DIDAGELOS MATTHAIOS

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LEIVADITIS VASILEIOS

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CIRCULATORY SYSTEM

Parts of the Circulatory System :

The circulatory system consists of three parts :

  • Heart
  • Vessels
  • Blood

Heart is a muscular organ that pumps blood through the body

Vessels are any of the arteries, veins and capillaries that carry blood through the body:

  • Arteries are tubes that carry blood away from the heart.
  • Veins are tubes that return blood to the heart.
  • Capillaries are the connection between arteries and veins

Bloodis a fluid that carries oxygen and nutrients to the tissues and takes away carbon dioxide and metabolic wastes from them.

How the circulatory system works :

The circulatory system has three subsystems:

  • Systemic circulation
  • Pulmonary circulation
  • Coronary circulation

The systemiccirculation starts from the left ventricle and ends at the right auricle. Blood, from the left ventricle, passes through the aortic valve, travels through the aorta, common arteries, arterioles and reaches the capillaries. In the capillaries, nutrients and oxygen are released to the body cells while carbon dioxide and metabolic wastes enter the circulation.

From the capillaries start the venules. Venules continue as veins and end up as superior and inferior vena cava.

The pulmonarycirculation starts from the right ventricle and ends at the left auricle. Blood, from the right ventricle, passes through the pulmonary semilunar valve, travels through the pulmonary artery and reaches the lungs. There blood is supplied with oxygen and releases the CO2. After this oxygen-rich blood returns through the pulmonary vein to the left auricle.

The coronarycirculation consists of the right and left coronary arteries which start from the ascending aorta. Then blood is gathered from the major, minor and middle coronary veins and ends up at the right auricle. This is the way the cardiac muscle is supplied with blood.

The Heart

External structure of the heart:

The heart is surrounded by a hymen which is called pericardium,a tough, double-layered sac. The inner layer of the pericardium, known as the epicardium, rests directly on top of the heart muscle.The outer layer of the pericardium is attached to the breastbone and other structures in the chest cavity and helps hold the heart in place. Between the two layers of the pericardium is a thin space filled with a watery fluid that helps prevent these layers from rubbing against each other when the heart beats.

The heart is contracted by a special muscle which is called myocardium. Myocardium has common properties with both the skeletal and the smooth muscles:

The Three Types of Muscles
Type of Muscle
/
Smooth Muscle
/
Cardiac Muscle
/
Skeletal Muscle
Appearance
/ Smooth / Striated / Striated
Voluntary or Involuntary / Involuntary / Involuntary / Voluntary
Function
/ Controls movement of internal organs. / Controls contractions of the heart. / Moves bones. Skeletal muscles work in pairs. When one contracts, the other relaxes.
They are attached to bone by bands of tissue called tendons.

As we notice,

Both skeletal and cardiac muscle are striated and

Both smooth and cardiac muscle are involuntary.

Internal structure of the heart:

The inner surfaces of the heart's chambers are lined with a thin sheet of shiny, white tissue known as the endocardium. The same type of tissue, more broadly referred to as endothelium, also lines the body's blood vessels, forming one continuous lining throughout the circulatory system. This lining helps blood flow smoothly and prevents blood clots from forming inside the circulatory system.

How the heart works:

The heart, a muscular organ, positioned behind the ribcage and between the lungs. Its size is about a clenched fist and it weights 280-350gr at the male adult and about 230-280gr at the female adult. The human hearthas four chambers. The upper two chambers, the right and left auricles, are the receiving chambers for blood. They collect blood that pours in from veins. The lower two chambers, the right and left ventricle, are the pumping chambers for blood.The right and left sides of the heart are separated from each other by a wall of tissue. Each side pumps blood through a different circuit of blood vessels: The right side of the heart sends oxygen-poor blood to the pulmonary circulation, while the left side of the heart sends oxygen-rich blood both to the systemic and coronary circulation.

Blood returning from a trip around the body has given up most of its oxygen and picked up carbon dioxide in the body's tissues. This oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right auricle of the heart. The right auricle conducts blood through the tricuspid valveto the right ventricle, and the right ventricle pumps blood into the pulmonary artery through the pulmonary semilunar valve.

