2025-03-06 BIO-151 - Blood

Overview of Blood Functions

Blood has several key functions in the body, including:

• Transporting:

  • Gases: Oxygen (O2) and carbon dioxide (CO2). Hemoglobin in red blood cells binds to O2 for transport from the lungs to tissues, while CO2 is transported from tissues to the lungs for exhalation. Oxygen is vital for cellular respiration, the process by which cells produce energy, while CO2 is a waste product that must be removed to maintain pH balance in the body.

  • Nutrients: Absorbed from digested food travel through the bloodstream to cells needing energy or materials for growth and repair. This includes glucose, fatty acids, vitamins, and minerals.

  • Waste Products: Byproducts of cellular metabolism, such as urea and creatinine, are collected in the blood and transported to the kidneys or liver for processing and excretion. Efficient removal of waste products is essential for maintaining homeostasis and preventing harmful accumulation in tissues.

Blood also plays a vital role in:

• Production of Vitamin D: The skin produces a signal in response to sunlight, prompting the liver to produce Vitamin D, which is vital for calcium absorption in the intestines and bone health. This process is integral for maintaining bone density and overall skeletal function.

• Hormonal Regulation: Hormones secreted by endocrine glands (like insulin, adrenaline, and cortisol) travel through the bloodstream to target organs and tissues, where they regulate various biological functions such as metabolism, stress response, and growth. For example, insulin regulates blood glucose levels, while cortisol is involved in the body's stress responses.

• Homeostasis: Blood helps maintain a stable internal environment. This includes regulating:

  • pH Balance: Blood pH is tightly controlled between 7.35 and 7.45; buffers in the blood, primarily bicarbonate ions, neutralize excess acids or bases to maintain pH. Disruptions in blood pH can lead to acidosis or alkalosis, which can impair various bodily functions.

  • Body Temperature: Blood flow adjusts via vasodilation and vasoconstriction to either release or retain heat, helping to regulate core body temperature. This thermoregulation is especially important during physical activity or exposure to extreme temperatures.

Blood Composition and Cellular Components

Blood consists primarily of:

• Red Blood Cells (RBCs):

  • Contain hemoglobin, which carries up to 98.5% of oxygen and about 23% of carbon dioxide in blood. Each RBC includes around 270 million hemoglobin molecules, ensuring efficient transport.

  • RBCs live for about six months and are continuously produced from stem cells in the bone marrow. Loss of RBCs occurs through rupturing in the spleen and other organs.

  • Primary function: gas transport, enabling efficient delivery of oxygen to, and removal of carbon dioxide from, the body's tissues.

• White Blood Cells (WBCs):

  • Crucial for immune response; different types involved:

    • Granulocytes: Include basophils (involved in allergic responses), eosinophils (combat multicellular parasites and play a role in allergic reactions), and neutrophils (phagocytic cells that respond first to infection, particularly bacterial infections). Granulocytes play a vital role in the body's innate immune response.

    • Agranulocytes:

      • Lymphocytes: T cells (cell-mediated immunity) directly attack infected cells or cancer cells, whereas B cells (humoral immunity) produce antibodies that neutralize pathogens.

      • Monocytes: Larger, longer-living cells that differentiate into macrophages and dendritic cells, which are key players in phagocytosis, antigen presentation, and orchestrating the immune response against pathogens.

• Platelets:

  • Not true cells but cellular fragments derived from megakaryocytes; essential for blood clotting and wound healing by forming temporary plugs at injury sites. Platelets contain granules that release important clotting factors.

Hematopoiesis

Definition: Formation of blood cells from stem cells in the bone marrow, which is a critical process to sustain healthy blood levels.

Key terms:

• Proerythroblast: Precursor to red blood cells (erythrocytes), undergoing several maturation stages before becoming mature RBCs.

• Myoblast: Precursor cells that give rise to granulocytes; they undergo a series of divisions and differentiations to mature into fully functional white blood cells.

• Lymphoblast and Monoblast: Precursor cells responsible for the production of lymphocytes and monocytes, respectively. Lymphoblasts will eventually develop into B cells and T cells, vital for immune responses, while monoblasts mature into monocytes.

Gas Transport and Hemoglobin Structure

• Hemoglobin Composition: Composed of four heme groups, each containing an iron atom that binds to one oxygen molecule, and four globin proteins, forming a quaternary structure crucial for its function. Each heme group can bind one oxygen, allowing each hemoglobin molecule to carry four molecules of oxygen, facilitating efficient oxygen transport.

• Oxygen Transport: Each heme can bind one oxygen molecule, allowing each hemoglobin molecule to carry up to four oxygen atoms to tissues, releasing them as needed during cellular respiration.

