The Circulatory System The Circulatory System
Two of the body's most important functions are transportation and protection. The systems that provide these vital services for the body are the circulatory system and the lymphatic system.
The LYMPHATIC SYSTEM
The lymphoid organs are classified as either primary or secondary. The primary lymphoid organs are the thymus and bone marrow. The secondary lymphoid organs consist of the spleen, lymph nodes, tonsils, and Peyer patches of the small intestines.
All of the lymphoid organs link the haematologic and immune systems in that they are sites of residence, proliferation, differentiation, or function of lymphocytes and mononuclear phagocytes (monocytes and macrophages).
The CIRCULATORY SYSTEM
Plasma
Blood plasma is the liquid part of the blood (or blood minus its formed components). The most abundant type of solute in the plasma is a group of plasma proteins:
Blood serum is plasma minus its clotting factors such as fibrinogen.
The continuous movement of the blood keeps the formed elements suspended throughout the plasma where they are available to carry out their principal functions:
Formed elements
There are three main types and several subtypes of cellular components, or formed elements, in the blood:
Components of the Blood |
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| Plasma (55%) | Formed Elements (45%) | ||||
| Proteins (7%) |
Water (91%) |
Other Solutes (2%) |
Platelets | Leukocytes | Erythrocytes |
| albumin's (58%) | ions |
neutrophils (60-70%) | |||
| globulins (38%) | nutrients | lymphocytes (20-25%) | |||
| fibrinogen (4%) | waste products | monocytes (3-8%) | |||
| gases | eosinophils (2-4%) | ||||
| regulatory substances | basophils (0.5-1%) | ||||
BODY CELL TYPES
Platelets
Platelets are not cells, but are rather disc shaped cytoplasmic fragments. They are formed from megakaryocytes. Platelets are essential for coagulation and control of bleeding. They lack a nucleus and therefore have no DNA and are not able to perform mitotic division.
Leukocytes
Leukocytes are white blood cells that defend the body against infection and remove debris, including dead or injured host cells of all kinds.
Leukocytes are named according to structure as being either granulocytes or agranulocytes, and according to the function as either phagocytes or immunocytes. The granulocytes, which include neutrophils, basophils and eosinophils, are all phagocytes. Of the agranulocytes, the monocytes and macrophages are phagocytes whereas the lymphocytes are immunocytes (cells that create immunity).
Granulocytes:
The granulocytes are named because of the many membrane bound granules in their cytoplasm. The granules contain enzymes which are capable of killing microorganisms and destroying debris ingested by the process of phagocytosis.
The neutrophil's cytoplasm has small lysosomal granules and central nucleus with two to five distinct lobes. These cells are the chief phagocytes of early inflammation.
Agranulocytes:
The agranulocytes - monocytes, macrophages, and lymphocytes - differ from the granulocytes in that they do not contain lysosomal granules in their cytoplasm.
Monocytes and macrophages are classified as agranulocytes. Both of these cells are active participants in the immune and inflammatory response because they are very powerful phagocytes. They also ingest dead or defective host cells - in particular, blood cells. Monocytes are essentially immature macrophages. After monocytes are formed and released by the bone marrow, they enter the bloodstream and circulate for about 36 hours while they mature into macrophages.
Lymphocytes are known as the primary cells of the immune response. Most of this form of cell are located in the lymphoid tissues. Only a very small portion circulate in the blood. There are many types of lymphocytes; the most important of which are the T-cells, B-cells and mature B cells (plasma cells).
Erythrocytes
Erythrocytes are the most abundant cells in the blood, occupying about 48% of the blood volume in men and 42% in women. Erythrocytes are primarily responsible for tissue oxygenation. Because erythrocytes are unable to undergo mitotic division, these cells live out their life span (about 120 days) in the circulatory system, die, and are replaced by a new erythrocyte.
Erythrocytes have distinct size and shape and are ideally suited to the function of 'carriers of gases'. In short, erythrocytes are small discs with two distinct properties:
The flattened, biconcave shape provides for a surface area that is optimal for diffusion in and out of the cell. Reversible deformity enables the erythrocyte to change its shape and squeeze through the microcirculation and then return to normal.
| Body Cell | Function |
| Erythrocyte (RBC) | Oxygen and carbon dioxide transport |
| Neutrophil | Immune defense (phagocytosis) |
| Eosinophil | Defense against parasites |
| Basophil | Inflammatory response |
| B-lymphocyte | Antibody production (precursor of plasma cells) |
| T-lymphocyte | Cellular immune response |
| Monocyte | Immune defense (phagocytosis) |
| Platelet | Blood clotting |
QUICK FACTS
The formation of new blood cells is called hemopoiesis. Red bone marrow forms all types of blood cells except some lymphocytes and monocytes.
Most adults have between 4-6 L of blood which normally accounts for about 7-9% of total body weight.
Blood is alkaline with a pH between 7.35-7.45.
DEVELOPMENT of BLOOD CELLS
Blood cell production, known as hematopoiesis occurs in the liver and spleen of the fetus, but after birth it normally only occurs in the bone marrow. Hematopoiesis is a two stage process that involves mitotic division, or proliferation, and maturation, or differentiation. Hematopoiesis continues throughout life to replace blood cells that grow old and die, are killed by disease, or are lost through bleeding.
MECHANISMS of HEMOSTASIS
Hemostasis means arrest of bleeding. Mechanisms of hemostasis maintain a relatively steady state of blood volume pressure, and flow through injured blood vessels after vascular damage and bleeding. These important components of hemostasis are platelets, blood proteins (clotting factors) and the vasculature.
Hemostasis involves a complex sequence of events:
Each of these events involve platelets, clotting, and the blood vessels (vasculature).
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