The Taxis of the Human Body
While the cardiovascular system may be thought of as the interstate highway system of the body, the blood and its many components are most definitely the vehicles, carriers, transporters, and workers that keep the body supplied with essential materials. Additionally, some of these components act as garbage trucks, removing waste and keeping tissues healthy.
Cellular Function
The oxygen-carrying red blood cells (RBCs, also called erythrocytes) are packed with the oxygen-binding molecule hemoglobin, which give the cells their red color. They shuttle oxygen from the lungs to the tissues of the body where it can be used. RBCs are essential for the processing of CO2 and its transportation in the plasma as bicarbonate.
Cell Formation
Erythropoiesis is the process of forming RBCs from precursor cells. In an adult, RBCs are produced in the marrow of the long bones. However, in some instances, the liver and even the spleen may function in erythropoiesis.
The number of RBCs in the blood remains fairly constant as the newly formed RBCs equal the number of old RBCs removed from circulation daily. This balance is under close hormonal control to ensure that the numbers do not increase or decrease beyond a functional range.
When the body detects decreases in the amount of available and transported oxygen (hypoxia), the hormone erythropoietin (EPO) is released from the kidneys and leads to increased synthesis of RBCs in the bone marrow. These additional cells aid in the transportation of sufficient oxygen to the tissues. Once the number of RBCs is balanced with available oxygen and oxygen use in the body, EPO levels decrease and RBC formation returns to normal levels.
Cell Structure
As RBCs develop in the bone marrow, the cells become progressively smaller and redder in color as the cytoplasm fills with hemoglobin. Toward the end of their development, round RBC precursor cells called orthochromatophilic erythroblasts abandon their nucleus, which allows the cytoskeleton of the RBC (now called a reticulocyte) to adopt its typical and final bi-concave shape.
This shape maximizes the number of hemoglobin molecules that can bind to oxygen. If the cell remained round, the molecules at the very center would never encounter oxygen because they would be too far away from the plasma membrane.
Hemoglobin
Hemoglobin is made of 4 protein molecules joined together. Adult hemoglobin is composed of 2 alpha and 2 beta chains, each of which has an amino acid structure called a heme group capable of binding to a molecule of inorganic iron. The iron molecule, in turn, binds to oxygen in a reversible manner (that is, the oxygen can attach and release). Therefore, each hemoglobin molecule binds to 4 molecules of oxygen. When oxygen is bound to hemoglobin (oxyhemoglobin), RBCs turn a red color. Conversely, when oxygen is unloaded from hemoglobin (deoxyhemoglobin), the cells turn bluish in color.
Destruction
As RBCs age and approach their 120-day life expectancy, their plasma membrane becomes more rigid and the cell as a whole is less flexible. Recall that a capillary is only about the diameter of a single RBC (8 micrometers). Therefore, RBCs must be flexible enough to squeeze through capillaries in single-file fashion. Otherwise they could block the capillaries. As the cells circulate, they pass through the spleen where they must squeeze through barrel-shaped sinuses. Healthy RBCs traverse the spaces of the sinusoids effectively and in the process have debris cleaned from their surfaces. However, older, rigid RBCs are shredded as they are forced through the narrow space, and are thus destroyed and removed from circulation.
The spleen also contains an abundance of resident macrophages, the vacuum cleaners of the body. These phagocytic cells remove the cellular debris and waste material.
Blood Groups
Marker proteins and carbohydrates on the surface of red blood cells are formed into groups, which allow for identification and matching of blood cells from donor to recipient in clinical and emergency cases.
ABO Group
The most familiar of the blood groups, and the one which most refer to when asked their “blood type,” is the ABO blood group.
All RBCs have the same foundational protein on their cell surface. This base molecule is called the H antigen (an antigen is a molecule that can be recognized by the body’s immune system). If no additional carbohydrate modifications to this H antigen are present, these cells form O-type blood. A person with A-type blood will have an additional n-acetylgalactosamine on the H antigen, while a person with B-type blood will have an additional galactose carbohydrate on the base H antigen. Since ABO genetics is a form of codominant inheritance, a person with AB blood type would have some H antigens with the A-type carbohydrate, while other H antigens would have the B-type carbohydrate. There would never be both carbohydrate antigens on the same H antigen.
How can antibodies develop against an antigen an immune system has never encountered?
Many scientists have attempted to explain this problem by hypothesizing that either environmental agents similar to the blood groups or viruses that deliver these antigens infect every human in the early months and years of childhood and stimulate the production of these antibodies.
Rh Factor
The rhesus (Rh) blood group is so named because of the monkeys in which it was first identified. Rather than being a single antigen, several different genes can be expressed on the red blood cell surface and result in a person being Rh positive (the most common Rh antigen is RhD). In fact, the vast majority of the human population is Rh positive. Only when no Rh antigen is on the RBC surface is the blood considered Rh negative. Antibodies against Rh factor will only be produced if and when the blood of an Rh-negative person comes into contact with Rh-positive blood. In normal life, this would be a rare event for most people. However, for Rh-negative females who may become pregnant with babies carrying the Rh factor, it could present complications.