• Carbon Dioxide Transport: 23% of CO2 binds to the globin portion of hemoglobin, while the majority (approximately 70%) is transported as bicarbonate ions in the plasma after conversion by carbonic anhydrase within red blood cells. This conversion is essential for maintaining the acid-base balance in the body.

pH Regulation and Carbon Dioxide

• Carbon Dioxide Role: Acts as a vital buffer in the blood; it aids in maintaining pH through the formation of bicarbonate ions, providing a critical line of defense against acidity in body fluids. The balance of CO2 levels directly influences respiratory rates.

• Acid-Base Balance: Too little CO2 can disrupt this balance, leading to conditions such as respiratory alkalosis, while excess CO2 can lead to acidosis.

• Carbonic Acid-Bicarbonate Buffer System: Plays a crucial role in stabilizing pH by adjusting bicarbonate (HCO3-) and carbonic acid (H2CO3) levels in response to metabolic demands. This buffer system allows for rapid responses to pH changes, maintaining a relatively constant pH in the blood.

Erythropoiesis and Erythropoietin

• Erythropoiesis: The process of red blood cell production, predominantly occurring in the bone marrow, stimulated by the hormone erythropoietin, which is produced in the kidneys in response to low oxygen levels (hypoxia). Erythropoietin acts on progenitor cells in the marrow to increase the rate of red blood cell formation.

• Effects of High Altitudes on Red Blood Cell Production: Quick adaptation includes increased erythropoietin release, which stimulates marrow activity, boosting RBC count to enhance oxygen transport to tissues, mitigating the effects of reduced atmospheric oxygen. This physiological response is critical for maintaining oxygen homeostasis when exposed to hypoxic conditions.

Blood Clotting and Hemostasis

• Hemostasis: The process through which the body stops bleeding, involving three main stages:

  • Vascular Spasm: Immediate constriction of blood vessels at the injury site to reduce blood flow and limit blood loss. This spasm is mediated by the release of chemicals from platelets and endothelial cells.

  • Platelet Plug Formation: Platelets adhere to the exposed collagen and fibrous tissue at the injury site, aggregating to form a temporary seal. This aggregation also signals the release of more chemicals to recruit additional platelets.

  • Coagulation: A complex cascade of events converts prothrombin to thrombin, leading to the conversion of fibrinogen to fibrin, forming a stable blood clot that reinforces the platelet plug. Coagulation is regulated by a series of clotting factors, which can be activated by intrinsic or extrinsic pathways.

• Anticoagulants: Substances like heparin (produced by basophils) and antithrombin (produced in the liver) that prevent unwanted clotting by inhibiting thrombin activity and other factors in the clotting cascade. Anticoagulants are important in preventing thrombosis (unwanted blood clots).

Blood Types and Immune Responses

• ABO Blood Grouping: Blood types A, B, AB, and O (the latter being the universal donor type).

• Determined by the Presence or Absence: of specific antigens (A and B antigens) on the surface of red blood cells which can trigger an immune response if transfused incompatible types are introduced.

• Rhesus Factor: Indicates whether the D antigen is present on RBCs (Rh-positive) or absent (Rh-negative), which is critical in transfusion compatibility and pregnancy. Rh incompatibility can pose significant risks during pregnancy and requires careful monitoring.

• Hemolytic Disease of the Newborn: Occurs when Rh-negative mothers produce antibodies against Rh-positive fetal blood cells if there is a mix during pregnancy or delivery, potentially causing hemolysis (destruction) of fetal red blood cells. This condition can lead to serious health complications for the newborn.

Blood Lab Work and Diagnostic Measures

• Hematocrit: Measures the ratio of RBCs to total blood volume, indicating overall blood composition and health and is crucial in diagnosing various anemias and conditions affecting red blood cell production.

  • Normal Values: Males: ~50%; Females: ~45% (variations based on age and health).

• Blood Doping: A practice involving the illicit increase of hematocrit levels to enhance athletic performance by improving oxygen delivery to muscles. This practice raises ethical concerns in sports and can have severe health risks.

• White Blood Cell Count: Monitoring normal range percentages for various types of WBCs (neutrophils, lymphocytes, eosinophils, etc.) is critical for diagnosing immune responses, identifying potential disorders or infections, and assessing overall health status. A deviation from normal ranges can indicate an infection or immune disorder.

Summary of White Blood Cell Functions

• Neutrophils: Phagocytic cells targeting bacteria, playing a first line of defense in the immune response; often they are the first responders to sites of inflammation or infection, engulfing pathogens and debris.

• Lymphocytes: Responsible for adaptive immune response; B cells produce antibodies, while T cells kill infected cells and coordinate the immune response. Specificity allows for targeted elimination of pathogens after prior exposure.

• Monocytes: Differentiate into macrophages, which are key players in phagocytosis, antigen presentation, and orchestrating the immune response against pathogens. Macrophages are also involved in wound healing and tissue repair.

These detailed notes should help reinforce your understanding of blood functions and compositions as you prepare for the exam.