Chapter 6: Cardiovascular System - Blood

6.1 Blood: An Overview

Learning Outcomes:

• List the functions of blood in the human body.

• List the formed elements of blood and their functions.

• Describe the composition of plasma.

6.2 Circulation of Blood

The heart pumps approximately 75 ml of blood per contraction. With an average heart rate of 70 beats per minute, the total volume of blood pumped per minute reaches about 5,250 ml, effectively circulating the entire blood supply throughout the body to provide oxygen and nutrients to tissues and organs. The circulatory system consists of the heart, blood vessels (arteries, veins, and capillaries), and the blood itself, working together to maintain homeostasis.

6.3 Functions of Blood

Blood has several critical categories of functions:

• Transport: Carries oxygen via hemoglobin in red blood cells (RBCs) to tissues, transports nutrients from the digestive tract, removes metabolic wastes such as carbon dioxide to the lungs for exhalation, and delivers hormones from endocrine glands to target organs.

• Defense: Blood plays a crucial role in the body's immune defense mechanisms. It contains white blood cells (WBCs) that identify and neutralize pathogens like bacteria and viruses. Some WBCs, such as neutrophils, phagocytize (engulf and digest) microbes, while others, like lymphocytes, produce antibodies to target specific antigens. Additionally, blood clotting mechanisms prevent excessive fluid loss upon injury.

• Regulation: Blood helps maintain body temperature by redistributing heat from warmer to cooler parts of the body. Plasma proteins are vital for regulating osmotic pressure, creating an equilibrium for fluid distribution between blood vessels and surrounding tissues. The blood's buffering system maintains a constant pH of around 7.4, essential for optimal enzyme function and metabolic reactions.

6.4 Composition of Blood

Blood is a specialized liquid connective tissue consisting of formed elements and plasma:

• Formed Elements:

  • Red Blood Cells (RBCs): Also known as erythrocytes, they are responsible for transporting oxygen from the lungs to the body and returning carbon dioxide from the body to the lungs.

  • White Blood Cells (WBCs): Also called leukocytes, they are less numerous than RBCs and play key roles in the immune response.

  • Platelets: Known as thrombocytes, these are cell fragments derived from megakaryocytes in the bone marrow and are essential for blood clotting and wound healing.

• Plasma: A fluid matrix that constitutes about 55% of blood volume, it is primarily 91% water, with the remaining 9% composed of salts and organic molecules, including nutrients, hormones, gases, and waste products.

6.5 Plasma Composition

The plasma is crucial for maintaining osmotic pressure and transporting essential substances. Key components include:

• Solutes: Nutrients (like glucose, amino acids, and lipids), wastes (urea and creatinine), hormones (insulin and adrenaline), and proteins (the most abundant organic molecules).

6.6 Types of Plasma Proteins

• Albumins: The most abundant plasma protein, they regulate osmotic pressure and transport various small molecules (like bilirubin, fatty acids, and some hormones).

• Globulins: Involved in immune responses and transport functions, they include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: An inactive precursor to fibrin, which plays a key role in blood clotting; when activated, it helps form the fibrous framework for blood clots.

6.7 Red Blood Cells and Gas Transport

Hemoglobin (Hb) is the protein in RBCs that binds oxygen, giving blood its characteristic red color. Each hemoglobin molecule can bind up to four oxygen molecules via its heme portion. The oxygenated form of hemoglobin is termed oxyhemoglobin, while deoxygenated is referred to as deoxyhemoglobin, which releases oxygen as needed by tissues.

6.8 Carbon Dioxide Transport

Carbon dioxide generated by cellular metabolism is transported in three ways:

• 7% dissolved in plasma directly.

• 23% binds to hemoglobin forming carbaminohemoglobin.

• 70% is transported as bicarbonate ions in plasma, formed by the reaction of carbon dioxide and water, which helps regulate blood pH.

6.9 Production and Lifespan of RBCs

RBCs are produced in the red bone marrow through a process called erythropoiesis. During maturation, they lose their nucleus and organelles, and have an average lifespan of about 120 days. Erythropoietin (EPO) is a hormone released by the kidneys in response to low oxygen levels in the blood, stimulating increased RBC production to enhance oxygen delivery.

6.10 Disorders of Red Blood Cells

• Jaundice: Occurs due to the accumulation of bilirubin resulting from the breakdown of hemoglobin, often seen when liver function is impaired.

