Blood: Structure and Function

Overview of Blood Functions

Blood is a vital connective tissue that performs essential functions in the human body, including transport, regulation, and protection.

• Transport: Blood carries oxygen, carbon dioxide, nutrients, hormones, and metabolic wastes throughout the body.

• Regulation: Blood helps maintain homeostasis by regulating body temperature, pH, and fluid volume.

• Protection: Blood contains cells and proteins that defend against infection and prevent blood loss through clotting.

Key Components:

• Plasma: The liquid matrix of blood, composed mainly of water, proteins, electrolytes, and nutrients.

• Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments involved in clotting).

Erythrocytes: Specialized for oxygen and carbon dioxide transport via hemoglobin.

Platelets: Small cell fragments crucial for initiating hemostasis and stopping bleeding.

Hemostasis: The process that prevents blood loss after vessel injury.

Transfusion: Medical procedure to replace lost blood and restore oxygen-carrying capacity.

Hemostasis: Prevention of Blood Loss

Introduction to Hemostasis

Hemostasis is a rapid, localized, and tightly regulated process that stops bleeding when a blood vessel is injured. It involves a series of steps that utilize clotting factors and substances released by platelets and damaged tissues.

• Vascular Spasm

• Platelet Plug Formation

• Coagulation (Blood Clotting)

Step 1: Vascular Spasm

Vascular spasm is the immediate constriction of a blood vessel in response to injury, reducing blood flow and loss.

• Triggered by direct injury to vascular smooth muscle, chemicals released by endothelial cells and platelets, and pain reflexes.

• Most effective in smaller blood vessels.

• Reduces blood flow until other hemostatic mechanisms activate.

Step 2: Platelet Plug Formation

Platelets adhere to exposed collagen fibers at the injury site, forming a temporary plug.

• Von Willebrand Factor (VWF): Stabilizes platelet-collagen adhesion.

• Activated platelets swell, become sticky, and release chemical messengers:

  • ADP: Attracts more platelets to the site.

  • Serotonin and Thromboxane A2: Enhance vascular spasm and platelet aggregation.

• Positive feedback loop: More platelets stick and release chemicals, amplifying the response.

• Effective for small vessel tears; larger breaks require coagulation.

Step 3: Coagulation (Blood Clotting)

Coagulation reinforces the platelet plug with fibrin threads, forming a stable clot.

• Involves a cascade of reactions using clotting factors (procoagulants), mostly plasma proteins synthesized by the liver.

• Clotting factors are numbered I-XIII.

• Vitamin K is essential for synthesizing several clotting factors.

• Most factors circulate in inactive form and are activated during coagulation.

Phases of Coagulation

• Phase 1: Pathways to Prothrombin Activator

  • Intrinsic Pathway: Triggered by factors within the blood (e.g., exposed collagen, activated platelets).

  • Extrinsic Pathway: Triggered by tissue factor (TF) from damaged tissue.

  • Both pathways converge at Factor X, which combines with Ca2+ and Factor V to form prothrombin activator.

• Phase 2: Pathway to Thrombin

  • Prothrombin activator converts prothrombin to thrombin.

• Phase 3: Pathway to Fibrin Mesh

  • Thrombin converts fibrinogen to fibrin.

  • Fibrin forms a mesh that traps blood cells and stabilizes the clot.

  • Factor XIII (fibrin stabilizing factor) cross-links fibrin, strengthening the clot.

Retraction and Fibrinolysis

After clot formation, the clot retracts and is eventually dissolved.

• Clot Retraction: Platelets contract, pulling torn vessel edges together and squeezing out serum.

• Fibrinolysis: Plasminogen is converted to plasmin, which digests fibrin and dissolves the clot.

• Endothelial growth factors stimulate repair of the vessel wall.

Factors Limiting Clot Growth or Formation

Mechanisms exist to prevent excessive clotting and ensure clots remain localized.

• Removal and dilution of clotting factors by flowing blood.

• Inhibition of activated clotting factors by anticoagulants (e.g., antithrombin III, heparin).

• Healthy endothelium secretes substances (NO, prostacyclin, vitamin E quinone) that prevent platelet adhesion.

Hemostasis Disorders

Thromboembolic Disorders

These disorders involve undesirable clot formation within blood vessels.

• Thrombus: A stationary clot that can block circulation and cause tissue death.

