Blood: Components and Functions

Overview of the Cardiovascular System

The cardiovascular system is composed of the heart, blood vessels (arteries, veins, capillaries), and blood. Blood is a fluid connective tissue essential for transporting substances throughout the body and maintaining homeostasis.

• Arteries carry blood away from the heart.

• Veins return blood to the heart.

• Capillaries are sites of exchange with tissues.

• Blood is the fluid connective tissue that circulates through these vessels.

Functions of Blood

Blood performs several critical functions in the body:

• Transport of gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation of pH and ion composition of interstitial fluids.

• Restriction of fluid losses at injury sites (clotting).

• Defense against toxins and pathogens (immune response).

• Stabilization of body temperature.

Physical Characteristics of Blood

• Temperature: 38ºC (100.4ºF), slightly above normal body temperature.

• Viscosity: 5x more viscous than water.

• pH: Slightly alkaline (7.35–7.45).

• Volume: 7% of body weight; 5–6 L in adult males, 4–5 L in adult females.

Composition of Blood

Whole Blood: Plasma and Formed Elements

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

• Plasma: 55% of blood volume; contains water, proteins, and solutes.

• Formed Elements: 45% of blood volume; includes red blood cells, white blood cells, and platelets.

Plasma

Plasma is the liquid component of blood, similar in composition to interstitial fluid.

• 92% water

• 7% plasma proteins (albumins, globulins, fibrinogen, enzymes, hormones)

• 1% other solutes (electrolytes, organic nutrients, wastes)

Plasma Proteins

• Albumins (60%): Maintain osmotic pressure, transport fatty acids and hormones.

• Globulins (35%): Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen (4%): Functions in blood clotting; converted to fibrin.

• Other proteins (1%): Enzymes and hormones.

Most plasma proteins are synthesized in the liver, except antibodies (produced by plasma cells) and peptide hormones (produced by endocrine organs).

Formed Elements of Blood

Overview of Blood Cells

Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

• Red blood cells (RBCs): 99% of formed elements; transport oxygen.

• White blood cells (WBCs): Immune defense.

• Platelets: Cell fragments involved in clotting.

Red Blood Cells (Erythrocytes)

RBCs are specialized for oxygen transport and make up the majority of blood cells.

• Contain hemoglobin, which binds O2 and CO2.

• Normal count: 4.5–6.3 million/μL (males), 4.2–5.5 million/μL (females).

• Hematocrit: Percentage of RBCs in blood (46% males, 42% females).

• Biconcave shape increases surface area for gas exchange.

• Mature RBCs lack nucleus and ribosomes; lifespan ~120 days.

Hemoglobin Structure and Function

Hemoglobin is a protein with four subunits, each containing a heme group with iron that binds oxygen.

• Normal hemoglobin: 14–18 g/dL (males), 12–16 g/dL (females).

• Fetal hemoglobin binds oxygen more readily than adult hemoglobin.

• Anemia results from reduced hematocrit or hemoglobin, impairing oxygen delivery.

RBC Formation and Recycling

RBCs are produced in red bone marrow through erythropoiesis, stimulated by erythropoietin (EPO). Old RBCs are recycled by macrophages in the spleen, liver, and bone marrow.

• Iron from heme is recycled; bilirubin is excreted in bile.

• Jaundice results from bilirubin buildup.

Blood Types and Compatibility

Blood Type Determination

Blood types are based on surface antigens (A, B, Rh) present on RBCs.

• Type A: Antigen A

• Type B: Antigen B

• Type AB: Antigens A and B

• Type O: Neither antigen

• Rh positive: Rh antigen present

• Rh negative: Rh antigen absent

Agglutinogens and Agglutinins

Agglutinogens are surface antigens; agglutinins are antibodies in plasma that react with foreign antigens, causing agglutination (clumping).

• Type A: Anti-B antibodies

• Type B: Anti-A antibodies

• Type AB: No anti-A or anti-B antibodies

• Type O: Both anti-A and anti-B antibodies

• Rh−: Only sensitized Rh− blood has anti-Rh antibodies

Hemolytic Disease of the Newborn

Occurs when an Rh− mother carries an Rh+ fetus. Maternal anti-Rh antibodies can cross the placenta and attack fetal RBCs in subsequent pregnancies.

• Prevented by administering RhoGAM to the mother.

White Blood Cells (Leukocytes)

Types and Functions of WBCs

WBCs defend against pathogens, remove toxins, and attack abnormal cells. Five main types exist, each with distinct functions.

• Neutrophils: Phagocytize bacteria; most abundant.

• Eosinophils: Attack parasites, involved in allergic responses.

• Basophils: Release histamine and heparin; allergic response.

• Monocytes: Become macrophages; engulf pathogens.

• Lymphocytes: Adaptive immunity; B and T cells.

WBC Disorders

• Leukopenia: Low WBC count.

• Leukocytosis: High WBC count.

• Leukemia: Cancer of WBCs; extreme leukocytosis.

Platelets and Blood Clotting

Platelets (Thrombocytes)

Platelets are cell fragments essential for blood clotting. They circulate for 9–12 days and are stored in organs like the spleen.

• Normal count: 150,000–500,000/μL

• Mobilized during circulatory crisis

Hemostasis and Clotting Mechanisms

Hemostasis is the process of stopping blood loss. It involves three phases:

1. Vascular phase: Vascular spasm and release of chemicals.

2. Platelet phase: Platelets adhere to exposed collagen and endothelium.

3. Coagulation phase: Fibrinogen is converted to fibrin, forming a clot.

Calcium and vitamin K are essential for clotting. Thrombin forms a positive feedback loop to accelerate clotting.

Clot Retraction and Fibrinolysis

• Clot retraction pulls vessel edges together, stabilizing the injury site.

• Fibrinolysis dissolves the clot after repair is complete.

• Anticoagulants (e.g., antithrombin-III, heparin) regulate clotting.

Blood Disorders

• Thrombocytopenia: Low platelet count.

• Hemophilia: Deficiency of clotting factors.

• Thrombophilia: Excessive clot formation (e.g., DVT).

• Sickle cell anemia: Abnormal hemoglobin; RBCs break down easily.

• Thalassemias: Inability to produce functional hemoglobin subunits.

Blood Infections and Cancers

• Bacteremia: Bacteria circulate in blood.

• Sepsis: Bacteria multiply and produce toxins in blood.

• Viremia: Viruses circulate in blood.

• Malaria: Parasitic infection affecting RBCs.

• Leukemias: Cancers of blood-forming tissues; myeloid and lymphoid types.

Blood Collection and Testing

Obtaining Blood Samples

• Venipuncture: Blood drawn from superficial veins (e.g., median cubital vein).

• Veins are easier to locate and have lower pressure than arteries.

Blood Typing Table

The following table summarizes blood types, antigens, and antibodies:

Additional info: Universal donor is Type O−; universal recipient is Type AB+.

Blood Typing Test Results Table

Summary Table: Blood Cell Types