Blood Composition

Blood makes up approximately 7% of the weight in the human body. Blood is made up of clear yellow liquid plasma and formed elements, which it transports through the body. Blood cells are derived from hematopoietic stem cells, immature cells that can mature into any type of blood cell—red blood cells, white blood cells, or platelets—and are produced in bone marrow through the process of hematopoiesis.

Plasma is the clear yellow liquid part of the blood that carries water and nutrients to the tissues of the body. Plasma contains water, glucose (sugars), fatty acids (lipids), amino acids (proteins), and electrolytes (salts). Water makes up 91 percent of plasma, and electrolytes are the most common solutes. The primary function of plasma is to transport blood cells, nutrients, and waste products throughout the body. Plasma also carries the other components of blood—red blood cells, white blood cells, platelets, antibodies, clotting proteins, hormones, and other proteins—that help the body to maintain its fluid balance.

Plasma comprises approximately 55 percent of the blood. Proteins make up 7 to 9 percent of the plasma. The proteins are albumins, globulins, and fibrinogen. Albumin, which is synthesized in the liver, is the most common protein in blood, making up about 70% of the plasma protein and helping to maintain vascular pressure by pulling water into the capillaries. Globulin is primarily responsible for transport, and globulins are broken up into three different types—alpha, beta, and gamma globulins—and make up 36% of the plasma. Alpha and beta globulins are also made in the liver and transport lipids as well as fat-soluble vitamins through the bloodstream. Gamma globulins play a role in the immune system. Fibrinogen is also produced in the liver and aids in the clotting process of blood; it makes up about 4% of blood plasma. Serum is similar to plasma in that it is a clear liquid part of the blood, but it is the part of the blood that can be separated from clotted blood (i.e., it is plasma without the proteins involved in blood clotting).

Because plasma has such important functions in the body, donated plasma is used to treat people with chronic diseases and disorders such as hemophilia A and B and some cancers such as leukemia. Since plasma contains antibodies that fight infections, it is given to people with immunodeficiency because their own body cannot protect them from infections. It is also used as a treatment for severe burns to reduce the inflammatory response. Plasma is used to replace blood volume in patients suffering from blood loss without having to wait for blood typing. Plasma is obtained from donors when they give blood. The donation process can take several hours, and the screening for donors is very stringent. Because of this, plasma donors are often paid for their donation.

An erythrocyte is a red blood cell, a cell that transports oxygen from the lungs to the body for use in cellular respiration. Erythrocytes have the iron-containing protein hemoglobin needed to bind oxygen for transport. The carbon dioxide produced when the body's cells make energy through cell respiration is transported to the lungs, where it is eliminated. Most of the carbon dioxide travels to the lungs as bicarbonate; some travels either attached to hemoglobin in red blood cells or simply dissolved in plasma.

Erythrocytes are known for their reddish color and disk-shaped appearance with a flattened center that makes them resemble a doughnut. They are described as biconcave because they are concave on both sides. Erythrocytes are the most common and abundant cells in the blood, making up approximately 45% of blood volume. The percentage of whole blood volume that is made up of erythrocytes is known as hematocrit and is commonly used in the medical industry to measure red blood cell levels. Adult humans have roughly 20 to 30 trillion red blood cells, making up approximately 70% of all cells by number.

Production of erythrocytes occurs primarily in the red bone marrow. Erythropoietin (EPO) is a hormone that stimulates the marrow to promote formation of red blood cells. Unlike many other cells, red blood cells do not have a nucleus, which makes them flexible and allows them to change shape and flow easily through the blood vessels of the body. The lack of a nucleus also shortens their lifespan. Erythrocytes have an average lifespan of 120 days, after which they become inflexible and get trapped in circulatory channels of the spleen (and liver), where they are engulfed by macrophages, white blood cells that "swallow" and digest cellular debris. The rupture of erythrocytes that causes the cytoplasm to expel into the surrounding fluid is called hemolysis. Some diseases and processes cause red blood cells to break down too soon. Conditions that can cause excess hemolysis include infection, medication, toxins, immune reactions, and medical treatments such as the use of a heart–lung bypass machine.

Red blood cells and oxygen are crucial to health because the cells of the human body depend on oxygen to survive. Hypoxia is a condition in which the body is deprived of oxygen supply. Hypoxia may affect the whole body or just a part of the body. Hypoxia is most often related to a medical condition but could also be induced by high altitudes or strenuous exercise. Symptoms of hypoxia include fatigue, numbness, and nausea. In severe cases, symptoms may include confusion, hallucinations, headaches, lack of consciousness, elevated heart rate (tachycardia), high blood pressure (hypertension), and possibly death.

