All cells in the body gets their nutrients from blood. Blood is a mixture of cells (the solid, corpuscular elements) and liquid plasma. The heart pumps blood through the arteries, capillaries and veins to provide oxygen and nutrients to every cell of the body. The blood also carries away waste products.
The adult human body contains approximately 5 liters (5.3 quarts) of blood which makes up 7 to 8 percent of total body weight. Approximately 2.75 to 3 liters of blood is plasma and the rest is the cellular portion.
Plasma is the liquid portion of the blood. Blood cells float in the plasma. Also dissolved in plasma are electrolytes, nutrients and vitamins, hormones, clotting factors, and proteins such as albumin and immunoglobulins (antibodies to fight infection). Plasma distributes the substances it contains as it circulates throughout the body.
The cellular portion of blood contains red blood cells (RBCs), white blood cells (WBCs) and platelets. The RBCs carry oxygen from the lungs; the WBCs help to fight infection; and platelets are parts of cells that the body uses for clotting.
All blood cells are produced in the bone marrow. As children, most of our bones produce blood, but as we age this gradually diminishes to just the bones of the spine (vertebrae), breastbone (sternum), ribs, pelvis and small parts of the upper arm and leg. Bone marrow that produces blood cells (a process called 'hematopoiesis' is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow.
All blood cell types come from the same type of cell, called the pluripotential hematopoietic stem cell. This group of cells has the potential to form any of the different types of blood cells and also to reproduce itself. This cell then forms stem cells that will form specific types of blood cells.
During development, the red cell eventually loses its nucleus and leaves the bone marrow as a cell called a reticulocyte. Initially, the reticulocyte contains some remnants of organelles. Eventually these organelles leave the cell and a mature erythrocyte is formed. RBCs last an average of 120 days in the bloodstream. When they age, they are removed by the liver and spleen.
A hormone called erythropoietin and low oxygen levels regulate the production of RBCs. Any factor that decreases the oxygen level in the body, such as lung disease or anemia, increases the level of erythropoietin in the body. Erythropoietin then increases production of RBCs by stimulating the stem cells to produce more RBCs and increasing how quickly they mature. Most erythropoietin is made in the kidneys. When both kidneys are removed, or when kidney failure is present, that person becomes anemic due to lack of erythropoietin. Iron, vitamin B-12 and folate are also essential to the production of RBCs.
Red blood cells, also called erythrocytes, are by far the most abundant cells in the blood. RBCs give blood its characteristic red color. In men, there are an average of 5,200,000 RBCs per cubic millimeter (microliter), and in women there are an average of 4,600,000 RBCs per cubic millimeter. RBCs account for approximately 40 to 45 percent of the blood. This percentage of blood made up of RBCs is a frequently measured number and is called the hematocrit. The ratio of cells in normal blood is 600 RBCs for each white blood cell and 40 platelets.
There are several unusual things about RBCs :
The primary function of red blood cells is to transport oxygen from the lungs to the cells of the body. RBCs contain a protein called hemoglobin that actually carries the oxygen.
In the capillaries, the oxygen is released to be used by the cells of the body. Ninety-seven percent of the oxygen that is carried by the blood from the lungs is carried by hemoglobin; the other three percent is dissolved in the plasma.
Hemoglobin combines loosely with oxygen in the lungs, where the oxygen level is high, and then easily releases it in the capillaries, where the oxygen level is low. Each molecule of hemoglobin contains four iron atoms, and each iron atom can bind with one molecule of oxygen (which contains two oxygen atoms, called O2) for a total of four oxygen molecules (4 * O2) or eight atoms of oxygen for each molecule of hemoglobin. The iron in hemoglobin gives blood its red color.
The normal concentration of hemoglobin in blood is 15.5 grams per deciliter of blood in men, and 14 grams per deciliter of blood in women. (A deciliter is 100 milliliters or one-tenth
of a liter.)
Besides carrying oxygen to the cells of the body, the RBCs help to remove carbon dioxide (CO2) from the body. CO2 is formed in the cells as a byproduct of many chemical reactions and enters the blood in the capillaries and is brought back to the lungs for release in exhaling. RBCs contain an enzyme called carbonic anhydrase that helps the reaction between carbon dioxide (CO2) and water (H2O) to produce carbonic acid. This then separates into hydrogen ions and bicarbonate ions, the former combining with hemoglobin and the bicarbonate ions going into the plasma. Seventy percent of the CO2 is removed in this way with most of the rest binding directly with hemoglobin for release into the lungs.
White blood cells (WBCs), or leukocytes, are a part of the immune system and help fight infection. They circulate in the blood so that they can get to any area where an infection has developed. In a normal adult body there are about 4,000 to 10,000 WBCs per microliter of blood. When the number of WBCs in your blood increases, this can be a sign of an infection somewhere in the body.
There are five main types of WBCs:
Neutrophils, eosinophils and basophils are called granulocytes because they have granules in them that contain digestive enzymes. Basophils have purple granules, eosinophils have orange-red granules and neutrophils have faint, blue-pink ones.
