The heart tissue itself can be subdivided into three layers: (from the outside in) epicardium, myocardium, and endocardium. The epicardium is the outermost layer of the
heart and consists of a loose connective tissue of fibroblasts, collagen fibers, and adipose tissue. It contains a stroma which houses coronary arteries and veins that are surrounded by a layer of fat. These coronary branches penetrate the myocardium.
The myocardium contains the main muscle mass of the heart and is mostly made up of striated muscle cells. Each of the cardiac muscle cells contain one central elongated nucleus with some central euchromatin and some peripheral heterochromatin. The two atria have a very thin myocardial layer which increases in thickness as you go from the atria to the right ventricle and into the left ventricle. The outer surface of the myocardium, next to the epicardium, is not composed of smooth muscle but is very smooth in texture. The inner surface of the myocardium is rough and is raised into trabeculations. The ventricular papillary muscles, which are for the attachment of the chordae tendinae, are extensions of the myocardium even though they are covered by the endocardium. The outer layer of the myocardium is superficial bulbospiral and swirls around the ventricle in a clockwise fashion. The middle layer is circular muscles that are the ventricular constrictors. The inner layer, which is deep bulbospiral, swirls around the ventricle in a counterclockwise fashion.
The layer underneath the myocardium is known as the enodcardium. It contains a continuous smooth endothelial layer that covers all the inner surfaces of the heart, including the valves. The outer layer of the endocardium, underneath the myocardium, is irregularly arranged collagenous fibers that may contain Purkinje fibers/cells. The inner part of the endocardium contains more regularly arranged collagen and elastic fibers than the outer layer. Some myofibroblasts are present in the endocardium which is thicker in the atria than in the ventricles. There is a subendothelial component of the endocardium underneath the endothelium. The component contains fibroblasts, scattered smooth muscle cells, elastic fibers, collagen fibers, and an amorphous ground substance that contains glycoproteins and proteoglycans.
The valves of the heart are attached to the cardiac skeleton and consist of chondroid, which is a material resembling cartilage. The base of each valve is supported by a fibrocollagenous ring. Each valve also has a dense fibrocollagenous central plate that is covered by simple squamous epithelium. Chordae tendonae connect with the valves at the edge of each cusp as well as underneath each cusp at one end and they attach to papillary muscles in the ventricles at the other end. Endocardial endothelium completely covers the papillary muscles, valves, and the chordae tendonae. The junctions between the cusps of each valve are known as commissures.
In order for the heart to maintain the nearly 60.000 miles of circulatory path, the heart itself must ‘feed’ itself with a supply of blood. The arteries that provide the nutrients are known as the coronary arteries. These arteries encircle the entire heart and originate just above the aortic valve. They drain the blood from the pockets formed by the cusps of the valves. The left coronary artery divides into the anterior descending artery. It carries blood down the front of the heart to both ventricles. As well to the circumflex artery which winds around the back of the heart to feed the left ventricle and atrium. There is a right coronary artery that curves around the heart, sending the marginal artery along the front for the right ventricle and atrium. The next artery is the posterior descending artery. It travels down the back of the of the heart, sending smaller arteries into each ventricle. The cardiac veins carry the blood back to the coronary sinus. It drains into the right atrium.
Many heart diseases and problems can be attributed to a problem with the coronary arteries. It occurs when the blood flow through the arteries (coronary) can be blocked by such things as blood clots (thrombosis), causing a heart attack if total blockage occurs. Atherosclerosis, which partial blocking may lead to angina pectoris coronary spasm. This temporarily cuts off blood supply. In some cases however, problems with the coronary arteries can be looked after. Specially designed catheters are used in cases of blocked arteries. With a percutaneous transluminal coronary angioplasty (PTCA) a catheter with a small balloon is feed into the target artery. It is then inflated to increase the area of flow for the blood. Other ways used to remove the plaque on the arteries is to use different catheters with different settings. A laser may be used to burn the plaque away, or using a shearing tool to scrape the plaque off. Instead of trying to open a blocked artery, another method of ‘fixing’ the problem is to bypass the clog and insert an alternate method for which the blood can get to the tissue. This process is called a coronary bypass surgery. In this procedure, veins are taken out from the leg, the saphenous vein, or from arteries near the collarbone and one end is attached to the source of oxygenated blood and the other end just after the clog.
