Chapter 17: Blood – Structure, Function, and Clinical Aspects

Blood: Overview and Circulation

Systemic and Pulmonary Circuits

Blood is a fluid connective tissue essential for transporting substances throughout the body. It circulates via two main circuits: the systemic circuit, which delivers oxygenated blood to tissues, and the pulmonary circuit, which exchanges gases in the lungs. - Systemic Circuit: Blood exits the heart through arteries, branches into capillaries for nutrient and gas exchange, and returns via veins. - Pulmonary Circuit: Blood flows from the heart to the lungs for oxygenation and back.

Blood Composition

Components of Blood

Blood consists of plasma (the non-living fluid matrix) and formed elements (living cells and cell fragments). - Plasma: Makes up ~55% of blood volume; contains water, proteins, nutrients, hormones, and waste products. - Formed Elements: Includes erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Formed Elements: Types and Functions

- Erythrocytes (RBCs): Transport oxygen. - Leukocytes (WBCs): Provide protection against pathogens. - Platelets: Essential for blood clotting.

Physical Characteristics and Volume

Properties of Blood

Blood is a sticky, opaque fluid with a metallic taste. Its color varies with oxygen content: scarlet when oxygen-rich, dark red when oxygen-poor. - pH: 7.35–7.45 - Volume: 5–6 L in males, 4–5 L in females

Functions of Blood

Distribution, Regulation, and Protection

Blood serves three primary functions: - Distribution: Transports oxygen, nutrients, hormones, and waste. - Regulation: Maintains temperature, pH, and fluid volume. - Protection: Prevents blood loss (clotting) and infection (immune cells).

Blood Plasma

Composition and Plasma Proteins

Plasma is 90% water and contains over 100 dissolved solutes. - Plasma Proteins: Most abundant solutes, produced mainly by the liver. - Albumin (60%): Maintains osmotic pressure, acts as a buffer, and transports substances. - Globulins (36%): Alpha and beta globulins transport; gamma globulins are antibodies. - Fibrinogen (4%): Key for blood clotting.

Formed Elements: Detailed Structure

Erythrocytes (Red Blood Cells)

Erythrocytes are biconcave, anucleate cells specialized for gas transport. Their shape increases surface area for efficient oxygen exchange and flexibility for passage through capillaries. - Hemoglobin: The main protein, binds oxygen reversibly. - Spectrin: Provides structural flexibility.

Hemoglobin Structure and Function

Hemoglobin consists of four polypeptide chains (two alpha, two beta) and four heme groups, each with a central iron atom that binds oxygen. - Oxyhemoglobin: Hemoglobin bound to O2 (ruby red). - Deoxyhemoglobin: Hemoglobin without O2 (dark red). - Carbaminohemoglobin: Hemoglobin bound to CO2.

Hematopoiesis: Blood Cell Formation

Location and Process

Hematopoiesis occurs in red bone marrow, where hematopoietic stem cells (hemocytoblasts) give rise to all formed elements. - Hormonal Regulation: Erythropoietin (EPO) stimulates RBC production in response to hypoxia. - Dietary Requirements: Iron, vitamin B12, folic acid, amino acids, and lipids are essential.

Erythropoiesis: Stages of RBC Production

The process involves several stages: myeloid stem cell → proerythroblast → erythroblast (basophilic, polychromatic, orthochromatic) → reticulocyte → mature erythrocyte.

Regulation of Erythropoiesis

RBC production is tightly regulated to maintain oxygen delivery and blood viscosity. EPO is released by the kidneys in response to low oxygen levels, not RBC count.

Fate and Destruction of Erythrocytes

Lifecycle and Breakdown

RBCs live for 100–120 days. Old cells are removed by macrophages in the spleen. Hemoglobin is broken down: iron is salvaged, heme is converted to bilirubin (excreted in bile), and globin is recycled as amino acids.

