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

Blood: Internal Transport System

Overview

Blood is a life-sustaining fluid tissue that serves as the primary transport vehicle of the cardiovascular system. It is essential for the delivery of oxygen, nutrients, and hormones, as well as the removal of metabolic wastes. Blood also plays critical roles in regulation and protection within the body.

Functions of Blood

Transport

• Oxygen and Nutrients: Blood delivers O2 and nutrients to body cells.

• Waste Removal: Transports metabolic wastes to lungs and kidneys for elimination.

• Hormone Distribution: Carries hormones from endocrine organs to target tissues.

Regulation

• Temperature: Maintains body temperature by absorbing and distributing heat.

• pH Balance: Maintains normal pH using buffers, including bicarbonate ions.

• Fluid Volume: Ensures adequate fluid volume in the circulatory system.

Protection

• Clot Formation: Plasma proteins and platelets initiate clot formation to prevent blood loss.

• Immunity: Blood carries antibodies, complement proteins, and white blood cells to defend against infection.

Composition of Blood

Main Components

Blood is the only fluid tissue in the body and is classified as a connective tissue. It consists of a nonliving fluid matrix called plasma and living cells known as formed elements.

• Plasma: Straw-colored, sticky fluid making up about 55% of blood volume.

• Formed Elements: Erythrocytes (RBCs), leukocytes (WBCs), and platelets.

When blood is centrifuged, it separates into three layers:

• Erythrocytes: Bottom layer (~45% of blood, measured as hematocrit).

• Buffy Coat: Middle thin layer (<1%, contains WBCs and platelets).

• Plasma: Top layer (~55%).

Physical Characteristics and Volume

• Appearance: Sticky, opaque fluid with metallic taste.

• Color: Scarlet red when oxygenated, dark red when deoxygenated.

• pH: 7.35–7.45.

• Body Weight: Blood makes up ~8% of body weight.

• Volume: Males: 5–6 L; Females: 4–5 L.

Blood Plasma

Composition

Plasma is about 90% water and contains over 100 dissolved solutes, including nutrients, gases, hormones, wastes, proteins, and inorganic ions. Plasma proteins are the most abundant solutes and are primarily produced by the liver.

Constituent	Description and Importance
Water	90% of plasma volume; dissolving and suspending medium for solutes of blood; absorbs heat
Electrolytes	Most abundant solutes by number; help maintain plasma osmotic pressure and normal blood pH
Albumin	60% of plasma proteins; carrier, blood buffer, contributes to plasma osmotic pressure
Globulins (alpha, beta, gamma)	Transport proteins and antibodies
Fibrinogen	Forms fibrin threads of blood clot

Constituent	Description and Importance
Nonprotein nitrogenous substances	By-products of cellular metabolism (urea, uric acid, creatinine, ammonium salts)
Nutrients (organic)	Absorbed from digestive tract; include glucose, amino acids, fatty acids, cholesterol, vitamins
Respiratory gases	Oxygen and carbon dioxide; O2 mostly bound to hemoglobin, CO2 transported as bicarbonate or bound to hemoglobin
Hormones	Steroid and thyroid hormones carried by plasma proteins

Formed Elements

Types and Characteristics

• Erythrocytes (RBCs): No nuclei or organelles; dedicated to gas transport.

• Leukocytes (WBCs): Complete cells with nuclei and organelles; function in immunity.

• Platelets: Cell fragments involved in clotting.

Most formed elements survive only a few days in the bloodstream and originate in bone marrow.

Erythrocytes (Red Blood Cells)

Structural Characteristics

Erythrocytes are small, biconcave, anucleate cells optimized for gas transport. Their shape provides a large surface area relative to volume, facilitating efficient gas exchange. They are filled with hemoglobin and lack mitochondria, relying on anaerobic metabolism.

Function and Hemoglobin

• Hemoglobin: Each molecule consists of four polypeptide chains (two alpha, two beta) and four heme groups, each with a central iron atom that binds O2.

• Oxygen Transport: Each hemoglobin can carry four O2 molecules; each RBC contains about 250 million hemoglobin molecules.

• Gas Exchange: O2 loading in lungs forms oxyhemoglobin; unloading in tissues forms deoxyhemoglobin. CO2 binds to hemoglobin to form carbaminohemoglobin.

