BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.3. Composition of Blood

14.3.2. Blood Cells

Erythrocytes (Red Blood Cells)

Human erythrocytes are small cells that, when mature, lack a Nucleus and have the shape of biconcave discs (Table 14.2 and Fig. 14.1, B). The average diameter of an erythrocyte is 7–8 µm (compared to an average animal Cell diameter of about 20 µm), and its thickness is roughly 2.2 µm. Their unique shape provides a higher surface-area-to-volume ratio than a sphere, thereby increasing the surface area available for gas exchange. The small thickness of individual erythrocytes greatly facilitates the diffusion of gases from the surface to The Cell interior. Due to the elasticity of their membrane, erythrocytes can fold like an umbrella, allowing them to squeeze through capillaries whose lumen is smaller than the cell's diameter. A single cubic millimeter of blood (a droplet volume of approximately 50 mm³) contains about 5 million erythrocytes. However, this figure varies depending on the individual's age, sex, and health status. Erythrocytes make up nearly half of the blood volume, which accounts for its exceptionally high oxygen-binding capacity—approximately 20 ml of O₂ per 100 ml, meaning the oxygen capacity of blood is 20% (v/v).

Class="center">Table 14.2. Formed elements of blood (drawings are not to scale)

Erythrocytes contain large amounts of Hemoglobin—an oxygen-carrying protein pigment that gives blood its red color. Erythrocytes lack nuclei. (They also lack Mitochondria, which not only frees up extra space for hemoglobin but also forces them to respire anaerobically—that is, without consuming the oxygen they transport.) Hemoglobin reversibly binds oxygen (turning into oxyhemoglobin) in regions of high concentration and releases it in areas of low concentration. Erythrocytes also contain the enzyme Carbonic anhydrase, which is involved in Carbon dioxide transport (see Section 14.8.4).

The lifespan of an erythrocyte in an adult human is relatively short—approximately three months (due to the absence of a nucleus to regulate cellular repair processes), after which they are destroyed in the spleen or Liver. The protein portion of the erythrocyte is broken down into Amino Acids, while iron is released from the non-protein (pigment) part of hemoglobin, known as heme, and stored in the liver as the iron-storing protein ferritin. This iron can later be reused to form new erythrocytes or to synthesize Cytochromes. The remaining portion of the heme molecule is degraded into two Bile pigments: red bilirubin and green biliverdin, which are excreted into the gut via bile.

Every second, between 2 and 10 million erythrocytes are destroyed and replaced in The Human Body. Each cell contains roughly 250 million hemoglobin molecules. Because hemoglobin is a large protein, one can imagine how intense Protein Synthesis must be in the body simply to maintain the RESPIRATORY FUNCTION OF the blood. The rate of erythrocyte destruction and replacement depends partially on atmospheric oxygen levels. If The amount of oxygen entering the blood from the Lungs is low, the bone marrow produces more erythrocytes than are destroyed in the liver. This is one of the ways we adapt to oxygen deficiency at high altitudes. Consequently, mountaineers before scaling a peak—and athletes before high-altitude competitions—require a period of acclimatization. The reverse is true when oxygen levels in the air are high.

14.2. List the Major Groups of substances transported by the blood.

Leukocytes

Leukocytes are larger than erythrocytes and are present in the blood in much smaller numbers (approximately 7,000 per 1 mm³ of blood). They play a vital role in defending the body against disease. Every leukocyte has a nucleus. Despite having a nucleus, their lifespan in Circulation rarely exceeds a few days. All of them are capable of amoeboid movement, allowing them to squeeze through capillary walls at endothelial junctions and migrate toward infected Tissues.

Leukocytes can only be seen under a Light Microscope if they are stained. Stained preparations clearly reveal two main groups of leukocytes: granulocytes (or granular leukocytes), which contain granules in their Cytoplasm, and agranulocytes (or nongranular leukocytes), which lack such granules.

GRANULOCYTES (72%). Like erythrocytes, these cells are formed in the bone marrow, but from different precursors. They are characterized by segmented nuclei of quite bizarre shapes, which is why they are also called polymorphonuclear leukocytes (from the Greek poly — many, and morphē — form). They include neutrophils, eosinophils, and basophils.

1. Neutrophils (phagocytes) make up approximately 70% of total leukocytes. They are capable of squeezing between the cells that form capillary walls and migrating through the intercellular spaces of various tissues toward infected body sites. Neutrophils actively phagocytose—that is, engulf and digest—pathogenic bacteria (Section 14.8.5).

2. Eosinophils are distinguished by the presence of cytoplasmic granules that stain red with eosin. They normally account for only 1.5% of total leukocytes, but their numbers increase during allergic states (such as asthma or hay fever). Eosinophils possess antihistamine properties. Their blood levels are regulated by Hormones secreted by the adrenal cortex in response to A wide variety of stress stimuli.

3. Basophils comprise 0.5% of the total leukocyte population. When stained with basic Dyes, such as methylene blue, blue granules become visible within these cells. Basophils synthesize heparin, a protein that prevents blood clotting, and histamine, which notably initiates inflammatory responses in damaged tissues to promote rapid healing. In certain allergic conditions, such as hay fever, extremely high secretion of histamine is observed.

AGRANULOCYTES (28%). These cells contain no granules in their cytoplasm. Whereas granulocytes have multi-lobed or segmented nuclei, agranulocytes possess a distinct single nucleus that is oval or bean-shaped, leading to their Classification as mononuclear cells (Fig. 14.1, A). There are two MAIN TYPES OF agranulocytes.

1. Monocytes (4%) are produced in the bone marrow and feature a bean-shaped nucleus. They spend only 30–40 hours in the bloodstream before migrating into surrounding tissues to become macrophages.

2. Macrophages phagocytose bacteria and other relatively large particles. As will be explained in Section 14.9, they facilitate the Immune Response by binding and Processing certain Antigens. Together with neutrophils, they form a body-wide phagocytic system that serves as the first line of defense against infection.

3. Lymphocytes (24%) are produced in the Thymus and lymphoid tissue from bone marrow-derived cells. They are spherical cells with a high nucleus-to-cytoplasm ratio (very little cytoplasm). Their capacity for amoeboid movement is limited. Lymphocytes are also found in Lymph and other body tissues. There are two main types: T AND B lymphocytes (Section 14.9). They induce or participate in immune responses (aiding in antibody production, graft rejection, and the destruction of tumor cells). The lifespan of an individual lymphocyte varies widely—from a few days to well over ten years.