Erythrocyte Disorders

Anemia and Polycythemia

- Anemia: Reduced O2 carrying capacity; caused by blood loss, low RBC production, or high RBC destruction. - Polycythemia: Excess RBCs increase blood viscosity; can be primary (bone marrow cancer) or secondary (high altitude, blood doping).

Leukocytes (White Blood Cells)

Types and Functions

Leukocytes are less than 1% of blood volume and are crucial for defense. They are classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

Granulocytes

- Neutrophils: Most numerous, phagocytic, respond to bacterial infection. - Eosinophils: Digest parasitic worms, modulate immune response. - Basophils: Release histamine, involved in inflammation.

Agranulocytes

- Lymphocytes: T cells (attack infected cells), B cells (produce antibodies). - Monocytes: Differentiate into macrophages, phagocytize pathogens.

Leukopoiesis: Formation of WBCs

Origin and Regulation

Leukopoiesis is stimulated by interleukins and colony-stimulating factors (CSFs). All WBCs originate from hematopoietic stem cells.

Platelets

Structure and Function

Platelets are cytoplasmic fragments of megakaryocytes, essential for blood clotting. They form temporary plugs and release clotting chemicals.

Platelet Formation

Platelet production is regulated by thrombopoietin.

Hemostasis: Stoppage of Bleeding

Three Steps of Hemostasis

Hemostasis involves vascular spasm, platelet plug formation, and coagulation. - Vascular Spasm: Vasoconstriction of damaged vessel. - Platelet Plug Formation: Platelets adhere to exposed collagen and release chemicals. - Coagulation: Fibrin threads reinforce the plug; clotting factors and Ca2+ are required.

Coagulation: Overview

Coagulation is a cascade of reactions converting fibrinogen to fibrin.

Blood Clotting Factors Table

Factor Number	Factor Name	Nature	Source	Pathway Function
I	Fibrinogen	Plasma protein	Liver	Converted to fibrin
II	Prothrombin	Plasma protein	Liver	Converted to thrombin
IV	Calcium ions (Ca2+)	Inorganic ion	Diet	Needed for all stages of coagulation
V	Proaccelerin	Plasma protein	Liver	Common pathway
XIII	Fibrin stabilizing factor	Plasma protein	Liver	Cross-links fibrin
...	...	...	...	...
Additional info:	Other factors (III, VII, VIII, IX, X, XI, XII) are also involved in various stages of the cascade.

Disorders of Hemostasis

Thromboembolic and Bleeding Disorders

- Thrombus: Clot in unbroken vessel; may block circulation. - Embolus: Floating clot; can cause embolism. - Thrombocytopenia: Low platelet count; causes spontaneous bleeding. - Hemophilia: Hereditary disorder; lack of clotting factors.

Transfusions and Blood Groups

ABO and Rh Blood Groups

Blood groups are determined by antigens (agglutinogens) on RBC membranes. - ABO Groups: A, B, AB, O; O is universal donor, AB is universal recipient. - Rh Groups: Rh+ (antigen present), Rh– (antigen absent).

Hemolytic Disease of the Newborn

Occurs when Rh– mother carries Rh+ fetus; anti-Rh antibodies can cross placenta and destroy fetal RBCs in subsequent pregnancies.

Diagnostic Blood Tests

Common Tests and Their Uses

- Hematocrit: Measures RBC percentage; used to diagnose anemia. - Blood Glucose: Assesses diabetes. - WBC Count: Indicates infection, allergy, or parasite. - Platelet Count: Assesses hemostasis. - Complete Blood Count (CBC): Evaluates overall blood health.

Developmental Aspects

Blood Formation in Fetus and Aging

Fetal blood cells form in yolk sac, liver, and spleen; red bone marrow becomes primary site by seventh month. Hemoglobin F in fetus has higher O2 affinity than adult hemoglobin A. Blood diseases increase with aging, often due to cardiovascular or immune disorders.