Blood is familiar as the vehicle for oxygen transport from lungs to tissues, and for transport of CO2 generated during respiratory metabolism in tissues to the lungs for release. Blood plays many other roles in organisms with circulatory systems. Blood transports major organic nutrients from the intestine (where they are absorbed) to the liver (where they are processed) and ultimately to the other organs. Organic waste products and excess mineral ions are conveyed by blood to the kidneys for excretion. The blood also serves to transport hormones and other chemical messengers from various endocrine glands to their specific target organs. Finally, blood contains cells and antibody proteins that defend against disease.
The human vascular system contains about 5 to 6 liters of blood. Nearly one-half its volume consists of cells: red blood cells (erythrocytes), which transport oxygen and carbon dioxide, and much smaller numbers of white blood cells (leukocytes), and blood platelets, which are part of the defense (immune) system.
Every human cell requires a steady supply of oxygen gas, O2. Every cell also requires a way to dispose of CO2 gas. Red blood cells(erythrocytes) do both jobs, carrying oxygen from the lungs to the cells, and CO2 from the cells to the lungs. The active agent within the red blood cells is the hemoglobin molecule, a globular protein, red in color, which has binding sites for O2 and CO2. The structure of hemoglobin is shown below.
The non-cellular portion of blood is called the blood plasma. Plasma is about 90% water by weight. The plasma proteins make up three-fourths of the weight of solutes in plasma. The different types of plasma proteins have a number of important functions. Among these is the capacity to transport important nutrients, such as lipids and fatty acids, as well as some trace metals, vitamins, and hormones. Antibodies in plasma help combat pathogen attack, and protease inhibitors protect against the degrading effects of proteases. Fibrinogen, the target of the blood-clotting cascade, is another abundant plasma protein. The remainder of the dissolved solutes consists of organic nutrients and metabolites, waste products, and inorganic salts. Table 1 shows the major components of normal human blood plasma and their general function.
Major Components of Blood Plasma
| Component | Concentration (g/100 mL) | Function |
| Proteins (Total) | 5.8-8.0 | |
| Serum albumin | 3.0-4.5 | Osmotic Regulation, Fatty acid transport |
| α-globulins | 0.7-1.5 | Transport of lipids, copper, thyroid hormone |
| β-globulins | 0.6-1.1 | Transport of lipids, iron, and other metals |
| γ-globulins | 0.7-1.5 | Antibodies |
| Fibrinogen | 0.3 | Blood clotting agent |
| Lipids (Total) | 0.4-0.7 | |
| Triacylglycerols | 0.4-0.7 | Fuel en route to storage |
| Phospholipids | 0.15-0.25 | Membrane components |
| Cholesterol & esters | 0.15-0.25 | Membrane components |
| Free fatty acids | 0.01-0.03 | Immediate fuel for muscles |
| Glucose | 0.07-0.09 | Transport form of carbohydrate from liver to peripheral tissues |
| Amino Acids | 0.035-0.065 | Protein synthesis precursors |
| Urea | 0.02-0.03 | Nitrogen excretion product from amino acid catabolism |
| Uric Acid | 0.002-0.006 | Nitrogen excretion product from purine metabolism |
One mL of blood contains about 5 billion (5 x 109) erythrocytes. These cells are essentially circulating containers for hemoglobin molecules. Blood is loaded with erythrocytes each cell is loaded with hemoglobin molecules. Most of the solid matter of the red blood cell is hemoglobin. To transport O2 and CO2 in the bloodstream, the body must make large numbers of erythrocytes and it must make a large amount of hemoglobin. Hemoglobin is synthesized inside the erythrocytes as they are formed in bone marrow. A healthy adult male synthesizes approximately 900 trillion (9 x 1014) molecules of hemoglobin per second to replace hemoglobin lost due to normal wear and tear. The synthesis of hemoglobin and of its container, the erythrocyte, must account for a large fraction of the body’s need for nourishment from the environment. Recall that hemoglobin is composed of four subunits. Each subunit contains a prosthetic group called heme.
Heme is a planar organic molecule and is small in relation to the protein subunit. A heme group is shown in figure 1. The heme ring contains one iron atom, which is in the +2 oxidation state in deoxyhemoglobin. This Fe(II) form in hemoglobin can bind one oxygen molecule. The heme subunit lies in a hydrophobic pocket defined by tube-like segments of alpha-helix.
Blood Types
Blood types are established by genetics and are determined by the proteins that are present in your blood. These proteins are called Agglutinogens and exist on the surface membranes of red blood cells. There are 3 genes for different blood types: A, B and O. But since we get our genes from our parents that means we get 2 genes (one from each parent) to determine our blood type. With 3 possible genes that means there are 6 variants:
AA or AO = Type A
BB or BO = Type B
OO = Type O
AB = Type AB
This means for blood typing there are four major blood types A, B, AB and O.
In addition to the genetic typing an additional factor is used to separate blood into groups call the Rhesus Factor. While studying Rhesus monkeys, scientists discovered a blood proteins that is present in some people's blood while absent in others. The presence or absence of this factor is called the Rhesus Factor and is given a + sign for its presence and a – sign for its absence. So now our 4 major blood types are further separated into 8:
A+ A-
B+ B-
AB+ AB-
O+ O-
In the population, blood types are not present in the same amounts:
Blood typing is the analytical process used to determine a person's blood type from whole blood. The area of study of blood types is called Serology. Serologists detect the ABO antigens by the use of antibodies specific to each blood types. Antibodies are protein molecules that have a lock and key type of mechanism which recognizes specific ABO antigens, binds to it and causes it to precipitate out of solution in clumps.
The serologist creates small samples of the blood evidence and then adds the different antibodies for each blood type to those samples. Based on which blood sample "clumps" up, he/she can determine the blood type. Similarly an antibody is also added to the samples to determine the presence or absence of the Rhesus factor. The image below depicts the results of the tests and how they would be interpreted to determine blood type.
Blood Type Test Results
Blood Evidence
There are 3 main types of blood evidence:
- Blood samples – This is blood that is directly drawn from a suspect or victim and can be analyzed for both type and can be used to extract DNA evidence.
- Blood droplets – This is blood that is left in a trail or in smears that indicates movement of either the victim or suspect at a crime scene. The drops can tell an investigator the direction, height from which they were dropped and sometimes even the weapon that was used. They can also be collected for typing and if not too degraded DNA.
- Blood spatter – This is blood that is propelled towards a surface in response to violence. Splatter can be used to determine the type of weapon used, the height of the assailant, movement in the crime scene and whether a body was moved after death.