The conducting system of the heart consists of four main components; the sinuatrial node (SA), the atrioventricular node (AV), the bundle of His, and the Purkinje fibers or cells. All the parts of this conducting system are composed of modified cardiac muscle cells. The SA node is located in the right atrium, at the point where the superior vena cava enters. The small muscle fibers of the SA node contain a central nodal artery
and desmosomes. The muscle fibers do not contain intercalated discs. The AV node is located in the medial wall, in front of the opening of the coronary sinus and above the tricuspid ring. Its small muscle fibers are more regularly arranged than those of the SA node. The AV node contains a rich nerve and blood supply. The bundle of His has a right (single bundle) and a left (branched bundle) bundle branch located underneath the endocardium. It is similar to the other components of the nerve system. The Purkinje fibers/cells can be found in clusters of about six cells which are located under the endocardium in the ventricles. The cytoplasm of Purkinje fibers appears pale under the microscope and contains many glycogen granules. In the diagram below, we can see what electrical impulses pass along the Purkinje fibers during a single heart beat. The stages can be seen on an electrocardiogram (ECG). The current that flows through the atria produces a peak in electrical impulse. This is known as the P wave. As the impulse crosses the AV node, the ECG becomes level. This is the PR segment. The impulse then passes the muscular ventricals, and a larger peak occurs, known as the QRS complex. When the electrical system recovers for the next impulse, the T wave appears.
We will see how ECGs can give important information about the health of ones heart. It can be seen in the following diagram of the ECGs of various persons with a normal ECG and ones with abnormal ECGs.
The principle function of the heart and circulatory system is to provide oxygen and nutrients and to remove metabolic waste products from tissues and organs of the body, as mentioned before. The heart is the pump that provides the energy necessary for transporting the blood through the circulatory system in order to let the exchange of oxygen, carbon dioxide, and other molecules through the thin-walled capillaries. The contraction of the heart produces changes in pressures and flows in the heart chambers and blood vessels. The mechanical events of the cardiac cycle can be divided into four periods: late diastole, atrial systole, ventricular systole, and early diastole.
In late diastole, the mitral and tricuspid valves are open and the pulmonary and aortic valves are closed. Blood flows into the heart throughout diastole thus filling the atria and ventricles. The rate of filling declines as the ventricles become swollen, and the cusps of the atrioventricular valves start to close. The pressure in the ventricles remains low throughout late diastole.
In atrial systole, contraction of the atria forces more blood into the ventricles, but approximately 70 percent of the ventricular filling occurs during diastole. Contraction of the atrial muscle that surrounds the openings of the superior and inferior vena cava and pulmonary veins, narrows their openings and the blood moving towards the heart tends to keep blood in the heart. However, there is some regurgitation of blood into the veins during atrial systole.
At the start of ventricular systole, the AV valves close. The muscles of the ventricles initially contract relatively little. Intraventricular pressure rises sharply as the muscles squeezes the blood in the ventricle. This period of isovolumetric ventricular contraction lasts about 0.05 seconds until the pressures in the ventricles exceed the pressure in the aorta and in the pulmonary artery and the aortic and pulmonary valves (semilunar valves) open. During this isovolumetric contraction, the AV valves bulge into the atria, causing a sharp rise in atrial pressure. When the semilunar valves open, the phase of ventricular ejection begins. Ejection is initially rapid, but slows down as systole continues. The intraventricular pressure rises to a maximum and then declines before ventricular systole ends. Late in systole, the aortic pressure is higher than the ventricular pressure, but for a short period, momentum keeps the blood moving forward. The AV valves are pulled down by the contractions of the ventricular muscle, and the atrial pressure drops.
In early diastole, after the ventricular muscle if fully contracted, the already falling ventricular pressure drops even more rapidly. This is the period known as protodiastole and it lasts about 0.04 seconds. It ends when the momentum of the ejected blood is overcome and the semilunar valves close. After the valves are closed, pressure continues to drop rapidly during the period of isovolumetric relaxation. Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open, allowing the ventricles to fill. Filling is rapid at first, then slows as the next cardiac contraction is about to happen. Atrial pressure continues to rise after the end of ventricular systole until the AV valves open. At this time it drops and slowly rises again until the next atrial systole.
Blood vessels are of great importance to the circulatory system, since it is in them that the life fluid is carries. Since there is a need of a transport system early on in life, within the embryo, these organs are among the first to develop. Blood vessels consist of a closed system of tubes which transport blood all over the body and back to the heart.
Arteries take blood away from the heart under high pressure. The pressure is exerted by the pumping of the heart itself. The blood is forced into these tubes, which are elastic, causing them to recoil. This sends blood in pulsing waves. Since there is high pressure and pulsating movements in the artery, it must be strong for the fast and efficient delivery of blood. The wall of the artery has three main layers. The inner surface is smoother endothelium covered by a surface of elastic tissues. The two form what is called the tunica intima. The tunica is thicker in arteries and consists of smooth muscle cells with elastic fibers. The outer layer is called the tunica advetitia. It is the strongest of the layers. It is made up if collagenous and elastic fibers. This layer helps the artery from over-expanding. There is also a small blood vessel, vasa vasorum, in this layer to provide walls of the larger arteries with nourishment.
