BLOOD

1. Major functions of blood 
2. Composition of blood
3. Body fluid compartments

1. Major functions of blood

Major functions of blood:

1. The formation of blood cells (hematopoiesis) occurs in the bone marrow continuously. During the life in the human body forms about 450 kg of erythrocytes, 5400 kg of granulocytes, 275 kg of lymphocytes and 40 kg of platelets. A critical balance must be maintained between the rate of blood cell production and the rate of blood cell loss from the circulation. The destruction of blood cells occurs in the bloodstream, in the spleen and liver continuously. 
2. Respiratory function of blood is the transfer of oxygen from the lungs to the tissues and transport of carbon dioxide in the reverse direction. Oxygen and carbon dioxide are contained mainly in the bound state. 
3. Trophic function is the transfer of nutrients (amino acids, lipids, mono- and disaccharides, vitamins, minerals, etc.) from the intestines to the organs and tissues.
4. Excretory function is the transport of toxic products of metabolism (urea, ammonia, bilirubin, urobilin, carbon dioxide, etc.), as well excess of salts and water, to the excretory organs (kidney, lungs, sweat glands and the digestive tract). 
5. Secretory function is synthesis of biologically active substances (plasma proteins, hormones) in the blood cells, bone marrow, liver, spleen.
6. The protective function - is the provision of immune response (humoral and cellular immunity), blood clotting, and anticoagulants processes. 
7. Thermoregulatory function is to maintain a constant body temperature, especially in conditions of high or low ambient temperature. Blood transports the heat from the hotter to the less heated parts of the body and organs. 
8. All functions of the blood are a homeostatic function (maintaining the constancy of the most important indicators of internal environment - pH, gases, electrolytes, cellular composition and other).

2. Composition of blood

Blood circulates within the cardiovascular system to provide a vehicle for the distribution throughout the body respiratory gases 

• nutrients 
• water electrolytes 
• hormones 
• antibodies 
• drugs, toxins 
• metabolic wastes 
• heat

A person who weighs 70 kg has approximately 5 L of blood (that is 7% of body weight), of which about 2 L is occupied by the formed cellular elements and 3 L is plasma.

The hematocrit is the percentage of blood volume that is occupied by red blood cells. It is normally about 45% for men and 40% for women. It is considered an integral part of a person's complete blood count results, along with hemoglobin concentration, white blood cell count, and platelet count.

There are three main cell types in circulating blood: 

1. Red blood cells (erythrocytes) are essential for the transport of O2 and CO2 in blood . There are approximately 5000×10 9 red blood cells per L of blood. 
2. Platelets (thrombocytes) are small cellular fragments that play a key role in hemostasis by adhering to sites of damage and through participation in the blood clotting system. There are approximately 300×10 9 platelets per L of blood. 
3. White blood cells (leukocytes) are the only fully functional nucleated cells in circulating blood. White cells play a defensive role in destroying infecting organisms and in the removal of damaged tissue. The total white cell count is 5×10 9 cells per L of blood. 

A scanning electron microscope image of a normal red blood cell, a platelet, and a white blood cell.

Plasma is the part of the extracellular fluid that is contained within the cardiovascular system. It is a dilute solution, which by weight is approximately 91% water, 7% protein, and 2 small dissolved solutes (e.g., ions, urea, glucose, amino acids, and lipids). 

Plasma without red blood cells, fibrinogen and several coagulation factors is called serum. 

Body fluid compartments 

The normal plasma concentrations of selected ions and small molecules are almost the same as those in the interstitial fluid because of the free exchange of water and small solutes across most blood capillaries. In contrast, most capillaries are impermeable to plasma proteins. The resulting difference in protein concentration between the plasma and the interstitial fluids creates a colloid osmotic (oncotic) pressure gradient that opposes the filtration of plasma out of the capillaries.

The Major Types of Plasma Proteins

Plasma Protein	Major Source	Examples and Functions
Albumin	Liver	Main component of plasma oncotic pressure; binding of various substances
α-Globulins & βGlobulins	Liver	Examples (there are many) include hormone-binding proteins and the iron carrier protein transferrin
Coagulation proteins	Liver	Examples include plasminogen, prothrombin, antithrombin III, and fibrinogen
Immunoglobulin s (antibodies)	Lymphoid tissue	Host defense reactions
Complement proteins	Liver	Host defense reactions The Major Types of Plasma Proteins