Blood contains a complex mixture of many types of compounds with various properties and functions. It is distributed throughout the whole body via the vascu-lar system, and exhibits the following functions:
1. Transport system of various compounds: blood cells, salts, ions, proteins and gas (mainly CO2and O2).
2. Defense system against hostile pathogens such as bacteria, virus, fungi: spe-cialized cells like lymphocytes, monocytes and granulocytes; antibodies;
components of the complement system.
3. The wound sealing and wound healing system, life-saving precautions in the case of injuries: blood cells, e.g. blood platelets; blood coagulation and fibrinolysis
4. The balance of heat distribution throughout the body, thus guaranteeing a constant body temperature.
There are two main parts comprising blood, the blood plasma and the blood cells.
1.1.1 Blood plasma
The blood plasma represents approximately of 55 % of the entire blood, it contains the following groups:
• Water. It is the main component of blood plasma, approximately 90 %.
• Mineral salts and ions.
• Low molecular weight components. For example, there are carbohydrates such as glucose and fructose, amino acids, nucleotides such as ATP1 and
1Adenosine triphosphate (ATP) is a nucleotide that consists of a purine base (adenine), a pentose sugar (ribose), and three phosphate groups. ATP is used in DNA synthesis and energy storage.
cAMP2, vitamins, hormones, fatty acids, lipids and triglycerides, bile acids, urea and ammonia and many more components.
• High molecular weight components. Peptides and proteins, oligosaccha-rides and polysacchaoligosaccha-rides, DNA and RNA.
• Gases in soluble form. Gases such as oxygen, carbon dioxide and nitric oxide are dissolved in blood.
Among of different water-soluble compounds blood plasma comprise the blood plasma proteins. According to N.G. Anderson and N.L. Anderson [18], plasma proteins can be classified into the proteins secreted by solid tissues, immunoglob-ulins, ’long-distance’ and ’local’ receptor ligands, temporary passengers, tissue leakage products, aberrant secretion and foreign proteins. Based on this classifi-cation these proteins can be further grouped into the following main subclasses [18]:
1. Blood coagulation and fibrinolysis. These kind of proteins comprise fib-rinogen, kinin and the angiotensin/renin system.
2. The complement system. Proteins of the complement system are part of the immune system and are involved in the initiation of the immune response.
3. The immune system. The main proteins of immune system are: Immunoglob-ulin G (IgG) which exists in four subclasses: IgG1 (60%), IgG2 (30%), IgG3 (4%) and IgG4 (6%); Immunoglobulin D (IgD); Immunoglobulin E (IgE), Immunoglobulin A (IgA) which exists in two forms: IgA1 (90%), IgA2(10%); Immunoglobulin M (IgM).
4. Enzymes. These are proteins, which accelerate or catalyse chemical reac-tions. Enzymes control and regulate many biochemical pathways such as blood coagulation and fibrinolysis, the complement system.
5. Lipoproteins. They represent the aggregates of lipids and proteins. They enable transport of water-insoluble lipids via the vascular system to the target cells. Lipoproteins play a key role in homeostasis of cholesterol.
2Cyclic adenosine monophosphate (cAMP) is derived from ATP and used as a intracellular signal transducer. It is comprised of a purine base (adenine), a pentose sugar (ribose), and one phosphate group.
6. Hormones. They are either peptides or proteins, modified amino acids or steroids. Hormones are present in blood plasma in very low concentration (∼pg/ml). They are released by many glands of the endocrine system such as the hypothalamus, the pituitary gland, the parathyroid, the thyroid and pancreas, and by tissues such as the stomach, intestine, liver and placenta.
7. Cytokines and growth factors. Cytokines are the soluble proteins, pro-duced by hematopoietic and nonhematopoietic cells. They are responsible for intercellular communication, and also involved in the development, dif-ferentiation and activation of cells. Growth factors play an essential role in stimulating cellular proliferation and differentiation. Besides, they act as signal transmission proteins between cells.
8. Proteins of transport and storage. These type of proteins includes the serum albumin family, where Human Serum Albumin is the major transport protein, it is also present in a very high concentration (∼ 35-50 mg/ml).
Another class of plasma proteins responsible for transport and storage are globulin family. These are heme-containing proteins, which involved into the binding and transport of oxygen.
1.1.2 Blood cells
The blood cells represents approximately 45 % of entire blood, they are primarily synthesized in the bone marrow. There three main groups of blood cells:
• Erythrocytesorred blood cells. They have a biconcave form (see Figure 1.1), representing flattened cells with a discoed shape. They contain no nu-cleus and have a diameter of∼ 7.5 µm, a thickness of ∼2 µm. Blood contains approximately 4-5 × 109 erythrocytes/ml of blood, representing approximately 96 % of all blood cells. They contain hemoglobin protein, which carries oxygen and defines the red color of blood.
• Leukocytesorwhite blood cells. They represent∼3 % of all blood cells, and appear in concentration of∼4-8×106leukocytes/ml of blood with an approximate size of 7-15µm. Leukocytes are composed by cells from the innate immune system and adaptive immune system, particularly lympho-cytes T and B.
A B C
Figure 1.1:Electron micrographs of Erythrocytes (A), Leukocytes (B), Platelets (C). Source: [1].
• Thrombocytesor platelets. They contain no nucleus and have a discoid shape with a diameter of approximately 1-3 µm. Blood contains approxi-mately 2-3×108 platelets/ml of blood. Platelets are essential in the heal-ing process of vascular injuries, they protect the body by stoppheal-ing bleedheal-ing.
During the coagulation cascade they close the site of injury via intercalation with the fibrin network close. During this process activated platelets change its shape from discoid to spherical, exposing spines termed pseudopodia, which become sticky, thus annealing the injured blood vessel. By clumping together platelets form clots.
Serum
In the next part we will use the notion of serum. This is a blood compound not including blood cells and proteins participating in blood coagulation.