The cardiovascular system and the lymphatic system form what is collectively called the circulatory system. Together these systems transport oxygen, nutrients, cell wastes, hormones, and many other substances to and from all cells in the body. In fact, "Each year, the heart pumps more than 1,848 gal (7,000 l) of blood through a closed system of about 62,100 mi (100,000 km) of blood vessels" (World of Health, 2009). The millions of cells in the human body take up nutrients and excrete wastes every minute of the day. Although the pace of this exchange may increase with activity or slow with rest, it still happens continuously regardless. These systems are important because if they stop, so does life. Of the two systems, the cardiovascular system is the primary transport operator while the lymphatic system aids it in its function. As blood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands, which then delivers these substances to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these substances to the lungs and kidneys, where they are excreted. Substances diffuse or are transported out of blood vessels to the cells through the interstitial or tissue fluid that surrounds cells.
The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. The system can be compared to a large muscular pump that sends a fluid through a series of large and small tubes. As blood circulates through the increasingly intricate system of vessels, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands (2007). It delivers these substances to the cells, picking up carbon dioxide and other wastes in return. The blood then takes these waste products to the lungs and kidneys, where they are excreted. Consistent with its primary function, the efficient circulation of blood, the cardiovascular system is interconnected with two other heart-centered systems: the cardiopulmonary system, which controls the relationship between the heart and the lungs, and the cardiorespiratory system, the interrelationship between the heart and the general breathing mechanisms in the body, including the exchange of oxygen and carbon dioxide that occurs within the lungs.
The heart is a hollow cone-shaped muscular organ located behind and slightly to the left of the sternum or breastbone. Nestled between the lungs, the heart sits within a protective bony cage formed by the sternum, ribs, and spine. The heart wall is made up of three layers: the epicardium, the myocardium, and the endocardium. The outer layer, the epicardium, is actually the thin inner layer of the pericardium. The middle layer, the myocardium, is a thick layer of cardiac muscle that contracts to force blood out of the heart. The inner layer, the endocardium, is a thin, glistening membrane that allows blood to flow smoothly through the chambers of the heart. The heart is divided into four chambers. A muscular septum or partition divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, the atria are thin-walled. They are the receiving chambers of the heart. Blood flows into them from the body. The atria then pump the blood downward to the ventricles, the lower heart chambers. The ventricles are the discharging chambers of the heart. Their walls are thicker and contain more cardiac muscle than the walls of the atria. This extra thickness enables the ventricles to contract and pump blood out of the heart to the lungs and the rest of the body.
The cardiovascular system is a complex and extensive network. The circulatory process begins with a pump action in the heart muscle, known familiarly as the heart beat. The heart is equipped with its own nervous system that controls its beating activity. This system, called the intrinsic conduction system, is located within the heart tissue. Nerve impulses sent out through the system cause parts of the heart to contract at various times. Each beat is a two-part action where the timing is regulated by the heart component known as the SA node, whose function is in turn tied to brain signals (2009). Blood pressure in the circulatory system is calculated as a function of the two components of the pulse and the resistance of the arterial wall.
The blood vessels form a closed transport system of tubes. The entire blood vessel system can be thought of as a series of connected roads and highways. Interestingly enough, "tubes measuring about 62,000 miles (99,780 kilometers) in length--two and a half times the distance around the equator of Earth" (World of Sports Science, 2007). Blood leaves the heart through large vessels that travel outward into the body. At various points these large vessels divide to become smaller vessels. Arteries, capillaries, and veins are the main parts of this transport system. Arteries are the vessels that carry blood away from the heart. Large arteries leave the heart and then branch into smaller ones that reach out to various parts of the body. These divide even further into smaller vessels called arterioles. Within the tissues, arterioles divide into microscopic vessels called capillaries. The exchange of materials between the blood and the cells occurs through the walls of the capillaries. Before leaving the tissues, capillaries merge to form venules, which are small veins. As these vessels move closer to the heart, they merge to form larger and larger veins. The main blood vessels differ in their structure. Although the walls of both arteries and veins are composed of three coats, they vary in thickness. Arteries have thicker inner and middle layers, which makes them more elastic. They can expand and contract easily when blood pumped from the heart surges through them. Veins, on the other hand, have thinner walls. Their thinner structure allows skeletal muscles surrounding them to contract and press against their flexible walls, squeezing the blood along as it returns to the heart. One-way valves in the walls of veins prevent backflow, keeping the blood flowing in one direction. The valves are most numerous in the legs, in which blood must flow against the force of gravity on its way back to the heart. Unlike arteries or veins, the walls of capillaries are only one cell thick. In most capillaries, these singular cells are not joined together tightly. Because of this loose structure, oxygen, nutrients, and wastes are able to pass easily between the blood and the surrounding interstitial fluid, a watery substance that fills the spaces between cells.
Blood is the fluid pumped by the heart through the blood vessels to all parts of the body. It is considered a form of connective tissue. As the name suggests, connective tissue in general connects body parts, providing support, storage, and protection. Found everywhere in the body, connective tissue is the most abundant type of the four types of tissues. Of all the tissues in the body, blood is unique because it is the only one that is fluid under normal circumstances. Blood has many functions in the body. It carries everything that must be transported from one place to another within the body: oxygen and nutrients to the cells, hormones to the tissues, and waste products to the organs responsible for removing them from the body. Blood helps to protect the body by clotting to seal minor cuts and by acting as a defense against foreign microorganisms. It also keeps the body at a constant temperature by carrying heat away from cells, and it regulates the ph level of the body.
There are three types of blood cells. Red blood cells, or erythrocytes, are the most numerous of these three types of blood cells.The main function of red blood cells is to transport this form of oxygen to the cells throughout the body. After the oxygen is transferred, the hemoglobin combines with the carbon dioxide given off by the cells. The red blood cells carry the hemoglobin back to the lungs, where some of the carbon dioxide is exhaled. Because red blood cells are constantly squeezing through tiny capillaries, their membranes receive much wear and tear. For this reason, each red blood cell lives only about 120 days or four months (2007). New red blood cells are continually being produced in the bone marrow to replace the old ones. Leukocytes are white blood cells. They are larger than red blood cells, contain a nucleus, and do not have hemoglobin. Leukocytes fight disease organisms by destroying them or by producing antibodies. Lymphocytes are a type of leukocyte that bring about immune reactions involving antibodies. Monocytes are large leukocytes. Their purposes are to ingest bacteria and to eliminate dead matter. Most leukocytes are able to squeeze through the capillary walls and migrate to an infected part of the body. Formed in the white/yellow bone marrow, a leukocyte's life ranges from hours to years depending on how it functions during an infection. Finally, platelets bring about clotting of the blood. Clotting stops the bleeding when the circulatory system is damaged. When an injury to a blood vessel causes bleeding, platelets begin the clotting process by sticking to the ruptured blood vessel. As they do so, they release chemicals that attract other platelets resulting in recovery.