Blood makes up about 8% of the human body weight. It contains erythrocytes, leucocytes, thrombocytes (platelets) and plasma. The volume percentage of all blood cells in the whole blood is about 45% of adults (hematocrit). The rest consists of liquid plasma (e.g. water, plasma proteins, electrolytes etc.).
The blood is composed of:
Plasma
Plasma is a straw-coloured fluid in which blood cells are suspended. It is made up of approximately 90% water as well as electrolytes such as sodium and potassium and proteins.
Red Blood Cells (Erythrocytes)
The main function of red blood cells is to carry oxygen. Red blood cells contain a protein called Haemoglobin. This combines with oxygen to form Oxyhaemoglobin. Each red blood cell has a lifespan of approximately 120 days before it gets broken down by the spleen. New cells are manufactured in the bone marrow of most bones. There are approximately 4.5-5 million red cells per micro-litre of blood.
White Blood Cells (Leucocytes)
There a number of types of white blood cells, although the function of all of them is to help fight disease and infection. They typically have a lifespan of a few days and there are only 5-10 thousand WBC's per micro-litre of blood.
Platelets (Thrombocytes)
Platelets are disc shaped cell fragments which are involved in clotting the blood to prevent the excess loss of body fluids.
Functions of blood
Messenger & waste removal: Blood is the most important transport medium in the human body. It transports gases (oxygen, carbon dioxide, nitrogen etc.) as well as nutrients (metabolism) and end products of cell metabolism. Hence the blood has the task of assuring the exchange of substances. It provides the tissues with blood gases and nutrients and in exchange transports end products (e.g. carbon dioxide, urea, uric acid, creatinine etc.) to the eliminating organs (lung, liver, kidney). Furthermore, it carries chemical messengers (hormones) to their target organs.
Acid-Base Balance: The acid-base homeostasis is regulated in the blood through the diffusion of gases between alveoli and blood in the lung (alveolar diffusion) oxygen diffuses from the alveoli into the blood due to the concentration gradient. It is taken up by the carrying protein hemoglobin (hem = iron-containing, globin = protein). Contrariwise carbon dioxide diffuses from the blood into the alveoli due to its higher blood concentration where it is breathed out.
Oxygen Supply & Carbon Dioxide Removal: The blood transports the oxygen from the alveoli to the remotest cells of the body. Because of the higher gas pressure in the plasma (relative to the cells), it diffuses to the tissues.
Carbon dioxide diffuses from the cells into the blood due to the higher gas pressure in the tissue. Here it undergoes a chemical reaction and forms carbonic acid (CO2 + H2O → H2CO3) which dissociates into hydrogen ion (H+) and bicarbonate (HCO3-). Thus the metabolism end product carbon dioxide is transported in the form of carbonic acid (or rather hydrogen ion and bicarbonate). In the lung, the above mentioned chemical reaction reverses and carbon dioxide is exhaled.
To sum it up the blood regulates the acid-base homeostasis by the gas exchange. The blood is also responsible for the homeostasis, e.g. balancing the water between the blood capillaries on the one hand and intracellular and extracellular space on the other hand. It also maintains a constant body temperature.
Coagulation: Coagulation factors (proteins) are solved in the blood and stop bleeding after a complex (cascade-like) activation of coagulation factors through damage to blood vessels finally leading to the building of thrombus (thrombogenesis). Simultaneously, fibrinogen/fibrin prevents the pathological development of blood clots in the blood vessels. Blood coagulation and fibrinolysis influence each other and maintain a sensitive equilibrium.
Coagulation of blood
Coagulation, is the process by which a blood clot is formed. The formation of a clot is often referred to as secondary hemostasis, because it forms the second stage in the process of arresting the loss of blood from a ruptured vessel. The first stage, primary hemostasis, is characterized by blood vessel constriction (vasoconstriction) and platelet aggregation at the site of vessel injury. Under abnormal circumstances, clots can also form in a vessel that has not been breached; such clots can result in the occlusion (blockage) of the vessel (see thrombosis).
Clotting is a sequential process that involves the interaction of numerous blood components called coagulation factors. There are 13 principal coagulation factors in all, and each of these has been assigned a Roman numeral, I to XIII. Coagulation can be initiated through the activation of two separate pathways, designated extrinsic and intrinsic. Both pathways result in the production of factor X. The activation of this factor marks the beginning of the so-called common pathway of coagulation, which results in the formation of a clot.
The extrinsic pathway is generally the first pathway activated in the coagulation process and is stimulated in response to a protein called tissue factor, which is expressed by cells that are normally found external to blood vessels. However, when a blood vessel breaks and these cells come into contact with blood, tissue factor activates factor VII, forming factor VIIa, which triggers a cascade of reactions that result in the rapid production of factor X. In contrast, the intrinsic pathway is activated by injury that occurs within a blood vessel. This pathway begins with the activation of factor XII (Hageman factor), which occurs when blood circulates over injured internal surfaces of vessels. Components of the intrinsic pathway also may be activated by the extrinsic pathway; for example, in addition to activating factor X, factor VIIa activates factor IX, a necessary component of the intrinsic pathway. Such cross-activation serves to amplify the coagulation process.
The production of factor X results in the cleavage of prothrombin (factor II) to thrombin (factor IIa). Thrombin, in turn, catalyzes the conversion of fibrinogen (factor I)—a soluble plasma protein—into long, sticky threads of insoluble fibrin (factor Ia). The fibrin threads form a mesh that traps platelets, blood cells, and plasma. Within minutes, the fibrin meshwork begins to contract, squeezing out its fluid contents. This process, called clot retraction, is the final step in coagulation. It yields a resilient, insoluble clot that can withstand the friction of blood flow.