When a blood vessel ruptures, platelets combine with fibrin, derived from fibrinogen, to form a clot. Red Blood Cells A drop of blood the size of a pinhead contains approximately 5 million red blood cells erythrocytes.
They are small biconcave disks without a nucleus and get their red colour from an iron-containing protein called hemoglobin.
Red blood cells carry oxygen throughout the body. The three classes of formed elements are the erythrocytes red blood cells , leukocytes white blood cells , and the thrombocytes platelets. Erythrocytes, or red blood cells, are the most numerous of the formed elements. Erythrocytes are tiny biconcave disks, thin in the middle and thicker around the periphery.
The shape provides a combination of flexibility for moving through tiny capillaries with a maximum surface area for the diffusion of gases. The primary function of erythrocytes is to transport oxygen and, to a lesser extent, carbon dioxide. Leukocytes, or white blood cells, are generally larger than erythrocytes, but they are fewer in number. Even though they are considered to be blood cells, leukocytes do most of their work in the tissues.
They use the blood as a transport medium. In humans, the surface glycoproteins and glycolipids on red blood cells vary between individuals, producing the different blood types, such as A, B, and O. Red blood cells have an average life span of days, at which time they are broken down and recycled in the liver and spleen by phagocytic macrophages, a type of white blood cell.
The role of white blood cells is very different than that of red blood cells: they are primarily involved in the immune response to identify and target pathogens, such as invading bacteria, viruses, and other foreign organisms.
White blood cells are formed continually; some only live for hours or days, but some live for years. The morphology of white blood cells differs significantly from red blood cells. They have nuclei and do not contain hemoglobin. The different types of white blood cells are identified by their microscopic appearance after histologic staining, and each has a different specialized function.
The two main groups, both illustrated in Figure Granulocytes contain granules in their cytoplasm; the agranulocytes are so named because of the lack of granules in their cytoplasm. Some leukocytes become macrophages that either stay at the same site or move through the blood stream and gather at sites of infection or inflammation where they are attracted by chemical signals from foreign particles and damaged cells.
Lymphocytes are the primary cells of the immune system and include B cells, T cells, and natural killer cells. B cells destroy bacteria and inactivate their toxins. They also produce antibodies. T cells attack viruses, fungi, some bacteria, transplanted cells, and cancer cells. T cells attack viruses by releasing toxins that kill the viruses. Natural killer cells attack a variety of infectious microbes and certain tumor cells.
One reason that HIV poses significant management challenges is because the virus directly targets T cells by gaining entry through a receptor. After the HIV virus replicates, it is transmitted directly from the infected T cell to macrophages. The presence of HIV can remain unrecognized for an extensive period of time before full disease symptoms develop. Blood must clot to heal wounds and prevent excess blood loss. Small cell fragments called platelets thrombocytes are attracted to the wound site where they adhere by extending many projections and releasing their contents.
These contents activate other platelets and also interact with other coagulation factors, which convert fibrinogen, a water-soluble protein present in blood serum into fibrin a non-water soluble protein , causing the blood to clot. Many of the clotting factors require vitamin K to work, and vitamin K deficiency can lead to problems with blood clotting. Many platelets converge and stick together at the wound site forming a platelet plug also called a fibrin clot , as illustrated in Figure The plug or clot lasts for a number of days and stops the loss of blood.
Platelets are formed from the disintegration of larger cells called megakaryocytes, like that shown in Figure For each megakaryocyte, — platelets are formed with , to , platelets present in each cubic millimeter of blood. They contain many small vesicles but do not contain a nucleus. The liquid component of blood is called plasma, and it is separated by spinning or centrifuging the blood at high rotations rpm or higher.
The blood cells and platelets are separated by centrifugal forces to the bottom of a specimen tube. These include several plasma proteins proteins that are unique to the plasma , plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarized in Table 1. In addition to proteins, plasma contains a wide variety of other substances.
These include various electrolytes, such as sodium, potassium, and calcium ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All of these nonprotein solutes combined contribute approximately 1 percent to the total volume of plasma. When more than a few drops of blood are required, phlebotomists perform a venipuncture, typically of a surface vein in the arm.
They perform a capillary stick on a finger, an earlobe, or the heel of an infant when only a small quantity of blood is required. An arterial stick is collected from an artery and used to analyze blood gases. After collection, the blood may be analyzed by medical laboratories or perhaps used for transfusions, donations, or research.
While many allied health professionals practice phlebotomy, the American Society of Phlebotomy Technicians issues certificates to individuals passing a national examination, and some large labs and hospitals hire individuals expressly for their skill in phlebotomy.
Blood is a fluid connective tissue critical to the transportation of nutrients, gases, and wastes throughout the body; to defend the body against infection and other threats; and to the homeostatic regulation of pH, temperature, and other internal conditions.
Blood is composed of formed elements—erythrocytes, leukocytes, and cell fragments called platelets—and a fluid extracellular matrix called plasma.
More than 90 percent of plasma is water. The remainder is mostly plasma proteins—mainly albumin, globulins, and fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes, and dissolved gases.
Because of the formed elements and the plasma proteins and other solutes, blood is sticky and more viscous than water. It is also slightly alkaline, and its temperature is slightly higher than normal body temperature.
Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Chapter The Cardiovascular System. Search for:. An Overview of Blood Learning Objectives By the end of this section, you will be able to: Identify the primary functions of blood in transportation, defense, and maintenance of homeostasis Describe the general characteristics and functions of blood.
Describe major blood disorders. Name the fluid component of blood and the three major types of formed elements, and identify their relative proportions in a blood sample Discuss the unique physical characteristics of blood Identify the composition of blood plasma, including its most important solutes and plasma proteins.
Practice Question Visit this site for a list of normal levels established for many of the substances found in a sample of blood. Show Answer There are values given for percent saturation, tension, and blood gas, and there are listings for different types of hemoglobin. They perform a wide variety of tests on various body fluids, including blood. The information they provide is essential to the primary care providers in determining a diagnosis and in monitoring the course of a disease and response to treatment.
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