Cells of the Blood
Blood is specialized fluid connective tissue. It travels through the circulatory system transporting gases, nutrients, wastes, and other macromolecules throughout the body. The main gases traveling in the blood are oxygen and carbon dioxide. Circulating macromolecules include hormones, nutrients, plasma proteins, and some humoral components of the immune system. Blood also acts as a buffer to maintain homeostasis and partakes in regulation of body temperature.
Blood is made up of two main components: blood cells and plasma. The plasma, an aqueous solution, is mostly made up of water but contains some important solutes. About 7-8% of the plasma is made up of the proteins albumin, globulins, and fibrinogen. Other solutes, only 1-2% of plasma, include electrolytes such as Na+ and Ca+, urea and uric acid, glucose, lipids, hormones, and gases. Blood cells are classified as either erythrocytes or leukocytes. Erythrocytes are commonly referred to as red blood cells or RBCs. Leukocytes are white blood cells, or WBC. A smaller third cellular constituent are the thrombocytes (platelets).
In routine laboratory testing, blood can be spun down in a centrifuge to separate the plasma from the blood cells. Spinning down the cells reveals the hematocrit of the blood. The hematocrit is a measure of the erythrocyte content of a blood sample. A normal reading is between 39-50%. Males have a slightly higher hematocrit than females. Leukocytes and platelets separate from the hematocrit and form a small buffy coat between the plasma and erythrocytes.
Red Blood Cells
Erythrocytes, or red blood cells (RBCs), have predictable dimensions which makes them a good “histologic ruler”. They are circular with a diameter of 7.8 micrometers. Erythrocytes are shaped like biconcave discs (donut). At their perimeter they are 2.6 micrometers thick, at the center they are only 0.8 micrometers thick. This shape increases the cell's surface area to allow for more oxygen binding. Because these dimensions are typically consistent, leukocytes can be identified by comparing their size to that of the erythrocytes.
The shape of erythrocytes depends largely on membrane proteins. Integral and peripheral membrane proteins maintain the biconcave shape of erythrocytes. There are more integral membrane proteins in the lipid bilayer than peripheral membrane proteins. They are classified as either glycophorin or band 3 proteins. Band 3 proteins bind to hemoglobin and act as an anchor for cytoskeletal proteins. Glycophorin proteins form the main attachments between the lipid bilayer and the cytoskeleton.
Hemoglobin is a globular protein found in erythrocytes. It is capable of binding and carrying several gases, including the vasodilator nitrous oxide, but its main function is to transport oxygen and carbon dioxide. Blood carries oxygen away from the lungs to the periphery and carbon dioxide from peripheral tissues back to the lung to facilitate gas exchange in the alveoli. A hemoglobin protein is made up of four polypeptide chains; two alpha chains and two beta chains, each featuring a heme group capable of binding oxygen. The heme group itself is shaped like a ring with an iron atom in the center.
Oxygen binds to hemoglobin through positive cooperation. That is, once an oxygen ion binds to a heme group, the iron atom is shifted in the ring which facilitates the binding of oxygen ions to the remaining three heme groups. The erythrocytes can then transport oxygen from the lungs to tissues throughout the body. There are approximately 250 million hemoglobin proteins in each erythrocyte therefore one red blood cell can carry around one billion oxygen ions at once.
Leukocytes, also called White Blood Cells (WBCs), are associated with the immune system. There are three main classifications of leukocytes: lymphocytes, granulocytes, and agranulocytes. Granulocytes, as their name suggests, are white blood cells that have granules present in their cytoplasm. Agranulocytes do not have these granules. Using a special histological staining method (Wright’s Stain), leukocytes can be seen under a microscope. This stain includes azure B to stain the granules in the cytoplasm of white blood cells so that they can be differentiated.
Leukocytes are classified by both the shape of their nuclei and the presence or absence of azurophilic granules in their cytoplasm. Neutrophils, basophils, and eosinophils are all polymorphonuclear granulocytes. Polymorphonuclear cells have nuclei with multiple lobes. Monocytes and lymphocytes are mononuclear agranulocytes. Mononuclear cells have a single nucleus. The buffy coat, seen when blood is spun down in a centrifuge to separate its cellular components, is generally made up of 60-70% neutrophils, 20-25% lymphocytes, 3-8% monocytes, 2-4% eosinophils, and less than 1% basophils.
