Chapter 16: Blood – Anatomy & Physiology Study Guide

Blood: Internal Transport System

Introduction to Blood

Blood is a specialized connective tissue that serves as the body's internal transport system, delivering essential substances and removing waste products. It plays a critical role in maintaining homeostasis, supporting immune function, and facilitating communication between organs.

• Functions of Blood:

  • Transport of oxygen, nutrients, hormones, and waste products

  • Regulation of body temperature, pH, and fluid balance

  • Protection against blood loss (clotting) and infection (immune response)

Composition of Blood

Major Components of Whole Blood

Blood consists of plasma and formed elements. Plasma is the liquid matrix, while formed elements include erythrocytes, leukocytes, and platelets.

• Plasma: 55% of whole blood; least dense component

• Buffy coat: Leukocytes and platelets; <1% of whole blood

• Erythrocytes: 45% of whole blood (hematocrit); most dense component

Blood Plasma

Blood plasma is a straw-colored, viscous fluid composed primarily of water, dissolved solutes, and plasma proteins. It serves as the medium for transporting formed elements and dissolved substances.

• Water: 90% of plasma volume; acts as solvent and temperature buffer

• Solutes: Electrolytes, plasma proteins (albumin, globulins, fibrinogen), nutrients, gases, hormones, wastes

• Plasma Proteins:

  • Albumin: Maintains osmotic pressure and transports substances

  • Globulins: Immune function and transport

  • Fibrinogen: Blood clotting

Component	Description and Importance
Water	90% of plasma volume; dissolving and suspending medium for solutes; absorbs heat
Electrolytes	Most abundant solutes; help maintain plasma osmotic pressure and normal blood pH
Plasma proteins	8% of plasma; includes albumin, globulins, and fibrinogen
Albumin	60% of plasma proteins; maintains osmotic pressure
Globulins	36% of plasma proteins; transport and immune functions
Fibrinogen	4% of plasma proteins; forms fibrin threads in blood clotting
Nutrients	Materials for cell metabolism
Respiratory gases	O2 and CO2
Hormones	Regulatory molecules

Formed Elements

The formed elements of blood include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. These elements are suspended in plasma and perform specialized functions.

• Erythrocytes: Transport oxygen and carbon dioxide

• Leukocytes: Defend against infection

• Platelets: Essential for blood clotting

Erythrocytes (Red Blood Cells)

Structural Characteristics of Erythrocytes

Erythrocytes are biconcave, anucleate cells optimized for gas transport. Their shape increases surface area for efficient diffusion and flexibility for passage through capillaries.

• Size: Approximately 7.5 μm in diameter and 2.5 μm thick

• Shape: Biconcave disc; lacks nucleus and organelles

Function of Erythrocytes

The primary function of erythrocytes is to transport oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs. This is accomplished by the protein hemoglobin.

• Hemoglobin: A globular protein composed of four polypeptide chains (two alpha, two beta) and four heme groups

• Oxygen binding: Each heme group contains iron, which binds oxygen reversibly

Production of Erythrocytes (Erythropoiesis)

Erythropoiesis is the process of red blood cell formation, occurring primarily in the red bone marrow. It involves differentiation from hematopoietic stem cells to mature erythrocytes.

• Stages: Hematopoietic stem cell → Proerythroblast → Erythroblast (various stages) → Reticulocyte → Erythrocyte

• Regulation: Controlled by erythropoietin (EPO), a hormone produced by the kidneys in response to hypoxia

Regulation of Erythropoiesis

Erythropoiesis is tightly regulated to maintain adequate oxygen-carrying capacity. EPO stimulates red bone marrow to increase RBC production when oxygen levels are low.

• Stimulus: Hypoxia (low O2 levels) triggers EPO release

• Feedback: Increased RBCs restore oxygen levels, reducing EPO production

Clinical Note: EPO Abuse

Artificial EPO is sometimes abused by athletes to increase stamina. However, excessive RBC production can lead to dangerous consequences such as blood thickening, clotting, stroke, or heart failure.