• Embolus: A thrombus that breaks free and travels through the bloodstream.

• Embolism: An embolus that obstructs a blood vessel (e.g., pulmonary embolism).

• Risk Factors: Atherosclerosis, inflammation, slow blood flow, immobility.

Anticoagulant Drugs:

• Aspirin: Inhibits thromboxane A2, reducing clot formation and heart attack risk.

• Heparin: Used clinically to prevent thrombosis.

• Warfarin and Direct Oral Anticoagulants: Interfere with vitamin K action, used in patients with atrial fibrillation.

Bleeding Disorders

• Thrombocytopenia: Low platelet count (<150,000/μL), leading to spontaneous bleeding and petechiae.

• Impaired Liver Function: Reduces synthesis of clotting factors; causes include vitamin K deficiency, hepatitis, cirrhosis.

• Hemophilia: Hereditary disorders due to deficiencies in clotting factors (most commonly factor VIII).

• Treatment: Transfusions of platelets or genetically engineered clotting factors.

Blood Transfusions and Blood Groups

Blood Loss and Replacement

The cardiovascular system compensates for blood loss by constricting vessels and increasing RBC production, but severe loss requires transfusion.

• Loss of 15-30%: Pallor and weakness.

• Loss of >30%: Potentially fatal shock.

• Immediate replacement with saline or electrolyte solutions restores volume but not oxygen-carrying capacity.

Transfusing Red Blood Cells

• Packed RBCs: Preferred for restoring oxygen-carrying capacity; plasma and WBCs are removed.

• Blood banks separate components; shelf life is limited.

• Blood groups must be matched to prevent transfusion reactions.

Human Blood Groups

Blood groups are determined by the presence or absence of specific antigens (agglutinogens) on RBC surfaces.

• More than 30 RBC antigens exist; ABO and Rh are most clinically important.

• Mismatched transfusions can cause agglutination and destruction of RBCs.

ABO Blood Groups

Universal Donor: Type O (no A or B antigens)

Universal Recipient: Type AB (no anti-A or anti-B antibodies)

Rh Blood Groups

• Rh factor (D antigen) present: Rh positive; absent: Rh negative.

• Anti-Rh antibodies form only after exposure to Rh+ blood.

• Important in pregnancy (see erythroblastosis fetalis).

Erythroblastosis Fetalis

Hemolytic disease of the newborn occurs when an Rh- mother carries an Rh+ fetus.

• First pregnancy: Mother exposed to Rh+ blood during delivery, develops anti-Rh antibodies.

• Subsequent pregnancies: Maternal anti-Rh antibodies cross placenta and destroy fetal RBCs.

• Treatment: Prebirth transfusions and administration of anti-Rh serum (RhoGAM).

Transfusion Reactions

Occur if mismatched blood is infused.

• Recipient's antibodies attack donor RBCs, causing agglutination and hemolysis.

• Symptoms: Fever, chills, low blood pressure, rapid heartbeat, nausea, vomiting.

• Treatment: Prevent kidney damage with fluids and diuretics.

Blood Typing and Matching

• Blood is typed for ABO and Rh antigens before transfusion.

• Cross-matching: Testing donor and recipient blood for compatibility.

Diagnostic Blood Tests

Common Blood Tests

• Hematocrit: Percentage of RBCs in blood; low values indicate anemia.

• Blood Glucose: High levels suggest diabetes.

• Leukocytosis: Elevated WBC count may signal infection.

• Microscopic Examination: Reveals variations in RBC size, shape, and color.

• Differential WBC Count: Proportions of each WBC type aid diagnosis.

• Thrombin Time and Platelet Count: Assess hemostasis.

• Comprehensive Metabolic Panel: Blood chemistry profile for liver and kidney function.

• Complete Blood Count (CBC): Checks formed elements, hematocrit, and hemoglobin.

Blood Donation

Blood Donation Statistics

• Only a small percentage of the population donates blood regularly.

• Blood services encourage donation to maintain adequate supply.

Study Tips for Anatomy & Physiology

• Review notes frequently and develop a study schedule.

• Focus on understanding concepts rather than memorizing facts.

• Use different study materials, flash cards, and take breaks.

• Study with others or teach concepts to reinforce learning.

Additional info: Some details (e.g., specific percentages, clinical applications) were inferred or expanded for completeness and clarity.