the baby can be born healthy. However, in the second and subsequent pregnancies, necessary precautions should be taken as there may be enough antibodies to be produced in the mother to harm the fetus. For this, a vaccine has been developed to be administered to the mother within 48-72 hours after birth. With this vaccine, Rh antigens that pass from the fetus to the mother are inactivated without stimulating the mother's defense system.

Relationship Between Lymph Circulation and Blood Circulation

As the blood passes through the artery end of the capillaries, it releases its contents into the tissue fluid, and when it approaches the end of the vein, it absorbs only 99 percent of this fluid. The remaining 1 percent of this fluid is not directly reabsorbed into the blood capillaries but enters the blood through the lymphatic system.

Lymphatic system; It consists of lymph capillaries, lymph vessels and lymph nodes. There is no special heart of lymph that allows lymph fluid to be pumped. Therefore, there is no lymph artery.

Like blood capillaries, lymph capillaries spread over a large part of the body and are closed at one end. Monolayer epithelial cell walls have high material permeability. Lymph capillaries are then connected to lymph veins.

Although the blood capillaries do not allow the proteins to exit into the tissue fluid, some protein and white blood cells pass into the tissue fluid. These proteins must be transported back into the blood. Proteins that exit the tissue fluid pass into the lymph capillaries and are transported to the blood via lymph. The fluid lost by the capillaries is called lymph or accumulation after entering the lymph capillaries by diffusion. Lymph fluid then enters the bloodstream. This combination of lymph and blood circulation also functions in the transfer of fats from the small intestine into the blood, as we see in the digestive system.

Lymph veins are larger in diameter than the veins in the bloodstream. Lymph veins also have valves that open in one direction just like the veins in the blood circulation. Since lymph is not pumped by the heart, the movement of the lymph is much slower than blood. In the lymph circulation, there are also lymph nodes that are made up of special clusters of cells where lymph vessels meet.

Lymph movement takes place from tissues and organs towards the heart, similar to those in veins. The contraction of the skeletal muscles, the pressure difference that occurs in the chest during breathing, the valves opening in one direction and the fluid from the back pushing the front fluid provide the movement of the lymph fluid. Lymph fluid is transported around the body in two ways and joins the bloodstream.

There may be disorders that prevent the lymph system from working. These disorders cause a large amount of fluid between the tissues to increase, causing a condition called edema. In case of edema, tissue fluid moves into the cell and causes swelling of the cells. If the edematous area is pressed with a finger, the pressed area will collapse like dough and it may become dent for a while.

The most important lymph nodes located on the pathways of lymph vessels are the spleen and tonsils. There are white blood cells in the lymph nodes that take part in body defense. As the body fights an infection, lymph nodes swell and become fragile as these cells multiply rapidly. This is why when we are sick, the doctor checks for swelling in the lymph nodes in your neck, armpits and groin.