• Anemia: Refers to a deficiency in the number or quality of RBCs or hemoglobin, leading to reduced oxygen transport; types include:

  • Iron-deficiency anemia: Caused by insufficient iron intake.

  • Pernicious anemia: Resulting from a lack of vitamin B12.

  • Folic-acid-deficiency anemia: Due to inadequate folic acid.

  • Hemolytic anemia: Characterized by premature destruction of RBCs.

  • Sickle-cell disease: A genetic disorder characterized by abnormal hemoglobin causing distorted (sickle-shaped) RBCs.

6.11 White Blood Cells (Leukocytes)

Leukocytes are larger, nucleated cells that protect the body against infections and foreign diseases. They are less numerous than RBCs but are essential for the immune response, capable of moving out of blood vessels to reach affected tissues.

6.12 Types of White Blood Cells

• Granular Leukocytes:

  • Neutrophils: The most abundant, phagocytize bacteria and fungi.

  • Eosinophils: Target parasitic infections and contribute to allergy responses.

  • Basophils: Release histamine during inflammation and allergic reactions.

• Agranular Leukocytes:

  • Lymphocytes: Include B cells (produce antibodies) and T cells (destroy infected cells).

  • Monocytes: Differentiate into macrophages and dendritic cells to respond to infections and help initiate immune responses.

6.13 White Blood Cell Disorders

• SCID (Severe immunodeficiency disease): A condition in which there is a severe defect in both T- and B-lymphocyte systems, leading to an increased vulnerability to infections.

• Leukemia: A type of cancer characterized by uncontrolled proliferation of immature WBCs, impairing normal blood cell function.

• Infectious Mononucleosis: Caused by the Epstein-Barr virus (EBV), it leads to an increase in atypical lymphocytes and symptoms of fatigue, fever, and sore throat.

6.14 Platelets and Blood Clotting

Platelets, or thrombocytes, are cytoplasmic fragments derived from large cells called megakaryocytes in the bone marrow. They play an essential role in hemostasis (the process of stopping bleeding) by adhering to the site of vessel injury and aggregating to form a temporary plug. Blood clotting factors, including prothrombin and fibrinogen, are proteins in plasma that are crucial for the clotting process.

6.15 Blood Clotting Process

When a blood vessel is injured, platelets aggregate at the site to form a temporary plug. The activation of prothrombin activator converts prothrombin into thrombin. Thrombin then catalyzes the conversion of fibrinogen to fibrin, which forms a mesh that stabilizes the clot. This temporary fibrin clot allows for tissue repair, and over time, plasmin enzymes dissolve the clot, restoring normal blood flow.

6.16 Blood Clotting Disorders

• Thrombocytopenia: A condition characterized by a low platelet count, leading to excessive bleeding and difficulty in clot formation.

• Thromboembolism: Occurs when blood clots travel through the bloodstream and obstruct blood vessels, which can lead to serious conditions such as stroke or pulmonary embolism.

• Genetic Deficiencies: Conditions such as Hemophilia A and B (deficiencies of specific clotting factors) and Von Willebrand disease (deficiency in vWF) disrupt normal clot formation, increasing bleeding risk.

6.17 Human Blood Types

Human blood types are characterized by specific surface proteins (antigens) found on RBCs. The ABO blood group system identifies four blood types: A, B, AB, and O, which denote the presence or absence of two main antigens (A and B). Blood compatibility is crucial during transfusions; mismatched transfusions can cause agglutination (clumping of blood cells), which can be life-threatening.

6.18 Rh Factor and Hemolytic Disease of the Newborn

The Rh factor refers to the presence of another protein on the surface of RBCs, designating Rh+ blood type if present. If an Rh- mother is exposed to Rh+ blood (typically during delivery), she may produce antibodies against the Rh factor. In subsequent pregnancies, these maternal anti-Rh antibodies can cross the placenta and attack the RBCs of an Rh+ fetus, leading to hemolytic disease of the newborn. Preventive treatment involves administering RhoGAM injections to Rh- mothers during and after pregnancy.

6.19 How Blood Veins Work with Other Systems for Homeostasis

The cardiovascular system interacts with numerous other body systems, including the lymphatic, respiratory, digestive, muscular, skeletal, endocrine, and urinary systems, to ensure homeostasis. For example, the respiratory system facilitates the exchange of gases (oxygen and carbon dioxide) with the blood, while the urinary system regulates water and electrolyte balance, which impacts blood pressure and volume.