Blood: Structure, Function, and Cellular Components

Fluids of the Body

Overview of Body Fluids

The cells of the human body are serviced by two main fluids: blood and interstitial fluid. These fluids are essential for the transportation, regulation, and protection of the body's internal environment.

• Transportation: Blood transports oxygen (O2), carbon dioxide (CO2), metabolic wastes, nutrients, heat, and hormones throughout the body.

• Regulation: Blood helps regulate pH and body temperature, maintaining homeostasis.

• Protection: Blood provides defense against disease and prevents loss of blood through mechanisms such as clotting.

Components of Blood

Blood Composition and Hematocrit

Blood is a specialized connective tissue composed of plasma and formed elements. The hematocrit is the percentage of blood volume occupied by red blood cells (RBCs).

• Plasma: Makes up about 55% of blood volume; it is a straw-colored, sticky fluid that is approximately 90% water.

• Formed Elements: Comprise about 45% of blood volume and include erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Plasma contains ions (Na+, Cl-), nutrients, wastes, and three main proteins:

• Albumin

• Globulin

• Fibrinogen

Component	Percentage	Main Function
Plasma	55%	Transport of nutrients, hormones, and waste; regulation
Red Blood Cells (RBCs)	~99% of formed elements	Oxygen and carbon dioxide transport
White Blood Cells (WBCs)	~1% of formed elements	Immune defense
Platelets	~1% of formed elements	Blood clotting

Formation of Blood Cells

Hematopoiesis

Blood cells are continually replaced through the process of hematopoiesis. This occurs in different locations during development and adulthood.

• Embryo: Blood cell formation occurs in the yolk sac, liver, spleen, thymus, lymph nodes, and red bone marrow.

• Adult: Hematopoiesis occurs only in red bone marrow of flat bones (sternum, ribs, skull, pelvis) and ends of long bones.

Regulation of blood cell differentiation and proliferation is controlled by hormones and cytokines, including:

• Erythropoietin (EPO): Stimulates RBC production.

• Thrombopoietin (TPO): Stimulates platelet production.

Red Blood Cells (Erythrocytes)

Structure and Function

Erythrocytes are the most numerous formed element in blood. They are specialized for oxygen transport.

• Shape: Biconcave disk, about 8 microns in diameter, which increases surface area for gas exchange.

• Organelles: Mature RBCs lack nuclei and most organelles, maximizing space for hemoglobin.

• Hemoglobin: Each molecule can carry four oxygen molecules from lungs to tissue cells.

• Normal RBC Count: Males: 5.4 million/drop; Females: 4.8 million/drop.

Hemoglobin Structure:

• Composed of four polypeptide chains, each with a heme group that binds oxygen.

Oxygen Transport Equation:

$\text{Hb} + \text{O}_2 \rightarrow \text{HbO}_2$

Homeostasis and Life Cycle of Erythrocytes

Regulation of Blood Oxygen Capacity

The body maintains blood oxygen capacity through feedback mechanisms involving erythropoietin and the life cycle of erythrocytes.

• Stimulus: Decreased oxygen delivery to kidneys triggers increased erythropoietin release.

• Effect: Erythropoietin stimulates red bone marrow to produce more RBCs, increasing oxygen-carrying capacity.

• Life Cycle: Old erythrocytes are removed by the liver and spleen; their components are recycled.

Feedback Regulation Equation:

$\text{Decreased O}_2 \rightarrow \text{Increased EPO} \rightarrow \text{Increased RBC production}$

White Blood Cells (Leukocytes)

Classification and Function

Leukocytes are less numerous than RBCs and are classified based on the presence of cytoplasmic granules.

• Granulocytes: Neutrophils, Eosinophils, Basophils (contain visible granules)

• Agranulocytes: Monocytes, Lymphocytes (lack visible granules)

• Normal WBC Count: 5,000 to 10,000 cells per drop of blood (about 1 WBC for every 700 RBCs)

Leukocyte Type	Classification	Main Function
Neutrophils	Granulocyte	Phagocytosis of bacteria
Eosinophils	Granulocyte	Combat parasites, modulate allergic responses
Basophils	Granulocyte	Release histamine, mediate inflammation
Monocytes	Agranulocyte	Phagocytosis, become macrophages in tissues
Lymphocytes	Agranulocyte	Immune response (B cells, T cells, NK cells)

Diapedesis and Chemotaxis in Leukocytes

Mechanisms and Importance

Diapedesis is the process by which leukocytes exit the bloodstream by squeezing between endothelial cells of blood vessels to reach sites of infection or injury. Positive chemotaxis refers to the movement of leukocytes toward higher concentrations of chemical signals released by damaged tissues or pathogens.

• Mechanism: WBCs roll along the endothelium, adhere to it, and migrate through the vessel wall into interstitial fluid.

• Importance: Enables immune cells to reach and combat infections or tissue damage efficiently.

• Cells Involved: Neutrophils and macrophages are especially active in diapedesis and chemotaxis.

Example: During bacterial infection, neutrophils migrate from blood vessels to the site of infection by following chemotactic signals.

Hemostasis

Prevention of Blood Loss

Hemostasis is the process that stops bleeding in a quick and localized fashion when blood vessels are damaged. It prevents hemorrhage and maintains blood volume.

• Vascular Spasm: Immediate constriction of blood vessels to reduce blood flow.

• Platelet Plug Formation: Platelets adhere to the damaged area and aggregate, forming a temporary plug (positive feedback mechanism).

• Blood Clotting (Coagulation): Formation of fibrin threads stabilizes the platelet plug and forms a clot.

Coagulation Equation:

$\text{Fibrinogen} \xrightarrow{Thrombin} \text{Fibrin}$

Summary Table: Blood Components and Functions

Component	Main Function	Key Features
Plasma	Transport, regulation	90% water, proteins (albumin, globulin, fibrinogen)
Erythrocytes	Oxygen transport	Biconcave, no nucleus, hemoglobin-rich
Leukocytes	Immune defense	Granulocytes and agranulocytes, diapedesis
Platelets	Clotting	Cell fragments, initiate hemostasis

Additional info:

• Blood is classified as a connective tissue due to its origin from mesenchyme and its function in connecting body systems through transport.

• Positive feedback in platelet plug formation amplifies the response to vessel injury, ensuring rapid clotting.