The fluid part of the blood, the plasma, contains dissolved ions, inorganic and organic molecules, which either are transported to the various organs or serve as a means of transportation for yet other substances. The plasma proteins include albumin, globulin and fibrinogen. The globulins are subdivided into alpha, beta and gamma globulins. The gamma globulins are responsible for the immune reactions, the alpha and beta globulins primarily for the transport of other substances.
In the plasma, protein substances are present mostly as anions, representing about a sixth of the plasma's buffer capacity.
They exert a pressure on the capillary barrier of about 25 mm Hg, where the permeability of the walls is very low for plasma proteins. On the other hand, in the venous part, water is removed from the tissues and brought back into circulation via this pressure.
Albumin and fibrinogen are produced in the liver, the globulins for the most part in the spleen and lymph nodes.
But the approximately 5.5 liters [6 quarts] of blood in an adult body are not all fluid; there are also the red and white corpuscles.
1.3.2.1 The Red Blood Count
Erythrocytes measure about 7.5 u and thus have a larger diameter than the capillaries, which means that they must be pushed through the capillaries, since they are not capable of motion on their own. Their life span is about 120 days. Males have about 5 million of them, females somewhat fewer. Their most important function is transporting hemoglobin, which in its turn transports oxygen. They shrink in hypertonic solutions and swell up in hypotonic ones, at which point hemoglobin can be released. They are manufactured in the bone marrow.
Thrombocytes are small blood platelets with a diameter of about 2 p. They originate in the bone marrow's megakaryocytes, contain numerous hormones and carry the clotting factors.
Fig. 3a: The blood's primary physiological cell types
1.3.2.2 White Blood Count
The white corpuscles (leukocytes) only spend part of their life in the blood. They spend much more time in the bone marrow, lymphatic tissue or connective tissue. Morphologically, there are various forms:
• Granulocytes: neutrophils, basophils, eosinophils
• Lymphocytes, plasma cells
• Monocytes
Neutrophilic granulocytes: These originate in the bone marrow and have a life span of about 30 hours. Normally 55–60 % of these cells are found in the blood. They are capable of phagocytosis and are attracted to bacteria and general inflammatory reactions, leaving the bloodstream through the vessel wall at the site of the disturbance. These are the nonspecific cellular defense cells. They carry enzymes which enable them to perform phagocytosis, i. e., they can ingest and digest foreign bodies. They can penetrate bodily membranes.
Eosinophilic granulocytes: These comprise about 2–3 % of the white blood count. Their numbers increase during allergies, and they can ingest and digest antigen-antibody complexes.
Basophilic granulocytes: These are rich in histamine and heparin (an anticoagulant substance). They make up about 1 % of the white blood count.
Lymphocytes: These measure about 8 u and make up about 30 % of the white blood count. Depending on their function, they have a life span of from a few days to a year. They have the most varied tasks in the immune reactions. They are produced in the bone marrow.
They also come in different sizes depending on function. During inflammatory reactions, their numbers increase in the white blood count.
They are sensitized by contact with antigens and become capable of dividing. This gives rises to a specific defense, a characteristic which is passed on to all descendant cells. A functional distinction is made between B lymphocytes and T lymphocytes.
B lymphocytes appear clumped together as lymph follicles in the lymph nodes. In the context of confrontations with antigens, there is a distinction made between primary and secondary follicles.
T lymphocytes are individually distributed throughout the lymph nodes. They can remain there for several days, but not for more than 24 hours in the blood. The difference between B and T lymphocytes can only be perceived using an electron microscope.
Plasma cells: These make up only 1 % of the blood, but are quite numerous in lymph nodes and tissue. Morphologically, they are quite similar to lymphocytes. Their cell bodies include vacuoles containing gamma globulin, which they can release into the blood plasma.
Monocytes: These have a diameter of 20 u and are thus larger than the others mentioned above. They are also capable of phagocytosis, in addition to which they can move about like amoebas.
1.4 Materials Transport
The transport of individual nutrients from the blood to the cells makes use of various mechanisms. This is due to the varied composition of the fluids in their respective compartments. The individual substances are present in varied concentrations; the quantity of molecules and their electric charge also varies.
1.4.1 Diffusion
Diffusion is the main factor in water distribution. The particles (molecules, ions) of a gas or dissolved substance tend, due to their own motion, to fill the available space. They spread out from locations of higher concentration to those of lower concentration, eventually attaining an even distribution. Of course, there is movement in the opposite direction as well, but in the end, the average movement toward locations of lower concentration prevails. The magnitude of this tendency is proportional to the concentration differential of a given substance between two locations.
However, the diffusion of ions depends on their electrical charge as well. In the presence of a partial differential between two locations in a solution, positively-charged ions drift towards areas of higher negative charge, whereas negative ions drift in the opposite direction.
The engine of diffusion is the thermal motion of molecules. In the body, diffusion takes place not only within fluid compartments, but also from one compartment to another, as long as the intervening barrier is permeable to the diffusing substance. Diffusion is temperature-dependent, with higher temperatures accelerating diffusion.
Fig. 3b: The blood's primary physiological cells
An additional factor is particle size: the larger the particle, the slower the diffusion rate. Diffusion time is proportional to the square of the distance, which is why it is only effective over short distances. Oxygen transport from the capillaries to the cells and C02 transport in the opposite direction both function according to this principle. [2]
1.4.2 Filtration
Filtration is when a fluid is forced through a membrane by a pressure differential between the two sides of the membrane. The amount filtered is proportional to the pressure differential and the surface area of the filter. The molecules to be filtered must be smaller than the membrane pores. Small particles filter through capillary walls into tissues when the hydrostatic pressure in the blood vessels is greater than that in the extracellular fluid volume.
But since we are always dealing here with complexly structured compartments, there is always some resorption. Fluids and small molecular substances thus work their way through capillary