Blood and Human Physiology

Blood

Components of Blood

• Blood consists of living and non-living components.

  • The living component consists of three types of cells.

  • The non-living component is the plasma.

Plasma Constituents

• Plasma Constituent and Function:

  • Water: Serves as a solvent and medium for transporting materials.

  • Albumin: Maintains osmotic pressure and transports substances.

  • Gamma Globulins: Are antibodies that play a role in immune response.

  • Fibrinogen: Involved in blood clotting.

  • Nutrients and Wastes: Transport of nutrients and metabolic waste.

  • Salts: Electrolyte balance.

  • Gases: Transport of oxygen and carbon dioxide.

Role of Blood Cells

• Erythrocytes (Red Blood Cells):

  • Carry oxygen from the lungs to body tissues.

  • Contain hemoglobin, an oxygen-binding protein.

• Leukocytes (White Blood Cells):

  • Five different types divided into two categories: Granulocytes and Agranulocytes.

  • Granulocytes:

  • Neutrophil:

    • A granulocytic WBC.

    • Functions to phagocytize bacteria.

    • Increased numbers indicate a bacterial infection.

  • Eosinophil:

    • A granulocytic WBC involved in allergy and parasitic worm destruction.

    • Has red cytoplasmic granules.

  • Basophil:

    • A granulocytic WBC involved in inflammation.

    • Contains blue cytoplasmic granules filled with histamine.

  • Agranulocytes:

  • Lymphocyte:

    • An agranulocytic WBC involved in immunity.

    • B Lymphocytes: Produce antibodies.

    • T Lymphocytes: Directly attack foreign cells.

  • Monocyte:

    • An agranulocytic WBC functioning in phagocytosis of bacteria in long-standing infections.

    • The largest leukocyte.

• Platelets (Thrombocytes):

  • Cytoplasmic fragments of a larger cell, the megakaryocyte.

  • Functions in hemostasis (stopping bleeding).

Hematopoiesis

• The process of blood cell formation.

• Occurs in red bone marrow from stem cells called hemocytoblasts.

• Differentiation to:

  • Common Myeloid Progenitor yields:

  • Erythrocytes

  • Platelets

  • Basophils

  • Neutrophils

  • Eosinophils

  • Common Lymphoid Progenitor yields:

  • T Lymphocytes

  • B Lymphocytes

Erythropoiesis

• The formation of red blood cells occurs in red bone marrow.

  • From hemocytoblast to reticulocyte maturity.

  • Reticulocyte Count: A measurement indicating the level of erythropoiesis occurring.

• Erythropoietin:

  • A hormone released from the kidney in response to low blood pO₂.

  • Stimulates erythropoiesis.

  • Key factors:

  • Hypoxemia: Inadequate oxygen transport in blood.

  • Stimulates increased RBC count and accelerated erythropoiesis.

• Conditions of Low pO₂:

  • Low RBC numbers (anemia).

  • High altitude due to lower atmospheric pressure.

  • Athletes: Use high-altitude simulation tents to induce increased erythropoiesis and enhance oxygen capacity.

Life History of Red Blood Cells

• RBCs lack a nucleus and do not carry out mitosis.

• Produced in red bone marrow (erythropoiesis), with a lifespan of approximately 120 days.

• Destruction occurs in small capillaries of the spleen and liver.

Breakdown of Hemoglobin

• Iron from heme is recycled.

• Globin's amino acids are also recycled.

Porphyrin Ring Breakdown

• The porphyrin ring is converted to bilirubin, which is lipid-soluble.

• Bilirubin is transported to the liver, modified with glucuronic acid, forming conjugated bilirubin (water-soluble) which is expelled with bile into the intestines and exits with stool.

Blood Types

• Antigens (Agglutinogens): Present on RBC membranes and act as 'nametags.'

• Antibodies (Agglutinins): Present in plasma; will destroy cells with non-self antigens.

  • Types: A, B, AB, O blood types.

• Rh System:

  • Rh+: Possesses the Rh antigen.

  • Rh-: Lacks the Rh antigen; may produce antibodies against Rh.

  • Risks occur when an Rh- mother is pregnant with an Rh+ baby, leading to erythroblastosis fetalis (hemolytic disease of the newborn).

Hemostasis

• Defined as the stoppage of bleeding, comprising three phases:

  1. Vasospasm: Reflexive narrowing of the blood vessels, mediated by serotonin; reduces blood loss.

  2. Platelet Plug Formation: Platelets adhere to exposed collagen fibers, stimulated by ADP and Thromboxane A2, leading to further platelet aggregation.

  3. Coagulation (Clotting): Involves intrinsic and extrinsic pathways that rely on calcium ($Ca^{2+}$) and clotting factors generated by the liver, with Vitamin K supporting the formation.

Pathways of Coagulation

• Intrinsic Pathway:

  • Occurs in 3-6 minutes from damage to the blood vessel wall, mainly stimulated by the platelet plug.

  • Involves clotting factors including XII, XI, IX, VIII, and X.

• Extrinsic Pathway:

  • Occurs in 15 seconds following damage to both the vessel wall and surrounding tissues.

  • Involves tissue thromboplastin, VII, and X.

Summary of Coagulation

• Clots form through a meshwork of fibrin trapping blood cells and are initiated by thrombin.

• Clot retraction occurs as the clot shrinks, bringing vessel edges closer, facilitating repair by PDGF secreted by platelets.

Dissolution of Clot

• TPA (Tissue Plasminogen Activator): Released by healed endothelial cells, leading to the activation of plasmin, which dissolves the clot.

Risks of Clots

• Potential dangers from clots include thrombi (stationary clots) and emboli (travelling clots), leading to conditions such as strokes, heart attacks, or pulmonary emboli, which may be life-threatening.

Conditions Affecting Clot Formation

• Clots may form due to conditions such as:

  • Atherosclerosis (rough tissue triggering clot formation).

  • Blood stasis (pooling which activates clotting factors).

Inhibition of Clot Formation

• Aspirin: Reduces thromboxane A2 to inhibit platelet plug formation.

• Antithrombin III: Binds and inactivates thrombin, inhibiting clot formation.

• Heparin: Increases antithrombin III activity, also preventing clot formation.