Chaper 19: Cardiovascular System: Blood

Chapter 19: The Cardiovascular System: The Blood

Functions and Properties of Blood

Overview

• Blood is a liquid connective tissue consisting of cells suspended in a liquid matrix called plasma. Its primary function is to transport substances throughout the body, providing essential support to organs and tissues.

Components of Blood

Formed Elements:

• Red blood cells (RBCs): Hemoglobin-containing cells that transport oxygen from the lungs to the tissues and carbon dioxide back to the lungs for exhalation. Their structure (biconcave shape) enhances gas exchange.

• White blood cells (WBCs): Immune system cells that defend against pathogens. WBCs can be further classified into granular and agranular types based on the presence of granules in their cytoplasm.

• Platelets: These cell fragments, derived from megakaryocytes in the bone marrow, play a crucial role in hemostasis (the cessation of bleeding) by initiating the clotting process.

Plasma Composition:

• Plasma primarily consists of water, proteins, and various solutes that serve as a transport medium for nutrients, gases, hormones, and waste products.

Blood Composition

Whole Blood Composition:

• Whole Blood constitutes about 8% of an individual’s body weight.

• Blood Plasma: Comprises 55% of blood volume, serving essential physiological functions.

Plasma Composition:

• Water: 91.5% - Functions as a solvent and transport medium, crucial for thermoregulation and maintaining homeostasis.

• Blood plasma proteins: 7%

  • Albumins: (54%) - Crucial for maintaining osmotic pressure, helping to transport hormones, fatty acids, and drugs through the bloodstream. They also contribute to blood viscosity.

  • Globulins: (38%) - Includes antibodies that are vital for immune function, as well as transport proteins that carry lipids and fat-soluble vitamins.

  • Fibrinogen: (7%) - A soluble plasma protein essential for blood clotting; it converts to insoluble fibrin during the clotting cascade, forming a mesh that traps cells and forms a clot.

• Other solutes: 1.5% (Includes electrolytes, nutrients, gases, and waste). Each solute plays a specific role in metabolic processes and maintaining fluid balance.

Formed Elements Count

• RBCs: Normal range is 4.8-5.4 million/µL, carrying oxygen to tissues.

• WBCs: Normal range is 5000-10,000/µL, crucial for immune surveillance and response.

  • Differential Counts:

    • Neutrophils: 60-70% - First responders to infection, important in phagocytosis.

    • Lymphocytes: 20-25% - Key players in adaptive immunity, including B cells (produce antibodies) and T cells (directly attack infected cells).

    • Monocytes: 3-8% - Differentiate into macrophages and dendritic cells, aiding in phagocytosis and antigen presentation.

    • Eosinophils: 2-4% - Combat multicellular parasites and are involved in allergic reactions.

    • Basophils: 0.5-1.0% - Release histamine during inflammatory responses, playing a role in allergy responses.

• Platelets: 150,000-400,000/µL, critical for initiating clot formation and maintaining vascular integrity.

Function of Blood

• Transport: Oxygen and carbon dioxide exchange, transmission of nutrients from digestive tract to cells, distribution of hormones, transportation of metabolic waste products to excretory organs, and regulation of body temperature through heat distribution.

• Regulation: Maintains homeostasis of body fluids, blood pH (around 7.35-7.45), temperature regulation, and water content in cells to ensure optimal physiological functions.

• Protection: Clotting mechanisms that prevent blood loss at injury sites and immune functions provided by WBCs against infections and foreign substances.

Substances in Blood Plasma

• Major Constituents:

• Water: (91.5%) is the main component that serves multiple functions including acting as a solvent, partaking in chemical reactions, and regulating body temperature.

• Blood Plasma Proteins (7%):

  • Albumins (54%): Maintain osmotic pressure and serve as carriers for hormones and drugs, particularly important for drug availability in the blood.

  • Globulins (38%): Function as antibodies in immune responses and include transport proteins that carry lipids and hormones.

  • Fibrinogen (7%): Plays an essential role in the coagulation cascade, serving as a substrate for fibrin production to form a stable clot.

• Other Solutes:

  • Electrolytes: Vital for proper electrical function in nerve and muscle cells, maintaining cell membrane potential and hydration.

  • Nutrients: Include amino acids, glucose, and fatty acids that are essential for cellular metabolism and energy production.

  • Gases: Such as oxygen and carbon dioxide are transported within the blood and are crucial for respiration and cellular metabolism.

  • Regulatory Substances: Various enzymes, hormones, and vitamins that are involved in metabolic pathways and physiological regulation.

  • Waste Products: Metabolic waste from protein breakdown such as urea, creatinine, and uric acid which are transported to kidneys for excretion.

Formation of Blood Cells

• Hemopoiesis/Hematopoiesis: Refers to the complex process of blood cell production occurring in the bone marrow from pluripotent stem cells. This process is stimulated by various growth factors and hormones in response to physiological needs.

• RBCs and Platelet counts: Generally remain steady, while WBC counts can fluctuate significantly depending on the body’s response to infection or inflammation.

• Lymphocyte Lifespan: Can range from years in memory cells to days in activated cells; other WBC types typically have lifespans from days to weeks, balancing between immediate response and long-term immunity.

• Origin & Development: Blood cells arise from multipotent stem cells located in the bone marrow, influenced by signaling molecules from the immune and endocrine systems.