The blood, oxygenated from the lungs, returns to the heart through the pulmonary veins, which empty into the left auricle. Blood passes from the left auricle through the mitruid valveinto the left ventricle, from where it is pumped out of the heart into the aorta through the aortic semilunar valve, the body's largest artery. Smaller arteries that branch off the aorta distribute blood to various parts of the body.

The four valves within the heart help prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow, but when blood pushes against the valves in the opposite direction, the valves close. Two of the valves are located between the auricles and ventricles, and are known as atrioventricular valves. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve, while the left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve.

Although the right and left halves of the heart are separate, they both contract in unison, producing a single heartbeat. The sequence of events from the beginning of one heartbeat to the beginning of the next is called the cardiac cycle. The cardiac cycle has two phases: diastole, when the heart's chambers are relaxed, and systole, when the chambers contract to move blood. During the systolic phase, the auricles contract first, followed by contraction of the ventricles. This sequential contraction ensures efficient movement of blood from auricles to ventricles and then into the arteries. If the auricles and ventricles contracted simultaneously, the heart would not be able to move as much blood with each beat.

During diastole, both auricles and ventricles are relaxed, and the atrioventricular valves are open. Blood pours from the veins into the auricles, and from there into the ventricles. In fact, most of the blood that enters the ventricles simply pours in during diastole. Systole then begins as the auricles contract to complete the filling of the ventricles. Next, the ventricles contract, forcing blood out through the semilunar valves and into the arteries, and the atrioventricular valves close to prevent blood from flowing back into the auricles. As pressure rises in the arteries, the semilunar valves snap shut to prevent blood from flowing back into the ventricles. Diastole then begins again as the heart muscle relaxes-the auricles first, followed by the ventricles-and blood begins to pour into the heart once more.

Arteries, Veins, Capillaries:

All these are the “tubes” the circulatory system uses in order to carry blood throughout the body. However there are some interesting differences between these pathways of blood transportation.

Arteries start from the left ventricle (with the ascending part of the aorta) and become smaller and smaller as they gradually change into capillaries. The main role arteries have to complete, is to help the motion of the blood and control its flow before it goes into the tiny capillaries. Arteries are able to perform this because of the smooth muscle fibers they have into their walls, which are thicker and stronger of the walls of the veins or of the capillaries.

Capillaries represent the junction between arteries and veins and start after the arterioles. Their wall consists of only one layer of cells, that is called endothelium. This is to make easier the gas exchange between blood cells and tissues (blood O2 tissuesCO2 blood)and also the exchange of metabolic products. Blood removes metabolic wastes from the tissues and supplies them with nutrients, achieving this with the help of two kinds of pressure:

the hydrostatic pressure, which pushes water and nutrients out at the beginning of the vessel

and the osmotic pressure that helps the water and the wastes enter within the vessel at its venous end.

Capillaries continue then as venules that give the greater veins. Veins return blood to the heart but they can also be used as blood deposits at certain parts of the body, such as the skin, the liver or other organs and provide increased amounts of blood when necessary, for example in a sudden blood loss. Their wall contains a few smooth muscle fibers and is weaker than the artery wall although their lumen is wider.

What is blood:

Blood is the only liquid tissue of the body. It consists of four major elements:

Red blood cells: These cells carry oxygen (O2) from lungs to body cells and carbon dioxide (CO2) from body cells back to lungs to be exhaled. They have no nucleus and live about 120 days. Their most significant protein is hemoglobin(Hb) which takes part in the gas exchange. Their shape is double concave and have great elasticity, so that they are able to pass through capillaries whose diameter is smaller than theirs. They are able to do this because of the protein spectrin that creates an flexible net at their inner surface. Their number is about 5.5 million per mm³ for males and about 4.5 millionper mm³ for females. Their number is controlled by a hormone called erythropoietin which is produced by the kidneys.

Platelets:They are small cytoplasmic fragments of mega cytocytes and help blood accumulation. Their principle role is to begin the process of coagulationwith a sequence of reactions whenever a vessel is damaged. Their number is about 300.000/mm³.

White blood cells (leukocytes): Their major role is to fight germs that infect the body. Their number is normally between 4.000-8.000/mm³ and they are divided into to basic categories:

Granular

Non granular

In the second category belong the monocytes and the lymphocytes. Lymphocytes become mature either in the bone marrow or in the thymoid gland and so they are called B- or T- cellsand take part in the body defence with a special reaction called immune response.