Neutrophils are the one of the body’s main defenses against bacteria. They kill bacteria by ingesting them in a process called 'phagocytosis'. Neutrophils can ingest from five to 20 bacteria in their lifetime. They have a multi-lobed, segmented or 'polymorphonuclear' nucleus and so are also called 'PMNs', 'polys' or 'segs.' Cells called 'bands' are immature neutrophils that are seen in the blood. When an infection is present, an increase of neutrophils and bands is seen.
Eosinophils kill parasites and have a role in allergic reactions.
Basophils are not well understood but function in allergic reactions. They release histamine, causing blood vessels to leak and attracts WBCs, and also heparin, which prevents clotting in the infected area so that the WBCs can reach the bacteria.
Monocytes enter the tissue through blood vessel walls and turn into larger 'macrophages.' These can phagocytize bacteria (up to 100 in their lifetime). They also destroy old, damaged and dead cells in the body, functioning as a sort of 'clean up crew.' Macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine, making up what is called the 'reticuloendothelial system.'
Neutrophils and monocytes use similar mechanisms to destroy invading germs. They squeeze through openings in blood vessels by a process called diapedesis and are attracted to certain chemicals produced by the immune system or by bacteria. They move toward areas of higher concentrations of these chemicals in what is called 'chemotaxis.' They kill bacteria by completely surrounding and digesting them with digestive enzymes.
Lymphocytes are complex cells that help direct the body’s immune system. T lymphocytes (T cells) are responsible for 'cell-mediated' immunity while B lymphocytes are responsible for antibody production (known as 'humoral immunity'). Lymphocytes are different from the other white cells because they have a memory of invading bacteria and viruses and can recognize them.
There are many types of T cells that have specific functions:
B cells become plasma cells when exposed to an invading organism or when activated by helper T cells. B cells produce large numbers of antibodies (also called immunoglobulins or gamma globulins). There are five types of immunogloulins: IgG, IgM, IgE, IgA and IgD. These are Y-shaped molecules that have binding site for only one specific antigen. Binding to antigens causes them to clump, be neutralized or break open. They also activate the complement system.
The complement system is a series of enzymes that help antibodies and other components of the immune system to destroy the invading antigen. They attract and activate neutrophils and macrophages, neutralize viruses and cause organisms to break open.
The average percentage of each type of WBC in the blood:
Platelets (thrombocytes) help blood to clot by forming something called a platelet 'plug.' The other way that blood clots is through the combination of protein coagulation factors. There are approximately 150,000 to 400,000 platelets in each microliter of blood (average is 250,000).
Platelets are formed in the bone marrow from very large cells called megakaryocytes. These break up into fragments which are then called platelets. Platelets do not have nuclei and do not reproduce, but are replaced by megakaryocytes when more are needed. Platelets generally last for an average of 10 days.
Platelets contain chemicals that assist clotting. These include:
Plasma is a clear, yellowish fluid that can sometimes appear milky after a very fatty meal or when people have a high level of lipids in their blood. Plasma is 90-percent water. Dissolved substances are circulate throughout the body and diffuse into tissues where they are needed. They diffuse from areas of high concentration to areas of lower concentration. Waste materials flow in the opposite direction, from where they are created in the cells into the bloodstream, where they are removed either in the kidneys or lungs.
Blood pressure tends to push fluid out of blood vessels which are a bit 'leaky.' Balancing this is something called 'oncotic pressure' (caused by proteins dissolved in blood), which tends to keep fluid inside the blood vessels. Protein molecules are much larger than water molecules and tend to stay in blood vessels because they have more difficulty fitting through the pores in capillaries. They tend to attract water to keep their relative concentration in blood vessels more in line with fluid outside the blood vessels. This is one of the ways the body maintains a constant volume of blood.
Plasma contains 6.5 to 8.0 grams of protein per deciliter of blood. Chief among these are albumin (60 percent), globulins (alpha-1, alpha-2, beta, and gamma globulins (immunoglobulins)), and clotting proteins (especially fibrinogen). Together with the maintenance of oncotic pressure, these proteins (especially albumin) transport substances such as lipids, hormones, medications, vitamins, and other nutrients. They function as part of the immune system (immunoglobulins), help blood to clot (clotting factors), maintain pH balance, and are enzymes involved in chemical reactions in the body.
'Electrolytes' are another large category of substances dissolved in plasma. They include:
These chemicals are absolutely essential to many body functions including fluid balance, nerve conduction and muscle contraction.
Other materials dissolved in plasma are carbohydrates (glucose), cholesterol, hormones and vitamins. Cholesterol is normally transported attached to lipoproteins such as low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs).
When plasma clots, the fluid left behind is called 'serum.' Some blood collected from patients is allowed to clot so that the cells and clotting factors fall to the bottom and the serum is left on top. Serum is then tested for numerous items to determine if any abnormalities exist.