Study Notes: Blood (Chapter 17) – Human Anatomy & Physiology

Blood: The Internal Transport System

Overview of Blood and the Cardiovascular System

Blood is a specialized fluid connective tissue that serves as the primary transport medium within the cardiovascular system. The cardiovascular system consists of the heart (pump), blood vessels (conducting hoses), and blood (fluid connective tissue).

• Blood transports gases, nutrients, hormones, and waste products throughout the body.

• It plays a critical role in maintaining homeostasis, including temperature regulation, pH balance, and protection against pathogens.

Functions of Blood

Blood performs three primary functions: transportation, regulation, and protection.

• Transportation: Delivers oxygen and nutrients to cells, removes metabolic wastes, and transports hormones.

• Regulation: Maintains body temperature, pH, and fluid volume in the circulatory system.

• Protection: Prevents blood loss via clotting and defends against infection through immune cells and proteins.

Composition and Physical Characteristics of Blood

Physical Properties

Blood is a sticky, opaque fluid with a metallic taste.

• Temperature: 38ºC (100.4ºF)

• Viscosity: High

• pH: Slightly alkaline (7.35–7.45)

• Volume: ~8% of body weight; males 5–6 L, females 4–5 L

Components of Blood

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

• Plasma: Straw-colored, sticky fluid, about 90% water, contains over 100 dissolved solutes.

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

Blood Plasma: Composition

Plasma is the nonliving fluid component of blood, containing water, proteins, electrolytes, nutrients, gases, hormones, and waste products.

• Albumin: Most abundant plasma protein; maintains osmotic pressure and acts as a carrier.

• Globulins: Transport proteins and antibodies.

• Fibrinogen: Forms fibrin threads for blood clotting.

Constituent	Description and Importance
Water	90% of plasma; dissolving and suspending medium for solutes
Electrolytes	Maintain osmotic balance, pH, and membrane potential
Plasma proteins	Albumin, globulins, fibrinogen; osmotic pressure, transport, immunity, clotting

Constituent	Description and Importance
Nonprotein nitrogenous substances	By-products of metabolism (urea, uric acid, creatinine)
Nutrients	Glucose, amino acids, fatty acids, vitamins
Respiratory gases	Oxygen and carbon dioxide
Hormones	Steroid and thyroid hormones carried by plasma proteins

Formed Elements of Blood

Types of Formed Elements

The formed elements include erythrocytes, leukocytes, and platelets.

• Erythrocytes (RBCs): Transport oxygen and carbon dioxide.

• Leukocytes (WBCs): Defend against infection.

• Platelets: Cell fragments involved in clotting.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are small, biconcave, anucleate cells specialized for gas transport.

• Biconcave shape increases surface area for gas exchange.

• Filled with hemoglobin (Hb), which binds oxygen.

• No mitochondria; ATP production is anaerobic.

Hemoglobin Structure and Function

Hemoglobin is a protein composed of four polypeptide chains (two alpha, two beta), each with a heme group containing iron.

• Each heme binds one O2 molecule.

• Each RBC contains ~250 million Hb molecules.

• Hemoglobin also binds CO2 (carbaminohemoglobin).

Erythropoiesis: Formation of Red Blood Cells

Erythropoiesis is the process of RBC formation, occurring in red bone marrow.

• Stem cell (hemocytoblast) differentiates into myeloid stem cell, then proerythroblast, and through several stages to mature erythrocyte.

• Reticulocyte count indicates rate of RBC formation.

Regulation of Erythropoiesis

Erythropoiesis is regulated by erythropoietin (EPO), a hormone released by kidneys in response to hypoxia (low O2).

• Balance between RBC production and destruction is critical.

• Dietary requirements include amino acids, iron, vitamin B12, and folic acid.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are destroyed by macrophages in the spleen, and their components are recycled.

• Heme is degraded to bilirubin, excreted in bile.

• Iron is stored or reused.

• Globin is broken down to amino acids.

Erythrocyte Disorders

Anemia

Anemia is a condition of reduced O2-carrying capacity. Causes include blood loss, insufficient RBC production, or excessive RBC destruction.

• Iron-deficiency anemia: Microcytic, hypochromic RBCs; treated with iron supplements.

• Pernicious anemia: Lack of intrinsic factor for B12 absorption; treated with B12 injections.

• Renal anemia: Lack of EPO; treated with synthetic EPO.