When an antigen (germ, polypeptide chain, nucleic acid) enters the body B-cells are differentiated into plasma cells which secrete antibodies in order to eliminate the antigen (humoral immunity).

There are three types of T-cells: TH (Helper cells), TC (Cytotoxic cells) and TS (Suppresor cells).

TCcells act as an army and attack germs or cells of the organism that have been infected by an antigen (cellular immunity).

TH cells act as activators for both B and TC cells.

TScells are used in order for the immune response to be suppressed.

Plasma: Plasma is a yellowish liquid that consists mostly of water and plays the role of the extracellular fluid for the blood cells. It also contains proteins and salts. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated. Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body. Plasma carries a large number of important proteins, including albumin, gamma globulin, and clotting factors.

  • Albumin is the main protein in blood. It helps regulate the water content of tissues and blood.
  • Gamma globulin is composed of tens of thousands of unique antibody molecules.
  • Clotting factors, such as fibrinogen, are involved in forming blood clots that seal leaks after an injury. Plasma that has had the clotting factors removed is called serum.

Functions of blood:

Summarizing we could tell that the main functions of the blood, that pose and its importance are:

  • Transportation of oxygen, carbon dioxide, hormones, metabolic products (proteins, sugars, fatty acids, salts: Na, Cl, Ca etc.)
  • PH’s regulation of the body’s liquids
  • Coagulation of blood in a blood loss
  • It takes part in the immune response and in the general defence of the body with its lymphocytes
  • It helps the regulation of the body temperature with the contraction or not of the vessels

Transportation: Erythrocytes carry the oxygen from the alveoli of the lungs to the tissues where they are supplied with carbon dioxide and release it at the lungs. Blood also carries hormones from one part of the body to another. Hormones are the chemical messengers of the body and regulate metabolism.

PH: Normalblood pH ranges from 7,38 to 7,42. It is regulated by four systems :

  • H2CO3/NaHCO3 = 1/20
  • NaH2PO4/NaHPO4 = 1/4
  • System of Hb/HbO2
  • System of plasma proteins

Coagulation: As soon as an artery or vein is injured, the platelets in the area of the injury begin to clump together and stick to the edges of the cut. They also release messengers into the blood that perform a variety of functions: constricting the blood vessels to reduce bleeding, attracting more platelets to the area to enlarge the platelet plug, and initiating the work of plasma-based clotting factors, such as fibrinogen. Through a complex mechanism involving many steps and many clotting factors, the plasma protein fibrinogen is transformed into long, sticky threads of fibrin. Together, the platelets and the fibrin create an intertwined meshwork that forms a stable clot. This self-sealing aspect of the blood is crucial to survival.

Immune response: In the blood we can find white blood cells which eliminate antigens that enter the body. Monocytes and neutrophils provoke the innate (non-specific) immunity, while B- and T- cells provoke the adaptive (specific) immunity.

Temperature regulation: When blood reaches the skin there is a heat loss otherwise blood cannot reach the skin due to the contraction of the blood cells.

Conditions associated with the heart:

Angina

Angina is chest pain caused by coronary heart disease, a partial blockage of the coronary arteries. If you have angina, your heart may not get enough blood, especially when you exercise or are under stress.

Signs and Symptoms

• Pressing or squeezing pain in the chest

• Pain lessens in a few minutes when you rest or take medication prescribed by your health care provider

What Causes It?

Coronary heart disease is the root cause of angina. Some risk factors for developing angina are older age, male sex, menopause, family history of angina, diabetes, smoking, high cholesterol, high blood pressure, obesity, sedentary lifestyle, and stress.

Diagnosis

You will have an electrocardiogram (EKG), during which electrodes will be fastened to your chest with a sticky gel. Your health care provider may also suggest a stress test, in which the EKG is taken while you walk on a treadmill or use a stationary bicycle. Your health care provider may recommend coronary arteriography, where a catheter is inserted through a small incision to inject a dye that makes your blood flow visible on an x-ray image. Any blockages in and around your heart will appear.

Treatment Plan

There are two main goals in treating angina. The first is to allow you to perform moderate exercise without pain. The second is to treat the underlying heart disease and prevent it from getting worse.