The Cardiovascular System: Blood – Structure, Function, and Composition

The Cardiovascular System

Overview

The cardiovascular system is responsible for the transport of substances throughout the body and consists of the heart, blood, and blood vessels. Blood plays a central role as the transport medium, while the heart acts as the pump, and blood vessels serve as conduits for circulation.

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

• Heart: Composed of various structures, responsible for blood flow and its own blood supply.

• Blood Vessels: Characterized by their structure and function in pulmonary and systemic circulation, supplying blood to all body regions.

Blood: Structure and Function

General Properties

Blood is a specialized connective tissue that serves as the main transport medium in the body. It is composed of cells (formed elements) suspended in a liquid matrix (plasma).

• Transport Functions:

  • Nutrients (e.g., glucose, amino acids, lipids)

  • Respiratory gases (oxygen and carbon dioxide)

  • Waste products (e.g., urea, ammonia)

  • Signaling molecules (hormones)

  • Cells of the immune system

• Regulatory Functions:

  • Regulates body temperature

  • Maintains pH and fluid balance

• Blood Circulation: Powered by the heart, blood delivers oxygen and nutrients to tissues and removes waste products. Diffusion occurs across capillary walls.

Blood as Connective Tissue

• Formed Elements: Blood cells (erythrocytes, leukocytes, platelets)

• Plasma: The liquid portion of blood, making up about 55% of total blood volume

• Origin: Develops from mesenchyme (embryonic connective tissue)

• Matrix: Nonliving fluid matrix (plasma) surrounds the cells

• Blood Volume:

  • Males: 5–6 liters

  • Females: 4–5 liters

Composition of Blood

• Plasma: Straw-colored, sticky fluid; approximately 90% water; contains over 100 dissolved substances.

• Buffy Coat: Thin layer composed of leukocytes (white blood cells) and platelets.

• Hematocrit: Percentage of blood volume occupied by red blood cells (RBCs).

  • Males: 47% ± 5%

  • Females: 42% ± 5%

Blood Plasma

Plasma is the liquid component of blood, serving as the medium for transporting nutrients, hormones, and waste products.

• Main Components:

  • Ions: e.g., 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 loss from blood vessels.

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

• Fibrinogen: Essential for blood clotting.

Formed Elements of Blood

Erythrocytes (Red Blood Cells, RBCs)

Erythrocytes are the most numerous blood cells, specialized for oxygen transport.

• Shape: Biconcave discs, 7.5 μm in diameter (slightly smaller than capillaries).

• Function: Transport oxygen via hemoglobin; also carry some carbon dioxide.

• Hemoglobin: Protein with four polypeptide chains, each containing an iron atom that binds oxygen.

  • Gives blood its red color due to iron oxidation.

• Cellular Features:

  • No nucleus or organelles (maximizes space for hemoglobin).

  • Lack mitochondria (do not consume the oxygen they transport).

  • Life span: ~100 days.

  • Originate from bone marrow.

• Normal Counts:

  • Females: 4.3–5.2 million cells/mm3

  • Males: 5.1–5.8 million cells/mm3

Erythrocyte Disorders

• Polycythemia: Excess of erythrocytes; can be caused by bone marrow cancer (polycythemia vera).

• Anemia: Low erythrocyte count or hemoglobin concentration.

• Sickle Cell Disease: Inherited disorder causing RBCs to assume a sickle shape due to abnormal hemoglobin; provides some resistance to malaria.

Adaptations in High-Altitude Populations

• Andean Populations: Higher hemoglobin concentrations for oxygen delivery.

• Tibetan Populations: Lower hemoglobin, but higher ventilation rate and muscle capillary density.

Leukocytes (White Blood Cells, WBCs)

Leukocytes are less numerous than RBCs and function primarily in defense against infection. They operate mainly outside the bloodstream in connective tissues.

• Normal Count: 4,800–11,000 cells/mm3

• Movement: Amoeboid movement; can exit capillaries (diapedesis).

• Origin: Bone marrow.

• Leukocytosis: Elevated WBC count (>11,000/mm3), often due to infection.

Types of Leukocytes

• Granulocytes (contain cytoplasmic granules, lobed nuclei, phagocytic):

  • Neutrophils: Most abundant; multilobed nucleus; first responders to infection; phagocytize bacteria; form pus.

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

  • Basophils: ~0.5% of WBCs; secrete histamine; mediate inflammation in allergies and parasitic infections.

• Agranulocytes (lack visible granules, spherical or kidney-shaped nucleus):

  • Lymphocytes: 20–45% of WBCs; key cells of the immune system; act against specific antigens.

    • T cells: Attack foreign cells directly.

    • B cells: Produce antibodies.

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

Thrombocytes (Platelets)

Platelets are cell fragments derived from megakaryocytes. They are essential for blood clotting and vessel repair.

• Plug tears in blood vessels

• Signal for vasoconstriction

• Initiate clotting process

Hematopoiesis

Blood Cell Formation

Hematopoiesis is the process by which all blood cells are formed. In the fetus, this occurs in the liver, spleen, and bone marrow; after birth, it is restricted to bone marrow.

• Red Marrow: Actively produces blood cells; found in spongy bone of axial skeleton, girdles, and proximal epiphyses of humerus and femur.

• Yellow Marrow: Dormant; can be converted to red marrow in emergencies; found in other regions of long bones.

• At birth, all marrow is red; conversion to yellow occurs with age.

• About 100 billion new blood cells are produced daily.

Bone Marrow Structure

• Framework consists of reticular connective tissue (network of reticular fibers in loose ground substance).

• Supports developing blood cells and contains macrophages for antigen capture and blood cleaning.

• Blood sinusoids allow mature blood cells to enter the bloodstream.

Blood Cell Lineages

• All blood cells originate from a hemopoietic blood stem cell (pluripotential hematopoietic stem cell).

• Two main progenitor lines:

  • Lymphoid stem cells: Give rise to lymphocytes.

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

Leukemia

Leukemia is a form of cancer involving abnormal proliferation of white blood cells. It is classified as either lymphoblastic or myeloblastic, depending on the cell lineage affected. Treatment may involve hematopoietic cell transplantation from peripheral blood, bone marrow, or umbilical cord blood.

Summary Table: Main Components of Blood

Component	Main Function	Relative Abundance
Plasma	Transport of nutrients, wastes, hormones; maintains osmotic balance	~55% of blood volume
Erythrocytes (RBCs)	Oxygen and carbon dioxide transport	~45% of blood volume
Leukocytes (WBCs)	Defense against infection	<1% of blood volume
Platelets (Thrombocytes)	Blood clotting	<1% of blood volume

Key Equations

• Hematocrit Calculation: $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$