The Path of Blood: An Examination of the Mammalian Circulatory System
The heart is known as the organ of life and love, the part of our body that keeps us going on a daily basis. It is red, powerful, and often described as the center of life itself. But there is a whole system and pathway that the blood the heart pumps must travel through. The heart on its own cannot sustain life in the way society often imagines it. Instead, an intricate system surrounds the heart and allows it to play its essential role in the human body.
The way that blood travels around the human body is called the mammalian circulatory system. Ift is a closed double circulation consisting of a heart, blood, and blood vessels. In other words, blood goes through the heart twice in one cycle and does not leave the boundaries of these vessels. The vessel types in this system are arteries, arterioles, capillaries, venules, and veins. The mammalian circulatory system is a one-way mass transport system of blood that carries essential nutrients and gases to other cells in the body. This system can be broken into two parts, the systemic circulatory system, where oxygenated blood goes to tissues all around the body, and the pulmonary circulatory system, where deoxygenated blood travels to the lungs. The transition point between these two is the pump of life, the heart.
The main vessels that will be delivering blood to and from the heart are the pulmonary artery, pulmonary vein, aorta, and vena cava. The cycle begins with the pulmonary artery which brings deoxygenated blood into the lungs from the right side of the heart. Once the red blood cells are oxygenated, the pulmonary vein brings them back to the heart, where they will enter the left side and then move to the aorta. The aorta is the primary artery in the human body, responsible for supplying blood to all the parts of the body. Arteries in general have to withstand a large amount of pressure because they are transporting blood all the way from the heart to the toes, and the aorta, specifically, has to carry an immense amount of pressure without collapsing within itself. The aorta branches out into arteries which form into arterioles and eventually thin capillary beds surrounding tissues. When the blood reaches the target area, it lets go of its oxygen molecule(s) and is returned, deoxygenated, to the heart through the primary vein in the human body, the vena cava.
Each type of blood vessel also has a specific function. There are two major groups of arteries, elastic and muscular arteries. The elastic arteries have more elastic fibersin the smooth muscle layer so that their walls can stretch and recoil to accommodate blood surging and maintain pressures, preventing bursting. Muscular arteries have a larger layer of smooth muscle tissue. These types of arteries are the ones that usually become arterioles, and have to lower pressure within themselves as their size shrinks. In contrast, veins carry deoxygenated blood from the systemic circulatory system to the heart to be sent to the pulmonary circulatory system later on. Lastly, capillaries are the smallest vessels that are only one red blood cell thick, and are in charge of getting as close as possible to tissues for fast productive exchange of materials.
From Southern Illinois University, the larger cylinder on the left is an artery and the one on the right is a vein.All blood vessels have different structures designed to support their functions. Arteries and veins both have three layers: endothelium (inner, thin, orange layer), smooth muscle (thicker, middle, pink layer), and tunica externa (outermost, navy layer). The empty space in the middle of the blood vessels is called the lumen, which should be narrower in arteries than in veins. Arteries also have thicker, stronger elastic fibers in their smooth muscle layer so that they can easily contract to withstand blood surges from the heart.
On the other hand, veins do not have to withstand blood surges but instead they must rely on skeletal muscle contractions to create pressure for them. To keep the blood from falling backwards down the veins, the endothelium has semilunar valves that only allow blood to move upwards. To understand this better check the way that movement is directed for veins in the mammalian circulatory system shown below. It is clearly shown how the blood in the veins has to move against gravity and they need semilunar valves to assist them.
The capillaries are tiny vessels that form capillary beds around every tissue in the body. They only consist of one layer, the endothelium, whose walls have small gaps allowing for leakage. This is what creates tissue fluid, which is essential for the human body. The proximity between the capillaries and tissues is what allows for efficient exchange of oxygen (O2) and carbon dioxide (CO2), the primary function of the circulatory system.
The bloodstream also consists of many different types of blood cells. These cells have a biconcave shape and only have a cytoplasm and no nucleus. They are the ones responsible for transferring O2 and CO2. They usually have a color resembling a light red when viewed from a light microscope. The three major groups of white blood cells include monocytes, neutrophils, and lymphocytes. Monocytes are the largest type of white blood cells with a kidney-shaped nucleus and cytoplasm surrounding it. They are responsible for cleaning up dead cells and are known for becoming macrophages later on. Neutrophils have a multi-lobed nucleus (3-5) and a cytoplasm with small granules. These abundant white blood cells are the first responders to viruses or bacteria but they are short-lived phagocytes. Lastly, lymphocytes are the smallest white blood cells with a round nucleus and overall shape. They fight infections by making antibodies and can be separated into two distinct groups, B cells and T cells.
As mentioned before blood plasma within the capillaries tends to leak out of capillaries. The liquid that is released into the body is called tissue fluid, the main difference between this liquid and blood plasma is where they are located within the body. Another distinguishing factor between these two is that tissue fluid has less proteins because they are usually too big to fit between the fenestrations in the capillaries. This liquid allows for the exchange of substances between cells and blood by acting as a transport method.
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