• Aplastic anemia: Bone marrow destruction; treated with transfusions or stem cell transplants.

• Hemolytic anemias: Premature RBC lysis; includes thalassemias and sickle-cell anemia.

Polycythemia

Polycythemia is an abnormal excess of RBCs, increasing blood viscosity.

• Polycythemia vera: Bone marrow cancer.

• Secondary polycythemia: Due to low O2 or increased EPO.

• Blood doping: Artificially increasing RBC count for athletic performance.

Leukocytes (White Blood Cells)

Structure and Function

Leukocytes are complete cells with nuclei and organelles, functioning in defense against disease.

• Can leave capillaries (diapedesis) and move through tissues (amoeboid motion).

• Leukocytosis: Elevated WBC count, normal response to infection.

Types of Leukocytes

Leukocytes are classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

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

• Eosinophils: Digest parasitic worms; modulate immune response.

• Basophils: Release histamine; rarest WBC.

• Lymphocytes: Crucial to immunity; T cells and B cells.

• Monocytes: Largest WBC; become macrophages in tissues.

Production and Life Span of Leukocytes

Leukopoiesis

Leukopoiesis is the formation of WBCs, stimulated by interleukins and colony-stimulating factors (CSFs).

• All leukocytes originate from hemocytoblasts.

• Lymphoid stem cells produce lymphocytes; myeloid stem cells produce other WBCs.

Platelets

Structure and Function

Platelets are fragments of megakaryocytes, essential for blood clotting.

• Contain chemicals for clotting (serotonin, calcium, enzymes, ADP).

• Form temporary plugs to seal vessel breaks.

Hemostasis

Steps of Hemostasis

Hemostasis is the process of stopping bleeding, involving three steps:

1. Vascular spasm: Vasoconstriction in response to injury.

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 thrombin and fibrin formation.

Blood Transfusions and Blood Typing

Blood Groups

Blood groups are determined by antigens (agglutinogens) on RBC membranes.

• ABO blood groups: Based on presence of A and B antigens.

• Rh blood group: Presence or absence of Rh antigen (D).

Transfusion Reactions

Mismatched transfusions can cause agglutination and destruction of donor RBCs, leading to serious complications.

Diagnostic Blood Tests

Common Tests

• Hematocrit: Measures RBC proportion.

• Blood glucose: Checks for diabetes.

• Differential WBC count: Assesses immune status.

• Complete blood count (CBC): Evaluates overall blood health.

Developmental Aspects of Blood

Fetal and Aging Blood

• 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 of aging include chronic leukemias, anemias, and clotting disorders.

Key Equations and Concepts

Hematocrit Calculation

• Hematocrit (%) = (Volume of RBCs / Total blood volume) × 100

Oxygen Transport by Hemoglobin

• Each hemoglobin molecule can bind four O2 molecules.

Blood Volume Estimation

• Blood volume (L) = 0.07 × body weight (kg)

Coagulation Pathways

• Intrinsic and extrinsic pathways converge at factor X activation, leading to thrombin and fibrin formation.

Sample Equation (Hemoglobin-Oxygen Binding)

$\text{Hb} + \text{O}_2 \rightarrow \text{HbO}_2$

Sample Equation (Hematocrit)

$\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total blood volume}} \times 100$

Sample Equation (Blood Volume)

$\text{Blood volume (L)} = 0.07 \times \text{body weight (kg)}$

Sample Equation (Coagulation Pathway)

$\text{Prothrombin} \xrightarrow{\text{Prothrombin activator}} \text{Thrombin} \xrightarrow{\text{Fibrinogen}} \text{Fibrin}$

Summary Table: Formed Elements of Blood

Element	Main Function	Key Features
Erythrocytes	O2 and CO2 transport	Biconcave, anucleate, hemoglobin-rich
Leukocytes	Defense against infection	Complete cells, various types
Platelets	Clotting	Cell fragments, contain clotting factors

Summary Table: Leukocyte Types

Type	Relative Abundance	Main Function
Neutrophils	50–70%	Phagocytosis of bacteria
Lymphocytes	25–45%	Immunity (T and B cells)
Monocytes	3–8%	Phagocytosis, become macrophages
Eosinophils	2–4%	Digest parasitic worms, modulate immunity
Basophils	0.5–1%	Release histamine, inflammation

Summary Table: ABO Blood Groups

Blood Type	Antigens Present	Antibodies Present
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B