Components of Blood
Components of blood include:
- Formed elements, which include red blood cells/erythrocytes/RBCs, white blood cells/leukocytes/WBCs, and platelets/thrombocytes
- Plasma, which is liquid intercellular material
- Volume, which is 45% formed elements/cells and 55% plasma
- Hematocrit (packed cell volume/PCV), which is the volume of a whole blood sample occupied by red blood cells. Normal is between 39-50% in men and 35-35% in women
Red blood cells are biconcave discs 7.8μm in diameter, edge thickness of 2.5μm, and central thickness of 0.8μm.
White blood cells or leukocytes:
- Granulocytes contain specific granules
- Neutrophils are the most numerous WBCs and are readily identified by the multilobed nucleus. Other terms used are polymorphs and polymorphonuclear leukocytes (PMNs, Segs, Polys).
- Eosinophils are about the same size as neutrophils. They have bilobed nuclei and have two types of granules: large, elongated specific and azurophilic (non-specific). In LM, the granules stain bright showing intense acidophilia and appear highly refractive. Their specific granules contain a crystalloid body that is readily seen with TEM. It is surrounded by a less electron dense matrix and causes the refractivity seen with LM. Specific granules also contain major basic protein (MBP - localized in the crystalloid body), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), eosinophil-derived neurotoxin (EDN), histaminase, and collagenase. MBP, ECP, and EPO have strong cytotoxic effects on protozoa and helminths, while EDN affects the nervous system function in parasites.
- Basophils are the least numerous of the WBCs, and are about the same size as neutrophils. They have lobulated nuclei and many basophilic cytoplasmic granules. Their specific granules contain heparin and histamine. This causes a hypersensitivity reaction and makes the basophils functionally similar to mast cells.
- Agranulocytes lack specific granules
- Lymphocytes are the main functional cells of the lymphatic/immune system. There are three types of lymphocytes: small (6-8μm - make up 90% of lymphocytes in blood), medium (8-10μm), and large (10-18μm). They make up 30% of all WBCs.
- Monocytes are the largest white cells in a smear (~18μm). In the blood, they are in transit from the bone marrow to bone tissues where they will differentiate into phagocytes of the mononuclear phagocytic system (MPS). They will remain in the blood for about 3 days. Monocytes have a large indented (C Shaped) nucleus, with the indent being adjacent to the center. Golgi and centrioles are present in the indent. Basophilic cytoplasm contains azurophilic granules.
Both types of WBCs have non-specific azurophilic granules, which contain myeloperoxidase, lysosomal acid hydrolase, and defensins. They destroy parasites.
Platelets or Thrombocytes are very small cell fragments that are non-nucleated. They are 2-4μm in diameter and formed in the bone marrow from the cytoplasm of megakaryocytes. Number in circulating blood random from 150,000-400,000/μL. Circulating platelets have a life space of 10 days. Their main function is hemostasis. The cytoplasm is divided into two zones:
- The outer hyalomere (poor stains) demonstrates three types of structures:
- The peripheral most marginal bundle consists of microtubules and microfilaments (actin and myosin), which helps maintain the shape
- An open canalicular system consists of vesicles connected to invaginations of the plasma membrane
- Dense tubular system consists of vesicles derived from the endoplasmic reticulum and stored calcium ions
- The inner granulomere (contains darkly staining granules) contains most cytoplasmic organelles (mitochondria, Golgi, ribosomes, and glycogen particles). There are four types of electron dense granules:
- Alpha granules
- Delta granules or dense granules
- Lysosomes
- Microperoxisomes
Platelets are involved with blood clot formation. Fibrinogen is converted to fibrin, and red blood cells are trapped in the fibrin forming a hemostatic plug.