When there is the transition between artery and arteriole, it is very gradual. It is when there is a thinning of the vessel wall and a decrease in the size of the passageway. A single layer of circular and/or spiral smooth muscle fibers now make up the tunica media. The tunica adventitia consists of tissue elements.
As the arterioles get smaller in size, the three coats become less and less definite.
As capillaries come together, there are small venules that form. There function is to collect the blood from the capillary bed, which is the network of capillaries. The venules are made up of small amounts of collagenous tissue. As venules increase in size, the seem to have the same characteristics for their walls as those of arteries.
The functions of veins is to conduct blood from the outer tissues to the heart. There is an endothelial lining that is surrounded by the tunica media. However, this all contains much less muscle and elastic tissue that is found in the arteries. Since there is a low blood pressure exerted in the vein, a mechanism is required to ensure that there is no back-flow of blood. So, valves are used in order to keep the flow in one direction.
The valves is the vein are formed by semilunar folds in the tunica intima. As blood flows towards the heart, the flaps of the valve are flattened against the inner wall, leaving an open passage for the blood to flow in. There are more valves in veins on the extremities of the body, since that is where there is the least amount of blood pressure. Veins seem to follow a parallel course to that of the arteries but there are more veins than arteries. Veins also have a greater capacity than arteries but with thinner walls. About 60% of the blood in the body is contained in the regular systematic circulation, with 40% in the veins.
The pulmonary trunk is the stem of the pulmonary arteries. It arises from the top surface of the right ventricle. It rises 4-5 centimeters above the surface before it divides into the right and left pulmonary arteries. These arteries go to the lungs. The right and left pulmonary arteries are short but have a large diameter. The walls are versatile enough to serve the stroke volume of the right ventricle.
The pulmonary artery has to operate under high pressure in order to function properly to handle the large amounts of deoxygenated blood that comes out of the right ventricle and to the lungs. In contrast, the pulmonary vein only works with blood under low pressure, as they return oxygen rich blood to the left atrium.
The blood itself is made up of many specialized cells that are suspended in a liquid medium which is plasma. The circulating blood is always supplying oxygen and nutrients as well as other metabolic molecules. Blood also takes material away from the cells that can be toxic such as carbon dioxide, or ‘good’ things such as chemical messengers. Blood itself is red in color and is denser and more viscous than water. It gets it red color from the hemoglobin in it. Hemoglobin is an iron containing protein. It brightens in color when it is carrying oxygen, known then as oxyhemoglobin, and darkens when oxygen is removed (deoxyhemoglobin). Red cells of blood makes up 45% of the blood and all the other cells such as white cells and platelets make up 1%. The rest is plasma. Plasma contains 90% water and it is freely exchangeable with other body cells. The main difference between plasma and the extracellular fluids is of the tissues is the high protein content of the plasma. The plasma also contains lipids, salts, glucose, amino acids, vitamins, hormones and waste products. The total amount of blood varies with age, sex, weight, body build as well as a few other factors. A rough average is about 60 milliliters per kilogram of body weight.
Blood cell formation is known as hematopoiesis. It takes place in hematopoietic tissue. In the embryo however, it occurs in the liver. The hematopoietic tissue is located in the bone marrow. Bone marrow is a mixture of developing as well as mature blood cells. All blood cells are born from primordial cells called multipotent hematopoietic stem cells. Through dividing and differentiating, these cells give rise to the major blood cells: red cells, phagocytic cells, megakaryocytes and lymphocytes. Arteries pierce the wall of the bone from the outside and enter the marrow. It then divides into fine branches combine into a large venous sac called the sinusoids. In the sinusoids, blood flows very slowly. The blood that had been developing in the bone marrow can enter the circulatory system by penetrating the walls of the sinusoids.
There are also components of blood that allow it to clot. Clotting is the solidification of blood where there is an injured blood vessel. When there is an injured blood vessel, platelets in the blood gather at the site of injury and stick to the wall of the vessel. When there is minor damage to the vessel, ruptured platelets seal the leak. When a more serious break occurs, the actual clotting process takes over. First, the ruptured platelets and the vessel wall release an enzyme known as thromboplastin. This enzyme begins a series of reactions. The result is the changing of prothrombin to a plasma protein thrombin. This enzyme then converts the soluble plasma fibrinogen into the insoluble form of fibrin. The fibrin acts as a net and traps red blood cells and platelets to form the clot. The newly formed clot stops the bleeding and contracts and hardens. The wound is later repaired by the growth of cells that replace the destroyed ones. A disease that is attributed with the inability for clot formation is known as hemophilia. It is caused by not having enough platelets and vitamin K in the body. Vitamin K is needed for the synthesis of prothrombin.