Fate and Destruction of Erythrocytes

Erythrocytes have a lifespan of about 120 days. Old or damaged RBCs are removed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down into heme and globin; heme is further degraded to bilirubin and excreted.

Erythrocyte Disorders

Anemia

Anemia is a condition characterized by insufficient RBCs or hemoglobin, resulting in reduced oxygen delivery to tissues. Causes include blood loss, decreased RBC production, or increased RBC destruction.

Sickle-Cell Anemia

Sickle-cell anemia is a genetic disorder caused by a single amino acid substitution in the beta chain of hemoglobin. This results in abnormal, sickle-shaped erythrocytes that can block blood vessels and cause pain.

• Normal hemoglobin: Glu at position 6

• Sickled hemoglobin: Val at position 6

Leukocytes (White Blood Cells)

General Structure and Functional Characteristics

Leukocytes are nucleated cells that defend the body against infection and foreign substances. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

Granulocytes Overview

Granulocytes contain visible granules in their cytoplasm and have lobed nuclei. They are involved in immune responses and inflammation.

• Neutrophils: Phagocytize bacteria; most abundant

• Eosinophils: Combat parasitic infections and modulate allergic responses

• Basophils: Release histamine; involved in inflammatory responses

Agranulocytes Overview

Agranulocytes lack visible granules and have simpler nuclei. They play roles in adaptive immunity and phagocytosis.

• Lymphocytes: B and T cells; responsible for specific immune responses

• Monocytes: Differentiate into macrophages; phagocytize pathogens and debris

Leukopoiesis: Formation of WBCs

Leukopoiesis

Leukopoiesis is the process of white blood cell formation, originating from hematopoietic stem cells in the bone marrow. It is regulated by cytokines and growth factors.

• Myeloid stem cells: Give rise to granulocytes and monocytes

• Lymphoid stem cells: Give rise to lymphocytes

Clinical Note: Hematopoietic Hormones

Hematopoietic hormones such as EPO and colony-stimulating factors (CSFs) are used clinically to stimulate bone marrow in patients undergoing chemotherapy or with immune deficiencies.

Platelets

Structure, Function, and Formation

Platelets are cell fragments derived from megakaryocytes. They are essential for blood clotting and repair of damaged blood vessels.

• Formation: Megakaryocytes fragment to release platelets into circulation

• Function: Form platelet plugs and release factors for coagulation

Hemostasis

Events of Hemostasis

Hemostasis is the process that stops bleeding through a series of steps: vascular spasm, platelet plug formation, and coagulation.

• Vascular spasm: Smooth muscle contracts to reduce blood flow

• Platelet plug formation: Platelets adhere to exposed collagen and aggregate

• Coagulation: Fibrin forms a mesh that stabilizes the clot

Blood Groups, Transfusions, and Diagnostic Tests

Transfusing Red Blood Cells: Antigens and Blood Groups

Blood groups are determined by the presence of antigens on the surface of erythrocytes. The ABO and Rh systems are clinically significant for transfusions.

• ABO system: Type A, B, AB, O based on antigen presence

• Rh system: Rh+ or Rh– based on D antigen

Clinical Note: Hemolytic Disease of the Newborn

Hemolytic disease of the newborn occurs when an Rh– mother develops antibodies against Rh+ fetal blood, leading to destruction of fetal RBCs in subsequent pregnancies.

Blood Typing and Cross Matching

Blood typing and cross matching are essential for safe transfusions, ensuring compatibility between donor and recipient blood.

Key Takeaways

• Blood is a vital connective tissue with transport, regulatory, and protective functions

• Plasma and formed elements are the main components

• Erythrocytes, leukocytes, and platelets each have specialized roles

• Hemostasis prevents blood loss and maintains vascular integrity

• Blood groups are critical for transfusion safety