Cardiovascular System: Blood – Structure, Function, and Hematopoiesis

Cardiovascular System Overview

Main Components

The cardiovascular system is responsible for the transport of substances throughout the body and consists of three main components: blood, the heart, and blood vessels.

• Blood: Includes plasma, formed elements, and is produced via hematopoiesis.

• Heart: Structures, blood flow, and blood supply to the heart.

• Blood Vessels: General characteristics, pulmonary and systemic circulations, and regional blood supply.

Blood

Functions and Properties

Blood is a specialized connective tissue that serves as the transport medium for essential substances and regulatory molecules.

• Transport medium for:

  • Nutrients

  • Respiratory gases (O2, CO2)

  • Waste products

  • Signaling molecules (e.g., hormones)

  • Cells of the immune system

• Blood circulation is powered by the heart's pumping action.

• Oxygen and nutrients diffuse across capillary walls to body tissues.

• Helps regulate body temperature.

Blood volume:

• Males: 5–6 liters

• Females: 4–5 liters

Additional info: Blood develops from mesenchyme and consists of cells surrounded by a nonliving matrix (plasma).

Composition of Blood

Main Components

Blood is composed of plasma, a buffy coat, and formed elements. The relative proportions are important for physiological function.

• Plasma: Liquid portion, ~55% of blood volume.

• Buffy coat: Thin layer containing leukocytes and platelets.

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

• Hematocrit: Percentage of blood volume occupied by RBCs.

  • Males: 47% ± 5%

  • Females: 42% ± 5%

Blood Plasma

Composition and Proteins

Blood plasma is a straw-colored, sticky fluid that is approximately 90% water and contains a variety of dissolved substances.

• Contains over 100 kinds of molecules:

  • Ions: Na+, Cl-

  • Wastes: Carbon dioxide, urea, ammonia

  • Nutrients: Sugars, lipids, amino acids

  • Proteins: Albumin, globulins, fibrinogen

Major Plasma Proteins

• Albumin: Maintains osmotic pressure, prevents water from diffusing out of blood vessels.

• Globulins: Includes antibodies and transport proteins for lipids, iron, and copper.

• Fibrinogen: Essential for blood clotting.

Formed Elements

Types and Characteristics

Formed elements are the cellular components of blood, each with specialized functions.

• Erythrocytes (RBCs)

• Leukocytes (WBCs)

• Platelets (Thrombocytes)

• Do not divide; lifespan ranges from hours to months.

• Erythrocytes and platelets are not true cells (lack nuclei and organelles).

Erythrocytes – Red Blood Cells (RBCs)

Structure and Function

Erythrocytes are specialized for oxygen transport and have a unique biconcave shape.

• Biconcave discs, 7.5 μm in diameter (capillary diameter: 8–10 μm).

• Most numerous formed element:

  • Females: 4.3–5.2 million cells/mm3

  • Males: 5.1–5.8 million cells/mm3

• Originate from bone marrow.

• No organelles or nuclei; lifespan ~100 days.

Hemoglobin

• Packed with oxygen-carrying hemoglobin.

• Hemoglobin molecule: four chains of amino acids, each with an iron atom (binding site for O2).

• Oxidation of iron gives blood its red color.

Oxygen Transport

• Pick up O2 at lung capillaries, release O2 at tissue capillaries.

• Biconcave shape increases surface area by 30%.

• 97% hemoglobin content.

• Lack mitochondria; do not consume O2.

Erythrocyte Disorders

• Polycythemia: Excess erythrocytes; polycythemia vera is a bone marrow cancer.

• Anemia: Low erythrocyte or hemoglobin levels.

• Sickle cell disease: Inherited disorder; RBCs become sickle-shaped due to defective hemoglobin, conferring some resistance to malaria.

Case Study: Tibetan and Andean Populations

Populations living at high altitudes have evolved different physiological adaptations for oxygen delivery.

• Andeans: Higher hemoglobin concentrations.

• Tibetans: Lower hemoglobin, higher ventilation rate, increased muscle capillary density.

