Red blood cells have a unique shape and inner components that allow
them to efficiently transport oxygen and direct the elimination of carbon dioxide.
Bone Marrow is the "blood cell factory" which is found filling up the cavities of bones. All blood cells originate and are produced from a single "stem cell" whose progeny grow and mature into different types of blood cells. This stem cell can and does renew itself as required by our body.
Red blood cells (erythrocytes) carry oxygen from your lungs to all parts of your body. If you don't have enough red blood cells you have anemia.
White blood cells (leucocytes) are the body's infection fighters. There are three main groups of white blood cells: granulocytes, lymphocytes and monocytes. Their job is to rid your body of disease-causing bacteria, viruses, fungi and to destroy the body's dead or defective cells. If we do not have enough white blood cells we are at risk of catching all types of infections.
Platelets are small cells that prevent bleeding and makes blood clot following an injury. When a blood vessel is damaged or cut, platelets rush to the area and clump together to plug the bleeding site. If we do not have enough platelets easy bruising, nose bleeds, prolonged bleeding from cuts, or internal bleeding from the bowel or bladder may occur.
The trillions of cells that compose the human body rely on red blood cells, a.k.a. erythrocytes to supply them with oxygen and help direct the elimination of waste carbon dioxide gas. Red blood cells have a unique structure and properties that allow them to fulfill these crucial missions. Red blood cells (RBCs) are the primary cells in human blood. They are biconcave discs, having a depressed center on both sides. These depressed centers allow the cells to have more cell membrane surface which can be exposed to diffusing oxygen while transiting the lungs. This structure also allows them to be more flexible when negotiating tight passages.
RBCs are about 7.8 micrometers in diameter (A micrometer is 1/1,000,000 of a meter). They have a flexible nature that allows them to bend and bounce back their original shape. This comes in handy when they must squeeze through the minute capillary alleyways between cells in the tissues.
Hemoglobin
Unlike most cells, mature RBCs of humans don’t have nuclei, mitochondria, or other organelles. Instead, they are packed full of a special substance called hemoglobin (Hgb), a complex molecule composed of protein and iron. Hgb is responsible for picking up oxygen, which diffuses across membranes in the small vessels of the minute lung sacks called alveoli. Hgb holds onto oxygen until it reaches areas of the body where it is in low concentration and then releases it to diffuse into local tissues.
RBCs are passive in nature, being swept along by the blood. They sustain their meager energy needs by a form of anaerobic respiration. Since they don’t have mitochondria, there are no worries they might gobble up the oxygen they are charged with transporting.
Management of Carbon Dioxide
RBCs also contain an enzyme called carbonic anhydrase which takes carbon dioxide and water and catalyzes the creation of a stable molecule called bicarbonate (HCO3-). Bicarbonate dissolves much better than carbon dioxide in the fluid part of blood and great quantities can be transported this way without needing to be in the small red blood cells.
Structural Abnormalities of RBCs
Sometimes structural changes occur in RBCs that indicate a possible pathological condition. For instance, one may observe enlarged RBCs with folate or vitamin B12 deficiency (macrocytic anemia). Small RBCs may be observed with iron deficiency or chronic blood loss (microcytic anemia). Sickle cell anemia is a condition where a genetic flaw leads to defective hemoglobin that changes shape under certain conditions. This shape change induces the cell to lose its original biconcave disc confirmation and take on a more sickled appearance.
Classification & Structure of Blood Vessels
Blood vessels are the channels or conduits through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. One system, the pulmonary vessels, transports blood from the right ventricle to the lungs and back to the left atrium. The other system, the systemic vessels, carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins.
Arteries
Arteries carry blood away from the heart. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues. Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries. About 10 percent of the total blood volume is in the systemic arterial system at any given time.
The wall of an artery consists of three layers. The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel.
Veins
Veins carry blood toward the heart. After blood passes through the capillaries, it enters the smallest veins, called venules. From the venules, it flows into progressively larger and larger veins until it reaches the heart. In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs. Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells.
The walls of veins have the same three layers as the arteries. Although all the layers are present, there is less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries, which is related to the fact that blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. Medium and large veins have venous valves, similar to the semilunar valves associated with the heart, that help keep the blood flowing toward the heart. Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity.
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