The Heart
The heart is just one part of the circulatory system. The heart is about 5.5 inches (14 centimetres) tall by 3.5 inches (9 cm) wide by 2.5 inches (6 cm) thick. Inside the heart, the walls are composed 3 layers. Those three layers are, epicardium, myocardium, and endocardium. The thin outer layer that is composed of fat, would be the epicardium. The perdicardium surrounds the the epicardium, it is a protective two-layer sac. The sac anchors the heart in position and holds fluid inside its layers. To help keep the heart running smoothly, this fluid lubricates the heart. The endocardium is the inner layer of the wall of the heart. This layer, is very smooth, it is made up of endothelial cells, these cells are also found in the inner linings of blood vessels. The endocardium keeps the blood flowing smoothly and controls the heart as it forms before a baby is born. The endocardium also helps to regulate heart beats. Myocardium is the thickest and strongest wall of the heart. It is the middle layer of the heart. Cardiac muscle is the muscle that the myocardium is made up of.
The Blood Vessels
What are blood vessels? Blood vessels carrry blood to and from the heart. They are a system of hollow tubes, that help to circulate blood. An interesting fact about the heart, would be if stretched out all the way, the blood vessels would extend for 60,000 miles! That would almost be enough to go around the world 3 times!! There are three different types of blood vessels, they are, arteries, veins, and capillaries. The difference between arteries and veins is that arteries carry blood away from the heart, and veins carry blood to the heart. The type of blood vessel that connects the two systems would be known as the capillaries. The blood vessels are composed of layers of flexible tissue. Below is a little bit about each of the three types of blood vessels.
Arteries
Of all three types of blood vessels the arteries are the strongest, thickest and most elastic of all vessels. Arteries are three-layered. The inner linning containing a thin smooth layer of endothelial cells is called the intima. The middle layer is made of strong muscle fibers that wrap around the vessels and can readily stretch to accommodate the pressure of pumping blood, this layer would be known as the media. The media layer expands, stys stretched out, and then relaxes when the heart beats. By touching the thin skin of the wrists, temples and behind the knees this movement can be felt. This movement is called the pulse. Lastly there is the outer layer of the arteries. This outer layer, called the externa layer is made of connective tissue that surrounds and protects the movements of the media layer. The aorta is the largest artery, after leaving the left ventricle of the heart it rises upward. It then forms an arch that reaches downward to the abdomen and separates into two arteries. All major arteries exiting the heart, except the plumonary artery branch off from the aorta. The right and left coronary arteries are the first to do so. Connected to the upward aorta are these vessels. Corneries carry to the heart, oxygen rich blood. The brachiocepahlic artery, the left common carotid artery, and the left subclavian artery are some of the other arteries that stem from the aortic arch. The thoracic and abdominal arteries are the arteries that branch from the descending aorta. These arteries split into large arteries that branch into medium sized arteries. Arterioles are the arteries that branch off into smaller arteries. Arterioles contril the flow of blood into the smallest vessels, the capillaries.
Capillaries
Tiny, thin-walled vessels that form a network throughout the body. These would be known as the 2nd of 3 vessels I will be talking to you about, the capillaries. In the capillary bed, which is a web of blood vessels, that blood's nutrients and oxygen supply are exchanged for carbon dioxide and cell waste. Capillaries are formed by a single layer of smooth endothelial cells. Outside of this layer is a protective coating. The pathways in the blood vessels are so narrow that only one red blood cell can travel though at a time.
Veins
The system of veins, or the venous system collects cell waste and carries it back to the heart. There is only one direction, that the blood circulates in when travelling around the body. In the smallest veins, called venules, the wastes and carbon dioxide exchanged in the capillaries and join the circulatory flow. The venous system consists of the venules, which are medium sized veins, larger veins, and the superior and inferior venae cavae, the two largest veins. Vein walls are not as strong as arteries, they are although very flexible. Vein walls are not as strong as arteries because, they do not need to withstand the force arterial blood pressure. Blood pressure is greatly reduced by the time blood reaches the veins. The venous circuit veins depend on the skeletal muscles in the arms, legs and lungs to make contractions that will help with the blood flow. To maintain the one-way flow of blood, and to keep the blood from pooling, the larger veins have a series of valves that open and close. As blood travels through the veins, it eventually empties into one of the two venae cavae, the superior vena cava or the inferior vena cava. Both empty into the right atrium.
Major Tissues in the Circulatory System
Epithelial Tissue
Epithelial tissue covers the whole body. It is ranged in one or more layers, and made up of cells closely packed. This tissues job, is to protect, or cover all of the internal and external body surfaces. Endothelium is the epithelial tissue that occurs on surfaces on the inside of the body. Inside of the human body, the epithelial cells are packed very tightly together. This means, that there is very little intercellular spaces inside. No matter when type of epithelial tissue, normally, it is separated from the underlying tissue by a thin sheet of connective tissue. This extra separating layer, is used to help with structural support for the epithelium.
Muscle Tissue
Muscle tissue is made of muscle cells, and it creates action or movement in your internal organs. Every muscle cell has a single nucleus in it. In cardiac muscle tissue, at its end it connects or touches another cell there is a specialized intercellular junction called an intercalated disc, this only occurs in cardiac muscle tissue. This tissue is responsible for pumping blood through the heart chambers and into the blood vessels.
Nerve Tissue
Nerve tissue is made up of nerve cells called neurons. Nerve tissue is the main component of the nerve system. A nerve is made up of many nerve cell fibres (neurons) bound together by connective tissue. Nerve cells are functionally connected to each other at a junction known as a synapse. Nerves are made to help you to do things, it is a form of touch. The other type of cell is neuroglia, or glial, cell. The word "neuroglia" means "nerve glue." These cells are nonconductive and provide a support system for the neurons. They are a special type of "connective tissue" for the nervous system. Nerves tell your brain to do something, for example, if you were very thirsty and you really wanted a drink, your nerves would send a message to your brain, telling your brain to pick up the glass of water. It's quite an amazing process if you actually think about it, and about how fast the nerves tell your brain to do something.
Connective Tissue
Connective tissues function primarily to support the body and to bind or connect together all types of tissue. This tissue also provide a mechanical framework (the skeleton) which plays an important role in locomotion. Connective tissue are relatively few cells which are widely separated from each other. Connective tissue is a kind of biological tissue that supports, connects, or separates different types of tissues and organs of the body. It is one of the four general classes of biological tissues—the others of which are epithelial, muscular, and nervous tissues. Although connective tissue exists in a number of forms, all types have three basic structural elements -- cells, fibres and intercellular substance.