Neutrophils make up over half of the volume of white blood cells. The cytoplasm stains lightly and contains small, lavender colored granules. They are about 12-15 micrometers in diameter and have dark-staining multilobed nuclei. There are three types of granules in the cytoplasm of neutrophils: specific or secondary granules, azurophilic granules, and tertiary granules.
Specific granules are the smallest and most numerous and contain several enzymes: phospholipase and type IV collagenase. Azurophilic granules are the neutrophils’ lysosomes. There are two types of tertiary granules that contain either phosphatases or metalloproteinases. Neutrophils are active phagocytes that engulf bacteria. They can be considered the first responders of the (innate) immune system as they are often the first leukocytes to be activated in response to a pathogen.
The large, rounded nucleus of a lymphocyte takes up most of the volume of the cell and stains very dark blue. The cytoplasm appears as a narrow, lightly stained rim around the large nucleus where it does not contain granules.
Lymphocytes, the main functional cells of the immune system, are further classified by function and origin. For example, NK (natural killer) cells destroy virus infected and cancerous cells, B lymphocytes are involved in the production of circulating antibodies, and T lymphocytes are involved in cell-mediated immunity. Immunohistochemistry is required to identify individual types of lymphocytes in microscopy.
Eosinophils contain many brightly staining granules in their cytoplasm that give them a characteristic pink color when viewed with a Wright’s stain. Their bilobed nuclei stain lightly in comparison to the granules. Eosinophils are also about 12-15 micrometers in diameter. Eosinophils release histaminase during allergic reactions and are associated with inflammatory responses and helminthic responses to parasitic and protozoan infections.
Basophils are the least abundant leukocytes in the blood; however, they are the largest granulocyte. The cytoplasm of basophils contains many blue granules that obscure the lightly stained bilobed nucleus. Basophils are associated with allergic responses as well: they release histamine and vasoactive agents to dilate blood vessels thereby intensifying the allergic reaction.
Monocytes are the largest leukocytes circulating in the blood at 12-20 micrometers in diameter. They are agranulocytes with a light and foamy cytoplasm. Monocytes are easily identified by their size and their large, kidney or horse-shoe shaped nucleus.
In the periphery, monocytes differentiate into phagocytic macrophages. Phagocytosis is a process in which a cell engulfs and ingest a macromolecule or microorganism. The macrophages differentiate further upon reaching their target tissues. Kupffer cells are found in the liver. Alveolar macrophages are found in the lung. The red pulp of the spleen contains splenic macrophages. Peritoneal macrophages are found free-floating in peritoneal fluid. Microglial cells are specialized macrophages found in the nervous system.
Thrombocytes, more commonly known as platelets, are small, formed elements found throughout the blood. Because they do not contain a nucleus or organelles, they are not considered to be true cells. Rather, they are cell fragments derived from megakaryocytes, polyploid cells found in bone marrow. At only 2-4 micrometers in diameter, they are significantly smaller than any other cellular structures in the blood. Functionally, platelets are essential for forming clots in response to tears in blood vessels.
Blood Clot Formation
Blood clots are formed in response to injury/trauma of a blood vessel. The four basic steps to coagulation are:
Platelets are activated by the exposed collagen
Platelet factors are released
More platelets are attracted by the platelet factors
Platelets come together to form a plug
In more detail, a tear in a blood vessel exposes collagen. Platelets bind to the exposed collagen through membrane proteins called integrins. This binding activates the platelets and they release serotonin, ADP, and platelets-activating factor. More platelets are recruited to the site of injury by the platelets-activating factor.
Platelets-activating factor simultaneously activates pathways to convert phospholipids in the membranes: thromboxane A2. Thromboxane A2 and serotonin act as vasoconstrictors and contribute to platelet accumulation. Ultimately, the recruited platelets form a platelet plug that will be converted to a clot through the coagulation cascade. The coagulation cascade is a series of reactions that results in the conversion of fibrinogen to fibrin and the formation of a cross-linked fibrin mesh that traps erythrocytes.