When there is a decreased delivery of oxygen, the kidneys release erythropoietin, which stimulates the red bone marrow to promote the formation of additional red blood cells. This cycle helps to keep oxygen in the body at optimum levels.

The body has several types of white blood cells, each called a leukocyte, to defend the body from infections. They can travel in and out of the bloodstream to help protect the body from disease, injury, and foreign substances. Leukocytes provide a defensive function in the body and help to fight against infection and invasion from foreign microbes. White blood cells are fewer in number than red blood cells, accounting for less than 1% of blood volume.

Unlike red blood cells, leukocytes have a nucleus and are clear in color. They are only visible under a microscope when they have been stained using a type of dye. Leukocytes are categorized based on the presence or absence of cytoplasmic granules, areas filled with cellular material surrounded by a membrane, and are identified by their abilities to be stained. A leukocyte that is granular in appearance when stained is called a granular leukocyte, or granulocyte. A leukocyte that does not have cytoplasmic granules, small enzyme-containing sacs, and does not absorb any stain at all is called an agranulocyte.

A granulocyte that absorbs pink stain is called an eosinophil. An eosinophil is a type of white blood cell that helps fight diseases and parasitic infections. A granulocyte that absorbs blue stain is called a basophil. A basophil plays an important role in the immune response by causing inflammatory reactions such as in allergic reactions, anaphylaxis, and asthma. Another type of leukocyte called a neutrophil is the most common type of granulocyte. Known as the "immediate-response" cell, a neutrophil is the first to show up at sites of injury. Neutrophils account for 55–70% of the total white blood cell count. Neutrophils are short-lived and must be constantly regenerated in order to protect against infection.

There are two types of agranulocytes—monocytes and lymphocytes. A monocyte is a type of leukocyte that can change as needed into a macrophage and is targeted to destroy specific foreign substances such as bacteria and viruses. It has a large cell with a kidney-shaped nucleus. A lymphocyte is a small white blood cell that functions as part of the immune system. It has a large nucleus and very little cytoplasm because the nucleus takes up most of the cell space. There are two major types of lymphocytes—T cells and B cells. T cells help to regulate the other immune cells and can also directly attack various infected cells and tumors. B cells produce antibodies—proteins that target bacteria, viruses, and other foreign substances.

A thrombocyte—also known as a platelet—is responsible for the blood clotting process. This process allows blood to thicken and clump up during times of severe injury to prevent excess bleeding. Platelets do not have a nucleus. They are made up of cytoplasm and are biconvex disks shaped like a lens. Platelets are found only in mammals (in other vertebrates, cells involved in blood clotting are nucleated). For diagnostic testing, a blood smear is used to study platelets for shape, size, quantity, and clumping ability. In a blood smear, healthy platelets appear as dark purple spots and are 20% the diameter of red blood cells.

The primary function of platelets is to stop bleeding to protect the body from blood loss, a process called hemostasis. During hemostasis, platelets gather at the site of injury to form a clot that will plug the hole and stop the bleeding. Hemostasis occurs in three stages when a blood vessel is ruptured—vascular spasm, platelet plug formation, and blood coagulation.

Hemostasis begins with a stimulus—a blood vessel is injured. When a blood vessel is damaged, it constricts. A vascular spasm, or constriction, is the blood vessel's first response to an injury, reducing the amount of blood flow through a wound to limit the amount of blood loss. The spasm response is most effective in smaller blood vessels with extensive damage. In this stage, platelets become activated, which causes them to become sticky. The stickiness allows platelets to cling to one another and to the damaged tissue to form a platelet plug (primary hemostasis). While clinging to the damaged vessel, platelets release chemicals in order to facilitate even more platelets sticking to the area. Proteins that aid and activate platelet plug formation are known as clotting factors.

Once the platelet plug has been formed by the platelets, the clotting factors create a collagen fiber called fibrin. Fibrin is produced when the enzyme thrombin acts on the protein fibrinogen found in blood plasma. Fibrin mesh is formed around the platelet plug to help hold the fibrin in place. Both red and white blood cells become entangled in the fibrin mesh, strengthening the clot. This third and final step reinforces clotting by pulling together the fibrin threads that help to bind platelets. Thus, this step holds the blood cells and platelets in place to seal and protect the wounded area. Although hemostasis is important for wound healing, it can sometimes cause health problems if the blood clot detaches from the injury site and travels through the circulatory system. On occasion, platelets can respond to an irregularity inside the vascular system rather than to an injury. This can cause an abnormal clot to form, called a thrombosis. A thrombosis may partially or fully obstruct blood flow to a particular area (ischemia). If the clot breaks away and obstructs a vessel, it is called an embolism. In the heart, lungs, or brain, an embolism could result in tissue damage, stroke, or even death.