Leukocytes – White Blood Cells (WBCs)

Structure and Function

Leukocytes are less numerous than RBCs and are essential for immune defense.

• Count: 4,800–11,000/mm3

• Protect against infectious microorganisms.

• Function outside bloodstream in loose connective tissue.

• Diapedesis: Movement of leukocytes out of capillaries.

• Originate in bone marrow.

• Leukocytosis: Elevated leukocyte count (>11,000/mm3).

Types of Leukocytes

• Granulocytes (neutrophils, eosinophils, basophils):

  • Have cytoplasmic granules, lobed nuclei, phagocytic.

• Agranulocytes (lymphocytes, monocytes):

  • Spherical or kidney-shaped nuclei.

• Relative abundance: Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils

Granulocytes

• Neutrophils: Most numerous; multilobed nucleus (2–6 lobes); phagocytize bacteria; first line of defense; form pus.

• Eosinophils: 1–4% of WBCs; large red-staining granules; release enzymes; end allergic reactions; digest parasites.

• Basophils: ~0.5% of WBCs; two-lobed nucleus; granules secrete histamines; mediate inflammation; similar to mast cells.

Agranulocytes

• Lymphocytes: 20–45% of WBCs; most important for immune response; dark purple nucleus; act against specific antigens.

  • T cells: Attack foreign cells directly.

  • B cells: Produce antibodies to mark foreign cells for destruction.

• Monocytes: 4–8% of WBCs; largest leukocytes; kidney-shaped nucleus; transform into macrophages (phagocytic).

Thrombocytes (Platelets)

Structure and Function

Platelets are cell fragments essential for blood clotting and vessel repair.

• Break off from megakaryocytes.

• Plug tears in blood vessels.

• Signal for vasoconstriction.

• Initiate blood clotting.

Hematopoiesis

Blood Cell Formation

Hematopoiesis is the process by which blood cells are formed, occurring primarily in the bone marrow after birth.

• Fetal hematopoietic organs: Liver, spleen, bone marrow.

• Postnatal: Bone marrow only.

• 100 billion new blood cells formed daily.

Bone Marrow Structure

• Framework made of reticular connective tissue.

• Network of reticular fibers forms soft, internal scaffolding (stroma).

• Contains macrophages for "blood cleaning" and blood sinusoids for mature cell entry into circulation.

Stem Cells and Lineages

• All blood cells originate from hemopoietic blood stem cells (pluripotent).

• Stem cells produce progenitor cells:

  • Lymphoid stem cells: Give rise to lymphocytes.

  • Myeloid stem cells: Give rise to all other blood cells.

• Genesis of erythrocytes begins with proerythroblasts.

Leukemia

Definition and Treatment

Leukemia is a form of cancer affecting blood-forming tissues, classified as lymphoblastic or myeloblastic.

• Treatment may involve hematopoietic cell transplantation from:

  • Peripheral blood stem cells

  • Bone marrow

  • Umbilical cord blood

Table: Major Blood Cell Types and Functions

Cell Type	Main Function	Relative Abundance
Erythrocytes (RBCs)	Transport oxygen and carbon dioxide	Most numerous
Neutrophils	Phagocytize bacteria; first line of defense	Most abundant WBC
Lymphocytes	Immune response; produce antibodies	Second most abundant WBC
Monocytes	Transform into macrophages; phagocytosis	Third most abundant WBC
Eosinophils	End allergic reactions; destroy parasites	Fourth most abundant WBC
Basophils	Release histamine; mediate inflammation	Least abundant WBC
Platelets	Blood clotting; vessel repair	Numerous (less than RBCs)

Key Equations

• Hematocrit Calculation:

$\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100\%$

• Oxygen Transport (Hemoglobin):

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

Additional info: The notes cover content relevant to Ch. 17 Blood and Ch. 18 The Cardiovascular System: The Heart, as well as aspects of Ch. 20 The Lymphatic System and Ch. 21 The Immune System.