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The part of our body that keeps us going is none other than the heart. The heart is a muscular pump that allows blood to flow everywhere: our brain, lungs, even our pinky toe has blood flowing through it due to the heart. Keeping a constant supply of blood flowing through our body is no easy task, which is why the cardiovascular system is complex. Starting in the right atrium, deoxygenated blood goes through the tricuspid valve and into the right ventricle. The right ventricle contracts, pushing the blood into the pulmonary artery using the right semilunar valve. From there blood goes in and out of the lungs, getting oxygenated. Blood returns to the heart through the pulmonary veins, which drain into the left ventricle. The bicuspid valve pushes blood into the left ventricle, and contractions move blood up through the aortic valve and into the aorta. The aorta branches off into three parts: the right brachiocephalic artery, the left common carotid, and the left subclavian. As the aorta descends, blood travels to the lower half of the body. As blood travels throughout the body, oxygen is used by the tissues, and thus deoxygenated blood arrives and the inferior and superior vena cavae, soon to be oxygenized again.

The circulatory system contains the heart, blood vessels, and blood, and is extremely important in transporting materials throughout the body. How do these substances move? Through the blood! Blood is the liquid medium that allows things to travel and absorb into various organs and tissues.

All in all, the circulatory system serves to:

  1. Transport: Blood carries oxygen to the lungs and picks up carbon dioxide as waste.

  2. Protection: Blood is key to our body’s immunity and infection resistance, as white blood cells, platelets, and antibodies are all part of our blood.

  3. Regulation: Blood helps our body maintain homeostasis through various mechanisms, such as heat regulation and pH balance.

 Generalities about Blood

Although it may be hard to believe, blood is actually connective tissue, made up of cells and extracellular matrix. The matrix, however, is plasma, a clearish fluid that makes up the majority of blood volume. The rest of the blood volume is composed of formed elements, which include red blood cells, white blood cells, and platelets. Plasma makes up about 55% of our blood, while white blood cells and platelets make up less than 1%, and red blood cells form the last 45%. 

Red Blood Cells

Red blood cells, also known as erythrocytes, have two main functions:

  1. Pick up oxygen from the lungs to be delivered to other tissues

  2. Pick up carbon dioxide from body tissues to be unloaded in the lungs

Thus, erythrocytes are extremely important to our body’s  metabolism, as without them, our body would not receive oxygen. 

 

 Red blood cells have a disk-like shape, with a thick outer rim and a thinner, flatter center. They lack mitochondria, and thus use aerobic respiration to produce ATP. 

 Erythrocytes are red because they contain hemoglobin, a red gas transport pigment. Hemoglobin is important as it transports oxygen. It does this by each globin protein chain binded to a heme group. This heme group then binds to the iron atom of heme. Each heme can carry one molecule of O2; thus, one hemoglobin molecule can transport a maximum of 4 O2 atoms. Thus, hemoglobin and RBC count are vital in determining the amount of oxygen that can be carried throughout the blood. Hematocrit is the percentage of whole blood volume composed of RBCs. Normal hematocrit levels in men are 42-52%, while levels in women are 37-48%.

 RBCs are produced through the process of erythropoiesis. Erythropoietin, a hormone, is secreted by the kidneys which stimulate increased production. However, RBC count are greatly managed homeostatically through a negative feedback mechanism. The drop of RBC count leads to a state of hypoxemia, or oxygen deficiency. To counteract this state, the kidneys detect this and reacts by increasing their output of erythropoietin. RBC count will then increase through erythropoiesis. In contrast, an excessive RBC count is called polycythemia. Because there is a delicate, fine line of RBC count, many diseases can occur, such as anemia, or sickle-cell disease.

 

Blood Type 

Blood type is dependent on antigens and antibodies. Antigens are any large molecule capable capable of binding to an antibody or immune cells. An antibody are proteins that are secreted to act on an immune response. Using antibodies and antigens, blood type is determined by the antigens bound onto the red blood cells, and the antigens in the surrounding fluid. The O- blood type is the universal donor, as it contains all antibodies, while the AB+ blood type is the universal acceptor, as it lacks anti-A and anti-B antibodies. Due to the antigens and antibodies, it is often difficult to perform blood transfusions, as antibodies could attack the red blood cells. This occurs in erythroblastosis fetalis, where a “negative” mother gives birth to two “positive” fetus’, and thus her blood will attack the baby’s blood. 

Leukocytes

There are five types of leukocytes, which are divided into two categories: granulocytes and agranulocytes.  

Granulocytes:

  1. Neutrophils: most abundant of WBCs. They are antibacterial cells that fight off bacterial infections. 

  2. Eosinophils: They phagocytize and dispose of inflammatory chemicals, antigen-antibody complexes, and foreign antigens. Eosinophils also destroy large parasites such as tapeworms.

  3. Basophils: the rarest of all WBCs and formed elements. They secrete histamine, a hormone that vasodilates blood vessel, and heparin, a hormone that inhibits blood clotting (anticoagulant). 

Agranulocytes:

  1. Lymphocytes: Eliminates antigens by releasing antibodies, cytotoxic T cells, or signaling helper T cells.

  2. Monocytes: Fights against inflammation and viral infections by transforming into macrophages. They also perform phagocytosis that destroy dead particles.

As leukocytes are such an integral part of immunity, various diseases and infections can occur when when their processes go awry, such as leukemia.

 

Platelets

Platelets are vital in the process of hemostasis, or the cessation of bleeding.  Hemostasis uses three mechanisms: 

  1. Vascular spasm: the broken vessel is quickly constricted to reduce injury. 

  2. Platelet plug formation: Platelets flowing through the vessel arrive at the location and detect the broken vessel. The platelets clump together to form a platelet plug, which reduces bleeding.

  3. Coagulation: Blood cells and platelets stick to fibrin, creating a mass that seals the tear in the blood vessel. 

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