Blood: Structure, Function, and Cellular Components (Anatomy & Physiology II, Chapter 19)

Blood: Structure and Functions

Overview of Blood Functions

Blood is a vital connective tissue that performs several essential functions in the human body. It transports substances, regulates physiological parameters, restricts fluid losses, defends against pathogens, and stabilizes body temperature.

• Transport: Blood carries gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation: Maintains pH and ion balance in interstitial fluids.

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

• Defense: Transports white blood cells and antibodies to fight infections.

• Temperature Stabilization: Absorbs and redistributes heat throughout the body.

Physical Characteristics and Sampling of Blood

Physical Properties

Blood exhibits distinct physical characteristics that are important for its function and clinical assessment.

• Temperature: 100.4°F (slightly above body temperature)

• Viscosity: 5 times more viscous than water

• pH: 7.35–7.45

• Volume: Males: 5–6 liters; Females: 4–5 liters

Blood Collection Methods

Blood is commonly collected for diagnostic purposes using venipuncture or arterial puncture.

• Venipuncture: Most common, taken from the median cubital vein; veins are easier to locate and have lower pressure.

• Arterial Puncture: Drawn from radial or brachial artery; used to assess gas exchange efficiency in the lungs.

Composition of Blood

Main Components

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

• Plasma: 55–60% of blood; yellowish fluid containing proteins, solutes, and water.

• Formed Elements: 40–45% of blood; includes red blood cells (RBCs), white blood cells (WBCs), and platelets.

Plasma Composition

• Plasma Proteins: Albumin (60%), Globulins (35%), Fibrinogen (4%), Regulatory proteins (<1%)

• Other Solutes: Electrolytes (Na+, K+), organic nutrients, organic wastes

• Water: Most abundant component (92%)

Formed Elements of Blood

Red Blood Cells (Erythrocytes)

RBCs are specialized for oxygen and carbon dioxide transport. Their unique structure enhances their function.

• Shape: Biconcave disk, thin center, thick edge

• Features: No nucleus or mitochondria; high surface area for gas exchange; flexible and stackable for capillary passage

• Lifespan: Approximately 120 days

Hemoglobin Structure and Function

Hemoglobin is the protein responsible for oxygen transport in RBCs. It consists of four polypeptide chains (2 alpha, 2 beta), each with a heme group containing iron.

• Oxyhemoglobin (HbO2): Oxygen-bound form; bright red color

• Deoxyhemoglobin: Oxygen released

• Carbaminohemoglobin: CO2 bound

• Each hemoglobin molecule can carry up to four O2 molecules

Abnormal Hemoglobin and Anemia

• Iron Deficiency Anemia: Not enough iron to make hemoglobin

• Pernicious Anemia: Low RBC production due to lack of vitamin B12 absorption

• Sickle Cell Anemia: Mutation in beta chain causes RBCs to become stiff and curved

• Thalassemia: Inability to produce adequate alpha or beta chains

RBC Recycling and Destruction

Old or damaged RBCs are broken down by macrophages in the liver and spleen. Hemoglobin is split into heme and globin; iron is recycled, and heme is converted to biliverdin and then bilirubin.

• Biliverdin: Green pigment

• Bilirubin: Yellow/orange pigment; excreted in bile

• Jaundice: Accumulation of bilirubin causes yellow skin

Blood Types and Antigens

Blood Typing

Blood type is determined by the presence or absence of surface antigens (A, B, Rh) on RBCs. Antibodies in plasma react with foreign antigens, causing agglutination.

• Type A: Surface antigen A, anti-B antibodies

• Type B: Surface antigen B, anti-A antibodies

• Type AB: Both antigens, no anti-A/B antibodies (universal recipient)

• Type O: No antigens, both anti-A and anti-B antibodies (universal donor)

• Rh factor: Rh+ (antigen present), Rh- (antigen absent)

Erythroblastosis Fetalis (Hemolytic Disease of the Newborn)

Occurs when an Rh- mother develops antibodies against Rh+ fetal blood, which can cross the placenta and attack the fetus in subsequent pregnancies.

Origin and Production of Formed Elements

Hematopoiesis

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow. Hemocytoblasts (pluripotent stem cells) differentiate into myeloid and lymphoid stem cells, which give rise to RBCs, WBCs, and platelets.

• RBCs: Hemocytoblast → myeloid stem cell → proerythroblast → erythroblast → reticulocyte → erythrocyte

• WBCs: Hemocytoblast → myeloid/lymphoid stem cell → progenitor → blast cell → mature leukocyte

• Platelets: Hemocytoblast → myeloid stem cell → megakaryocyte → platelets

White Blood Cells (Leukocytes)

Classification and Functions

WBCs are divided into granular and agranular types, each with specific roles in immunity.

• Granular Leukocytes: Neutrophils, Eosinophils, Basophils

• Agranular Leukocytes: Monocytes, Lymphocytes

Key Characteristics

• Amoeboid Movement: Enables mobility

• Diapedesis: Ability to migrate out of bloodstream

• Positive Chemotaxis: Attraction to chemical signals

• Phagocytosis: Neutrophils, eosinophils, monocytes engulf pathogens

Types of WBCs

• Neutrophils (50–70%): First responders, phagocytize bacteria, form pus

• Eosinophils (2–4%): Defend against parasites, involved in allergic responses

• Basophils (<1%): Release histamine and heparin, mediate inflammation

• Monocytes (2–8%): Become macrophages, present antigens to T-cells

• Lymphocytes (20–30%): T-cells (cell-mediated immunity), B-cells (antibody production), NK cells (kill abnormal cells)

Platelets and Hemostasis

Platelet Structure and Function

Platelets are cell fragments essential for blood clotting. They aggregate, adhere, and form plugs at injury sites.

• Lifespan: 7–10 days

• Properties: Agglutination, adhesiveness, aggregation

Hemostasis: Phases of Blood Clotting

Hemostasis is the process that prevents blood loss after injury, involving three phases:

• Vascular Phase: Vessel contracts to reduce blood flow

• Platelet Phase: Platelets adhere and form a plug

• Coagulation Phase: Cascade of reactions forms a fibrin clot

Blood Clotting Inhibitors and Disorders

• Anticoagulants: Antithrombin III, heparin, thrombomodulin, prostacyclin

• Hemophilia: Genetic disorder causing inadequate clotting factor production

• Embolus: Drifting blood clot; can cause infarction

• Thrombus: Clot inside vessel; associated with plaque

Key Definitions

• Plasma: Liquid component of blood

• Antibodies: Proteins that bind antigens

• Hemocytoblasts: Blood stem cells

• Hematocrit: Percentage of blood volume occupied by cells

• Oxyhemoglobin: Oxygen-bound hemoglobin

• Heme molecules: Iron-containing pigment in hemoglobin

• Anemia: Reduced oxygen-carrying capacity

• Jaundice: Yellow skin due to bilirubin accumulation

• Hypoxia: Low oxygen levels in tissues

• RhoGam: Medication to prevent Rh antibody formation

• Hypovolemic: Low blood volume

• Viscosity: Thickness of blood

• Macrophage: Phagocytic cell derived from monocytes

• Coagulation: Blood clotting process

• Embolism: Blockage by a clot

• Infarction: Tissue death due to blocked blood supply

• Thrombus: Stationary blood clot

Key Equations

• Hematocrit Calculation: $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

• Oxygen Carrying Capacity: $\text{O}_2 \text{ capacity} = \text{Hb concentration} \times 1.34 \text{ mL O}_2/\text{g Hb}$