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Human Anatomy and Physiology: The Blood Vessels and the Heart
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In general there are three types of blood vessels: arteries, veins and capillaries. Given an artery and a vein with similar inner diameters, the artery will have a thicker wall than the vein. Capillary walls have only one layer-the intima.

The nutrient blood supply provides blood through the usual exchange of materials between body cells and the blood. A collateral circulation is a special organization of blood vessels around a major joint or other area of the body. There are areas of the body where a single artery is the sole supply of blood, such an artery is called an end artery.

End arteries are most common in the brain and the heart. A portal vein is a venous blood vessel that begins with capillaries in one area and ends in capillaries or another area. The most important portal vein in the human body is the Hepatic Portal Vein.

For protection, arteries tend to be located deep within the structures of the body. Veins can be located at both deep and cutaneous levels. The capillaries are located throughout all tissues of the body. The networks of capillaries in the tissues are often called capillary beds.

The capillary beds make up the greatest cross-sectional area of the cardiovascular system. The wall of the capillary consists of a single layer of flat cells. The capillary beds are provided with pre-capillary sphincters that can reduce or completely stop the flow of blood into the capillaries.

The heart consists of four separate chambers. The two chambers at the top of the heart are called atria (singular: atrium). Down the middle of the heart, an interatrial septum separates the two atria. The two chambers at the bottom of the heart are called ventricles. Between the ventricles is a wall of tissue separating the left and right ventricles, this is called the interventricular septum. The walls of the chambers of the heart consist of three layers: the myocardium, the endocardium, and the epicardium, also known as the visceral pericadium. The chambers themselves are linked with a simple epithelium known as the endocardium.

By far the  most important of the three layers in the heart walls is the myocardium, the middle layer. The myocardium is thicker in the walls of the ventricles than the atria. The stroke volume is the amount of blood forced out of each ventricle in one contraction. The cardiac output is the volume of blood pumped out of the ventricles (right into the lungs, left into the systemic circulation) in one minute (expressed in liters per minute). Valves are structures that ensure that fluids will pass through them in one direction only. There are two sets of cardiac valves-the atrioventricular (AV) valves and the semilunar valves. The AV valves are found between the atria and the ventricles.

Chordae tendineae are a special anatomical arrangement which help prevent the backward flow of blood into the atria. When the ventricles contract (ventricular systole) and the AV valves have closed, the blood moves out into the great arteries through the aortic and pulmonary semilunar valves. When the ventricles relax (ventricular diastole), the back pressure of the blood in the great arteries forces the cusps of the semilunar valves to the center and seals off each opening. The NAVL of the heart are the nerves, arteries, veins and lymphatics which influence the actions of the heart.

Control of heart functions can be divided into the following areas: Extrinsic controls (control from outside the heart), Intrinsic controls (control from inside the heart), and Humoral control, where some substances transported by the blood can accelerate or slow the action of the heart. The sinoatrial (SA) node is a collection of impulse conducting fibers in the interatrial septum. The atrioventricuar (AV) node is a group of impulse conducting fibers just above the interventricular septum. Fibers descend from the AV node to form the Bundle of His, which branches into the right and left septal bundles. The septal bundles connect with the Purkinje fibers located throughout the ventricular walls. Impulses begin in the SA node, pass to the AV node, and then descend through the septal bundles and on the Purkinje fibers to stimulate the myocardium of the ventricular walls to contract.

The blood which supplies the heart tissue itself is called nutrient blood. The openings leading into the coronary arteries are located in the base of the ascending aorta, just above the semilunar valve ( Aortic valve). Many of the branches of the coronary arteries are of the end artery type. The blood from the tissues of the heart is collected by the cardiac veins. The Thesbian veins are many minute sinuses found in the myocardium of the ventricles.

There is a fibrous connective tissue structure within the substance of the heart. Each atrioventricular valve of the heart is surrounded by a dense fibrous ring. Each of the semilunar valves of the heart is located within a short fibrous cylinder. The fibrous pericardium is a very dense fibrous envelope. the parietal pericardium is the outer serous membrane. the visceral pericardium intimately covers the surface of the heart.

Blood is driven through the arteries by a combination of forces. When the left ventricle contracts-systole, it forces the blood into the aortic arch. When the ventricle relaxes-diastole, the wall of the aortic arch recoils and presses against the blood. Vasoconstriction is the actual contraction of the arterial walls. In the head and neck, gravity helps to move the blood down through the veins.

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Human Anatomy and Physiology: Cardiovascular System

The cardiovascular system is the primary circulatory system of the human body. It’s main function is transport. It also provides protection against foreign substances. The arteries of this system generally carry blood from the chambers of the heart to the tissues of the body, while the veins carry blood from the tissues of the body to the chambers of the heart. The blood circulation is a two cycle system which involves both the pulmonary cycle and the systemic cycle. In the former, the pulmonary cycle, blood circulates from the heart to the lungs and back to the heart.

 

In simple organisms such as unicellular and one or two layer organisms, materials can be transferred among cells by simple processes of diffusion. However, in large organisms, a system is needed for the distribution and collection of materials. The circulatory system is used by the human body to carry substances. Some substances are collected from the body cells for elimination. The body also produces hormones which are the products of the endocrine glands.

 

Some of the liquids of the body are blood, lymph or cerebrospinal fluid. Some of the vessels of the body include the blood vessels and lymph vessels. Blood is composed of the following: plasma and formed elements. Plasma makes up about 55 percent of the total blood volume. Water is also a part of our body composition. It has an ample heat-carrying capacity. Along with dissolved salts, plasma proteins help to maintain the tonicity of the plasma. Fibrinogen is important to blood clotting. The percentage by volume of red blood cells in the blood is called haematocrit. Haemoglobin, a special protein, is found within red blood cell cytoplasm. The normal, mature red blood cell is a biconcave disc. White blood cells are another of the formed elements of the blood, they are also known as leucocytes. There are several types of white blood cells including neutrophils, monocytes, phagocytes and lymphocytes. Platelets are also another type of formed element in the blood.

 

The blood is the vehicle for the cardiovascular system; it is used to transport substances around the body. Oxygen in the air fills the alveolus of the lung and carbon dioxide is produced during metabolic oxidation within the individual cell.

 

The blood carries glucose and oxygen around the body. When the hormone epinephrine is secreted by the Adrenal Gland, it is delivered to all parts of the body by the cardiovascular system. In periods when much energy is required, the body can use its stores of fat as a source of energy.

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