The blood circulatory system is unidirectional. Adult males have about 5.5 liters blood (slightly less in females).
Blood consists of :
If a test-tube of blood is centrifuged it separates into several distinct layers. The lowest layer, which is an orange or reddish color, contains the erythrocytes (about 45% of the volume). A thin layer (1%) known as the buffy coat is located above the layer of erythrocytes. This contains the leukocytes and platelets. The remaining upper clear fluid is the plasma (about 55%). The relative volumes of cells and plasma are described as the "hematocrit". Low hematocrit values can indicate blood disorders such as anemia.
Blood smears
A drop of blood spread thinly on a microscope slide and dried is known as a blood smear. In order to provide contrast and distinguish the various cell types (formed elements) the smear is stained. The various common blood staining techniques (Leishman, Giemsa, Wright) are all modifications of the Romanovsky (1891) staining method based on mixtures of methylene blue (basic dye) and eosin (acid dye). The methylene blue is oxidized by certain components to form azures (purple color), and the structures stained are described as azurophilic. Affinity for eosin is termed eosinophilia or acidophilia.
Formed elements of the blood
The erythrocytes of mammals lack nuclei (apart from in the fetus). Human erythrocytes are biconcave discs about 7.2mm diameter (similar to the dimensions of most capillaries). Abnormal variation is size is known as anisocytosis. Abnormal variation in shape is known as poikilocytosis. Erythrocytes are fairly flexible and can modify their shape to pass bends in the capillaries. Erythrocytes transported in the blood often appear bound together in "rouleaux", similar to stacks of coins.
The concentration of erythrocytes in adult males is about 5.5 million/mL and slightly less in females.
Erythrocytes are acidophilic and are stained a reddish color with eosin. The main function of the erythrocytes is the binding and transport of oxygen to the tissues. The main protein of erythrocytes is hemoglobin, to which oxygen is bound to form oxyhemoglobin. Erythrocytes circulate in the blood for about 120 days prior to removal of the aged erythrocytes and their destruction by macrophages mainly in the spleen and liver.
Erythrocytes are sensitive to osmotic pressure. In hypertonic solutions the erythrocytes shrink and become crenated, whereas in hypotonic solutions they swell and burst (hemolysis) leaving membrane "ghosts".
Leukocytes (white blood cells)
The leukocytes are involved in the defense of the body and participate in protection against foreign material including microorganisms. The leukocytes are classified into two major groups:
GRANULOCYTES
Neutrophils
Neutrophils (polymorphonuclear leukocytes or polymorphs) are the most common of the leukocytes and represent some 55-70% of all the circulating leukocytes. They have a diameter of about 12mm and are characterized by a segmented nucleus. In females a small "drumstick" is found (Barr's body) on the nucleus representing a condensation of the sex-chromatin.
Immature (juvenile) forms of neutrophils (about 1-2%) have a non-segmented rod-like or horseshoe-shaped nucleus ("band").
The cytoplasm of neutrophils is packed with small specific granules (up to 200 per cell) stained a salmon pink color in smears after Romanovsky-type staining. Some specific granules contain phagocytins, which are anti-bacterial substances involved intracellular digestion. The non-specific granules (stained weakly azurophilic) contain lysosomal enzymes and peroxidase.
Neutrophils are phagocytic cells involved as a first line of defence against invading microorganisms. They are important in inflammation and at sites of injury or wounds. Pus that develops in sites of infection is mainly composed of dead neutrophils.
Eosinophils
Eosinophils represent 1-4% of leukocytes. They typically have bilobed nuclei and fairly large (0.5-1.5mm) acidophilic granules. The color of the granules in stained smears is similar to that of erythrocytes. The granules are considered to be lysosomes and contain acid phosphatase, cathepsin and other lysosomal enzymes. At the ultrastructural level the granules are seen to possess a characteristic central crystalloid (internum). Eosinophils are involved in selective phagocytosis. An increase in the numbers of eosinophils is seen in allergic reactions or helminth parasitic infections.
Basophils
These represent only about 1% of the leukocytes. Their
nucleus is bilobed or S-shaped. They have very large, irregular basophilic
granules, that commonly obscure the nucleus. The granules (similar
to those of mast cells) contain histamine and heparin and
show metachromatic staining. In stained blood smears the granules stain
a violet color. Basophils release the contents of their granules in response
to antigens.
AGRANULOCYTES
Lymphocytes
Lymphocytes represent 25-30% of all the leukocytes. Lymphocytes are involved in the immune responses of the body. The most common lymphocytes are small cells (6-8mm diameter), with very dense regulnuclei, with relatively little cytoplasm. The different types of lymphocytes are considered in the framework of the lymphatic system.
Monocytes
Monocytes represent about 5% of the leukocytes. They possess oval or kidney-shaped nuclei, often eccentrically located. The nuclei are less intensely stained than those of lymphocytes. The cytoplasm is basophilic with small azurophilic granules (lysosomes). Monocytes are part of the Mononuclear Phagocyte System. They are non-terminal cells and can differentiate into phagocytic cells such as macrophages. Monocytes that enter the tissues do not return to the blood.
Blood platelets
Blood platelets are derived from the cytoplasm of megakaryocytes of the bone marrow. They are small (2-5mm) with a central core of small purple granules (granulomere) surrounded by an outer pale-blue hyalomere. At the periphery (as seen by electron microscopy) there are peripheral or marginal microtubules.
If a blood vessel is injured, the platelets are involved in the clotting reaction. They aggregate at the site of injury, change shape and help form the thrombus.
Neutrophil Lymphocyte Lymphocyte Neutrophil
Lymphocytes Monocyte Platelets Neutrophils
Neutrophil-Band Lymphocyte Neutrophil -drumstick Monocyte
