GAP Ch18 Cardiovascular System: Blood

Blood is a vital fluid connective tissue that plays a crucial role in the cardiovascular system, which consists of the heart and blood vessels. It functions as a transportation system for various substances throughout the body and contributes to multiple physiological processes.

Components of Blood

• Cellular Elements:

  • Erythrocytes (Red Blood Cells):

    • Biconcave in shape, enabling high surface area for efficient gas exchange.

    • Contains hemoglobin, a protein that binds oxygen and carbon dioxide, facilitating their transport.

    • Produced in the bone marrow and regulated by erythropoietin, a hormone released in response to low oxygen levels.

  • Leukocytes (White Blood Cells):

    • Critical for immune defense, consisting of various types that respond to infections and foreign substances.

    • Produced in bone marrow and lymphatic tissue, maintaining the body’s defenses against pathogens.

  • Platelets (Cell Fragments):

    • Small cell fragments derived from megakaryocytes, crucial for blood clotting and wound healing.

• Extracellular Matrix:

  • Plasma:

    • The liquid component of blood, making up about 55% of its volume.

    • Contains water (about 92%), proteins (such as albumin, fibrinogen, and globulins), electrolytes, hormones, gases like oxygen and carbon dioxide, organic nutrients, and metabolic waste products.

Functions of Blood

• Transport:

  • Delivers oxygen from the lungs to body tissues and transports carbon dioxide from tissues back to the lungs for exhalation.

  • Transports nutrients absorbed from the digestive tract to cells throughout the body.

  • Carries metabolic waste products to organs like the kidneys for elimination from the body.

• Hormone Transport:

  • Functions as a medium for transporting hormones secreted by endocrine glands to target organs, influencing physiological processes such as growth and metabolism.

• Defense Mechanism:

  • White blood cells (leukocytes) act against pathogens like bacteria and viruses as well as monitoring for and destroying cancerous cells.

  • Some leukocytes also produce antibodies that specifically identify and neutralize foreign invaders.

• Hemostasis:

  • Platelets play a crucial role in preventing excessive blood loss from injuries by forming a plug and activating the clotting cascade to stabilize the blood clot.

  • Various proteins in plasma participate in the coagulation process, ensuring rapid response to vascular injury.

• Homeostasis:

  • Blood helps maintain a stable pH level through the bicarbonate buffer system and regulates osmotic balance in body fluids.

• Thermoregulation:

  • Blood flow is adjusted based on body temperature needs, diverting more blood to the skin to dissipate heat or conserving heat by redirecting blood to core organs.

Composition of Blood

• Hematocrit:

  • Represents the proportion of blood volume occupied by red blood cells, generally around 45% in healthy adults, with variations based on age, gender, and hydration levels.

• Blood Plasma:

  • Comprises 92% water, with the remaining 8% containing proteins like albumin (maintains oncotic pressure), fibrinogen (involved in blood clotting), and globulins (involved in immune responses).

  • Also contains electrolytes (sodium, potassium, calcium), dissolved gases (oxygen, carbon dioxide), and organic compounds (glucose, amino acids).

Hemopoiesis (Blood Cell Production)

• Definition:

  • The intricate biological process through which new blood cells and platelets are produced.

• Location:

  • Predominantly occurs in the red bone marrow of certain bones (e.g., vertebrae, ribs, and pelvis) and is influenced by age, with major production expected during childhood and adolescence.

• Process:

  • Begins with hematopoietic stem cells (hemocytoblasts) that differentiate into specialized cell lineages influenced by various hormones such as erythropoietin (for red blood cells) and thrombopoietin (for platelets).

Erythrocytes (Red Blood Cells)

• Structure and Function:

  • Their unique biconcave shape maximizes surface area for gas exchange, ensuring optimal oxygen delivery to tissues and facilitating carbon dioxide removal.

  • Lifespan of approximately 120 days, after which they are phagocytosed by macrophages, particularly in the spleen and liver.

Leukocytes (White Blood Cells)

• General Characteristics:

  • Larger than red blood cells, with a nucleus and organelles, comprising a pivotal portion of the immune system, with a typical count of 5,000-10,000 cells per microliter of blood.

• Types of Leukocytes:

  1. Granulocytes:

     • Contain granules in their cytoplasm; include:

       • Neutrophils: Most abundant, first responders to infections, adept at phagocytosing bacteria.

       • Eosinophils: Combat parasitic infections and play roles in allergic responses, their numbers increase notably during allergic reactions.

       • Basophils: Release histamines and other chemicals during inflammatory reactions, playing a role in allergy and infection.

  2. Agranulocytes:

     • Have fewer granules; consist of:

       • Lymphocytes: Critical to adaptive immunity, subdivided into B cells (producing antibodies), T cells (attacking infected or cancerous cells), and Natural Killer cells (providing rapid responses to virally infected cells).

       • Monocytes: Differentiate into macrophages upon migration into tissues; essential for phagocytosis of pathogens and dead cells.

Platelets (Thrombocytes)

• Characteristics and Function:

  • Cell fragments originating from large megakaryocytes in the bone marrow, an essential component of the coagulation process, playing a key role in hemostasis by adhering to sites of vascular injury and aggregating to form plugs, while also releasing growth factors that promote tissue repair.

Hemostasis (Blood Clotting Process)

1. Vascular Spasm:

   • Immediate constriction of blood vessels reduces blood flow, minimizing blood loss.

2. Platelet Plug Formation:

   • Platelets adhere to the exposed collagen at the injury site and release chemical signals that recruit additional platelets to form a temporary plug.

3. Coagulation Cascade:

   • The complex series of enzymatic reactions involving intrinsic and extrinsic pathways leads to the conversion of fibrinogen to fibrin, forming a mesh that solidifies the clot and successfully seals the vessel.

Blood Typing and Compatibility

• ABO Blood Groups:

  • Defined by specific A and/or B antigens on the surface of red blood cells.

  • Blood type O is considered a universal donor as it lacks these antigens, while type AB is a universal recipient, having both antigens.

• Antibodies:

  • Present in plasma and target specific antigens; for instance, individuals with type A blood produce anti-B antibodies, preventing incompatible transfusions.

Rh Factor

• Understanding Rh Positive and Negative Blood Types:

  • Determined by the presence of D antigens on red blood cells.

  • Individuals who lack this antigen are considered Rh negative and may develop anti-Rh antibodies only after exposure to Rh positive blood, which is vital knowledge in pregnancy and transfusion medicine.

Summary

• Understanding the multifaceted functions, composition, and types of blood is essential for a comprehensive grasp of the cardiovascular system's role in maintaining homeostasis, facilitating substance transport, and defending against disease. Moreover, proper knowledge of hemostasis and blood typing is paramount in clinical settings, notably in blood transfusions and surgeries.