Blood: Structure, Function, and Components – Anatomy & Physiology Study Notes

Blood: Structure, Function, and Components

Overview of Blood

Blood is a specialized connective tissue essential for the transport of nutrients, gases, waste products, and cells throughout the body. It plays a critical role in maintaining homeostasis and supporting metabolic processes.

• Blood consists of a liquid component (plasma) and formed elements (cells and cell fragments).

• The heart acts as a pump, and blood vessels serve as conducting tubes.

The Functions of Blood

Blood performs several vital functions necessary for survival and health:

• Transport: Delivers dissolved gases, nutrients, hormones, and metabolic wastes.

• Regulation: Maintains pH and ion composition of interstitial fluids.

• Restriction: Restricts fluid loss at injury sites via clotting.

• Defense: Defends the body against toxins and pathogens.

• Temperature Regulation: Absorbs and redistributes heat to regulate body temperature.

The Composition of Blood

Whole Blood Components

Whole blood can be separated into:

• Plasma (liquid portion)

• Formed elements (cells and cell fragments):

  • Red blood cells (RBCs)

  • White blood cells (WBCs)

  • Platelets

Blood can be separated by spinning (centrifugation) or by allowing it to settle.

Characteristics of Whole Blood

• Temperature: 38°C (slightly above body temperature)

• Viscosity: 5x more viscous than water due to cells and proteins

• pH: Narrow range, typically 7.35–7.45

• Volume: Approximately 5 liters in adults (males slightly more)

Plasma Composition

Plasma is the liquid matrix of blood, making up 46–65% of whole blood. It is about 92% water and 8% solutes.

• Solutes include:

  • Plasma proteins (albumins, globulins, fibrinogen, hormones)

  • Electrolytes (Na+, K+, Ca2+, Mg2+, Cl-, HCO3-)

  • Nutrients

  • Wastes

Plasma Proteins

• Albumins: Most common protein (60%), maintains osmotic pressure.

• Globulins: Includes antibodies (immunoglobulins) and transport globulins for fats, iron, and hormones.

• Fibrinogen: Clotting factor; converts to fibrin during clotting. Serum is plasma without clotting factors.

• Hormones: Includes insulin, prolactin, TSH, LH, FSH, etc.

Most plasma proteins are synthesized by the liver.

Other Solutes

• Electrolytes: Essential for cellular activities and osmotic pressure.

• Organic nutrients: For ATP production and building materials.

• Waste products: Urea, uric acid, etc.

Formed Elements

Red Blood Cells (Erythrocytes)

RBCs are the most common formed element in blood, responsible for oxygen transport.

• Normal count: ~5 million/μL

• Anemia: Low RBC count

• RBCs lack organelles and nuclei, and have a biconcave disc shape for increased surface area and flexibility.

• Contain hemoglobin for oxygen transport.

• Life span: ~120 days

Hemoglobin

• Globular protein with red pigment (contains iron, Fe).

• Accounts for 95% of intracellular proteins in RBCs.

• Enables oxygen transport and helps carry CO2.

• Normal range: 12–18 g/dL

Structure of Hemoglobin

• 4 polypeptide chains, each with a heme group containing iron.

• Oxygen binds to Fe in the lungs (oxyhemoglobin), and is released in tissues (deoxyhemoglobin).

RBC Life Span and Production

• RBCs are produced in the red bone marrow from stem cells (hemocytoblasts) via erythropoiesis.

• RBCs lack nuclei and organelles, so they cannot repair themselves and are phagocytosed by macrophages after about 120 days.

• Hemoglobin is broken down; iron is recycled or transported in the blood bound to transferrin.

Regulation of Erythropoiesis

• Stimulated by erythropoietin (EPO) from the kidneys in response to hypoxia (low O2).

• Other requirements: vitamin B12, B6, folic acid, iron, and protein.

Blood Types

Blood types are determined by the presence of specific antigens on RBC membranes. The immune system distinguishes self from non-self using these markers.

ABO Blood Groups

Blood type	Antigen (on RBC)	Antibody (in plasma)	Blood typing: Mixed with serum containing
A	A	Anti-B	Anti A antibodies
B	B	Anti-A	Anti B antibodies
AB	BOTH A & B	Neither Anti A nor Anti B	Neither
O	NEITHER	Both Anti A & Anti B	Both

If mismatched blood is transfused, agglutination (clumping) and hemolysis (cell rupture) can occur, which is dangerous and potentially fatal.

Rh Factor

• Rh-positive (Rh+) has the antigen; Rh-negative (Rh-) does not.

• Rh- individuals can develop antibodies after exposure to Rh+ blood (sensitization).

• Rh antibodies are a concern in pregnancy (hemolytic disease of the newborn).

Leukocytes (White Blood Cells, WBCs)

WBCs are spherical cells (6000–9000/mm3) that defend the body against pathogens and remove toxins, wastes, and abnormal cells.

• WBCs have nuclei and organelles; some have cytoplasmic granules.

• Two groups: Granulocytes and Agranulocytes.

Granular Leukocytes

• Neutrophils: Most common (50–70%), phagocytic, multi-lobed nucleus, attack pathogens.

• Eosinophils: 2–4%, attack parasites, involved in allergies.

• Basophils: Less than 1%, release histamine, involved in inflammation and allergies.

Agranular Leukocytes

• Monocytes: Largest WBCs (2–8%), become macrophages in tissues.

• Lymphocytes: 20–30%, include T cells, B cells, and NK cells; provide specific immunity.

WBC Production and Differential Counts

• Produced in bone marrow from stem cells (hemocytoblasts).

• Differential count: Indicates the number of each type of WBC, useful in diagnosis.

• Leukopenia: Low WBC count; Leukocytosis: High WBC count.

Platelets (Thrombocytes)

Platelets are cell fragments involved in blood clotting (hemostasis).

• Contain granules with clotting chemicals.

• Form temporary plugs and help contract after clot formation.

Hemostasis (Blood Clotting)

Hemostasis prevents blood loss after vessel injury and involves three phases:

1. Vascular Phase

• Vessel wall contracts (vasoconstriction), reducing blood flow.

• Lasts about 30 minutes.

2. Platelet Phase

• Platelets adhere to damaged area, form a temporary plug.

• Release chemicals to attract more platelets (positive feedback).

3. Coagulation Phase

• Complex cascade involving clotting factors from platelets and endothelial cells.

• Two pathways:

  • Extrinsic pathway: Initiated by external trauma, fast.

  • Intrinsic pathway: Initiated by trauma inside the vascular system, slower.

• Both pathways lead to the common pathway:

  1. Activation of Factor X

  2. Formation of prothrombinase

  3. Conversion of prothrombin to thrombin

  4. Thrombin converts fibrinogen to fibrin

• Fibrin forms a mesh that stabilizes the clot.

• Vitamin K and Ca2+ are required for clotting factor synthesis and activation.

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract, pulling vessel edges together.

• Fibrinolysis: Enzymatic breakdown of the clot after healing (plasmin digests fibrin).

Clinical Considerations

• Anemia: Low RBC count or hemoglobin.

• Leukemia: Cancers of blood-forming tissues.

• Embolus/Embolism: Drifting blood clot that can block vessels, causing myocardial infarction (heart attack) or stroke.

• Hemophilia: Inability to clot due to missing clotting factors.

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.