Production of Erythrocytes (Erythropoiesis)

Erythropoiesis is the process of RBC formation, occurring in red bone marrow. It involves several stages, starting from hematopoietic stem cells (hemocytoblasts) and progressing through committed cells to mature erythrocytes.

Regulation of Erythropoiesis

• Erythropoietin (EPO): Hormone released by kidneys in response to hypoxia; stimulates RBC production.

• Dietary Requirements: Amino acids, lipids, carbohydrates, iron, vitamin B12, and folic acid are essential for erythropoiesis.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are removed by macrophages in the spleen, and their components are recycled. Heme is degraded to bilirubin, iron is stored or transported, and globin is broken down into amino acids.

Erythrocyte Disorders

Anemia

Anemia is characterized by abnormally low O2-carrying capacity. It can result from blood loss, insufficient RBC production, or excessive RBC destruction. Types include hemorrhagic, iron-deficiency, pernicious, renal, aplastic, hemolytic, thalassemias, and sickle-cell anemia.

Polycythemia

Polycythemia is an abnormal excess of RBCs, increasing blood viscosity. Causes include polycythemia vera (bone marrow cancer), secondary polycythemia (low O2 or increased EPO), and blood doping.

Leukocytes (White Blood Cells)

General Structure and Function

Leukocytes are complete cells that defend against disease. They can leave capillaries (diapedesis) and move through tissues by amoeboid motion and chemotaxis. Leukocytes are classified as granulocytes or agranulocytes.

Granulocytes

• Neutrophils: Most numerous; phagocytic "bacteria slayers"; multilobed nucleus.

• Eosinophils: Digest parasitic worms; bilobed nucleus; red granules.

• Basophils: Rarest; release histamine; deep purple granules.

Agranulocytes

• Lymphocytes: Second most numerous; crucial to immunity; T cells and B cells.

• Monocytes: Largest; differentiate into macrophages; phagocytic.

Leukocyte Formation

Leukopoiesis is stimulated by interleukins and colony-stimulating factors. All leukocytes originate from hemocytoblasts, branching into lymphoid and myeloid stem cells.

Platelets

Structure and Function

Platelets are fragments of megakaryocytes and are essential for clotting. They form temporary plugs in blood vessel breaks and are regulated by thrombopoietin.

Summary Table of Formed Elements

Hemostasis

Steps of Hemostasis

Hemostasis is the process of stopping bleeding and involves three steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen and release chemicals to attract more platelets.

3. Coagulation: Reinforces plug with fibrin threads; involves intrinsic and extrinsic pathways leading to fibrin mesh formation.

Blood Clotting Factors

Disorders of Hemostasis

Thromboembolic Disorders

• Thrombus: Clot in unbroken vessel; may block circulation.

• Embolus: Thrombus floating in bloodstream; can cause embolism.

• Anticoagulant Drugs: Aspirin, heparin, warfarin used to prevent clotting.

Bleeding Disorders

• Impaired Liver Function: Inability to synthesize clotting factors.

• Hemophilia: Hereditary bleeding disorders due to factor deficiencies.

• DIC: Disseminated intravascular coagulation involves both clotting and bleeding.

Blood Transfusions and Blood Groups

ABO Blood Groups

Blood groups are determined by the presence or absence of antigens (agglutinogens) on RBC surfaces. The ABO system is based on A and B antigens, and blood may contain anti-A or anti-B antibodies.

Blood Typing

Blood typing involves mixing donor blood with antibodies against common agglutinogens. Agglutination indicates the presence of specific antigens.

Diagnostic Blood Tests

Clinical Applications

• Hematocrit: Low values indicate anemia.

• Blood Glucose: Used to diagnose diabetes.

• Differential WBC Count: Helps diagnose infections and disorders.

• CMP and CBC: Comprehensive panels for overall health assessment.

Developmental Aspects of Blood

Fetal and Aging Blood

• Fetal Blood: Forms in yolk sac, liver, and spleen; hemoglobin F has higher O2 affinity.

• Aging: Blood diseases such as chronic leukemias, anemias, and clotting disorders are more common.

Additional info: This study guide covers all major aspects of blood as outlined in Chapter 17 of a standard Anatomy & Physiology college course, including structure, function, disorders, and clinical relevance. All images included are directly relevant to the adjacent content and reinforce key concepts.