Red Blood Cells (RBCs)

• Structure and Function: Biconcave discs optimize surface area for gas exchange, allowing maximum oxygen transport. They lack organelles to allow more space for hemoglobin, the oxygen-carrying molecule. The typical lifespan of RBCs is approximately 120 days.

• Hemoglobin: A tetrameric protein that binds up to four oxygen molecules; its quaternary structure changes in response to oxygen binding, facilitating transport.

Role in Regulation

• Nitric Oxide (NO) Release: Nitric oxide acts as a potent vasodilator released by endothelial cells, helping to lower blood pressure and increase blood flow to tissues.

• Carbonic Anhydrase Activity: Enzyme critical for the reversible conversion of carbon dioxide and water to bicarbonate and protons, thereby facilitating the transport of carbon dioxide from tissues to the lungs and helping to regulate blood pH.

Availability and Recycling

• Old RBCs are phagocytized by macrophages in the spleen and liver, which recycle components such as iron for future use in erythropoiesis while bilirubin is produced from the breakdown of hemoglobin, contributing to bile production in the liver.

• Erythropoiesis: The production of RBCs is stimulated by erythropoietin, a hormone produced by the kidneys in response to tissue hypoxia, effectively increasing RBC production to meet oxygen demands in the body.

White Blood Cells (Leukocytes)

• Overview: Nucleated cells essential for the immune system, classified based on the presence and type of granules in their cytoplasm into granular (neutrophils, eosinophils, basophils) and agranular (lymphocytes, monocytes) categories.

• Function and Lifespan: WBCs are crucial for combatting pathogens through various mechanisms. They migrate from blood into tissues through a process called emigration or diapedesis. Activation and proliferation of specific WBC types depend on the nature of the immunological challenge.

Platelets

• Overview: Small, disc-shaped cell fragments produced from the cytoplasm of megakaryocytes that reside in the bone marrow, crucial for hemostasis by aggregating at sites of vascular injury.

• Lifespan: Platelets have a relatively short lifespan of about 5 to 9 days. Their production is regulated by thrombopoietin, a hormone produced by the liver and kidneys.

Hemostasis (Blood Clotting)

• Phases:

  • Vascular Spasm: A rapid, reflexive constriction of blood vessels that occurs immediately upon injury, significantly reducing blood flow to the affected area.

  • Platelet Plug Formation: Activated platelets adhere to exposed collagen at the injury site and release various signaling molecules, which recruit additional platelets, forming a temporary plug.

  • Coagulation: A series of complex biochemical reactions involving clotting factors, leading to the conversion of soluble fibrinogen to insoluble fibrin; this forms a mesh that stabilizes the platelet plug and prevents further blood loss.

• Clotting Pathways:

  • Extrinsic Pathway: Triggered by external injuries (e.g., cuts), this pathway leads to rapid clotting via tissue factor (TF) released from damaged cells.

  • Intrinsic Pathway: This more complex pathway is activated by damage to the blood vessel wall and involves a series of reactions within the blood itself, resulting in slower clot formation.

• Coagulation Factors: More than a dozen factors, primarily produced by the liver, work in a cascading fashion. Each factor is critical for ensuring effective clot formation and maintenance of hemostasis, highlighting the precision needed in these biochemical processes.

Blood Groups and Blood Types

Classification: Blood types are determined by specific surface antigens (A, B, AB, O) on the RBC membrane. These antigens play a crucial role in blood transfusions and organ transplants by influencing immune responses, as the presence or absence of these antigens can trigger adverse reactions.

Blood Type Compatibility

• Type O is considered a universal donor due to its lack of A and B antigens, thus minimizing the risk of transfusion reactions for recipients of various blood types.

• Type AB is regarded as a universal recipient as it has no antibodies against A or B antigens in its plasma, allowing it to receive blood from any ABO type without adverse reactions.

Blood Typing Procedure

• Cross-matching blood samples is performed using antisera, which contain antibodies to A and B antigens, allowing clinicians to observe agglutination reactions directly and confirm blood type compatibility before transfusions.

Disorders: Homeostatic Imbalances

Common Conditions:

• Sickle Cell Disease: A genetic disorder resulting from a mutation in the hemoglobin gene, leading to the production of abnormal hemoglobin (HbS) that distorts red blood cells into a sickle shape. These misshaped RBCs can block blood flow in small vessels, causing pain and organ damage due to chronic oxygen deprivation.

• Anemias: Various types such as iron-deficiency anemia, characterized by a reduced number of RBCs or lower hemoglobin levels, leading to fatigue and weakness. Other types include pernicious anemia (caused by vitamin B12 deficiency) and hemolytic anemia (caused by the premature destruction of RBCs).

• Hemophilia: An X-linked genetic disorder leading to deficiencies in specific clotting factors (e.g., Factor VIII for Hemophilia A), resulting in prolonged bleeding episodes. Patients often experience spontaneous bleeding and require careful management to prevent complications.

• Leukemia: A broad term for several forms of cancer characterized by the uncontrolled proliferation of abnormal WBCs. This leads to bone marrow overcrowding, decreased production of healthy blood cells, and an increased risk of infections, anemia, and hemorrhaging. Different forms exist based on the speed of progression (acute vs. chronic) and the type of white blood cells involved (lymphoid vs. myeloid).