Blood Circulation

Functions of the Circulatory System

  • Transportation

    • Respiratory gases (O2 and CO2)

    • Nutrients

    • Wastes

  • Regulation

    • Hormonal balance

    • Temperature control

  • Protection

    • Blood clotting

    • Immune response

Major Components of the Circulatory System

  • Cardiovascular System

    • Heart: A four-chambered pump consisting of:

    • Right atrium

    • Right ventricle

    • Left atrium

    • Left ventricle

    • Blood Vessels:

    • Arteries: Carry blood away from the heart.

    • Arterioles: Smaller arteries leading to capillaries.

    • Capillaries: Site of material exchange.

    • Venules: Small veins collecting blood from capillaries.

    • Veins: Return blood to the heart.

  • Lymphatic System:

    • Components:

    • Lymphatic vessels

    • Lymphoid tissues

    • Lymphatic organs (spleen, thymus, tonsils, lymph nodes)

Constituents of Blood

  • Average Adult Volume: 5 liters

  • Types of Blood:

    • Arterial Blood:

    • Bright red, oxygenated (except for blood going to the lungs).

    • Venous Blood:

    • Dark red, deoxygenated (except for blood coming from the lungs).

  • Composition:

    • 45% formed elements: Blood cells and platelets

    • 55% plasma: Liquid component of blood

Composition of Blood

  • Fractions when centrifuged:

    • Plasma: ~55% of total blood volume, mostly water with:

      • Proteins: (Albumin, Globulins, Fibrinogen, Regulatory proteins)

      • Electrolytes: (Na+, K+, Cl-, Ca++)

      • Nutrients: (Glucose, amino acids, fatty acids)

      • Gases: (O2 and CO2)

      • Wastes: (Urea, creatinine, lactic acid, ammonia)

    • Erythrocytes (RBCs): ~44% of total blood volume

    • Buffy Coat: ~1% of total blood volume (Leukocytes and platelets)

Composition of Blood Proteins

  • Albumin: Regulates water movement

  • Globulins: Transport proteins and antibodies involved in immunity

  • Fibrinogen: Involved in clotting processes

  • Regulatory Proteins: Hormones and enzymes involved in various bodily functions

Composition Details by Weight

  • Water: 92% of plasma

  • Proteins: 7% of plasma

    • Albumin: 58%

    • Globulins: 37%

    • Fibrinogen: 4%

    • Regulatory proteins: 1%

  • Other Solutes: 1% of plasma including electrolytes, nutrients, respiratory gases, and waste products

Cellular Components of Blood

  • Formed Elements: Red blood cells (RBCs), white blood cells (WBCs), and platelets

    • Only WBCs are actual functioning cells

  • Hematocrit: Percentage volume of RBCs in blood

    • Normal range: 38%-46% in females and 42%-56% in males

Regulation of Plasma Volume

  • Plasma volume and concentration are under regulatory control

  • Fluid loss leads to hyperosmotic blood plasma

    • Result: release of ADH (vasopressin) which promotes water reabsorption

  • Increased water reabsorption leads to an increase in blood volume and dilution of concentrated plasma

Erythrocytes (Red Blood Cells)

  • Mature RBCs lack nuclei and organelles

  • Shape: Small, biconcave discs that increase surface area for gas exchange and decrease the distance for gas travel

Hemoglobin in Erythrocytes

  • Each RBC contains approximately 280 million molecules of hemoglobin (Hb)

  • Function: Transport O2 and CO2 in blood

    • Structure: Consists of 4 globin proteins, each with a heme group that contains iron. Iron binds one O2 molecule.

  • Each hemoglobin molecule can bind up to 4 O2 molecules

Classification of Leukocytes (White Blood Cells)

  • Two classes:

    • Granulocytes: Visible granules in the cytoplasm

    • Agranulocytes: No visible granules

Functions of Leukocytes

  • Agranulocytes:

    • Monocytes: 3-8% of WBCs, undergo diapedesis to become macrophages, which eat pathogens and debris

    • Lymphocytes: 20-25% of WBCs

    • B lymphocytes: Produce antibodies

    • T lymphocytes: Attack infected or cancerous cells

Platelets (Thrombocytes)

  • Cellular fragments of megakaryocytes, which are much larger than RBCs

  • Size: Platelets are about 1/4 the size of RBCs

  • Function: Involved in the clotting of blood

Hematopoiesis (Blood Cell Formation)

  • Most formed elements have a short lifespan; therefore, they are continuously produced through hematopoiesis

  • Begins with hemocytoblasts (stem cells) in red bone marrow

  • Two cell lines arise from hemocytoblasts:

    • Myeloid Line: Produces RBCs, megakaryocytes, and all WBCs except lymphocytes

    • Lymphoid Line: Produces lymphocytes

Erythropoiesis (Red Blood Cell Production)

  • Regulated by erythropoietin produced by the kidneys in response to low oxygen levels

  • Sequential maturation phases from myeloid stem cell into mature erythrocyte:

    • Myeloid stem cell → Proerythroblast (large, nucleated cell) → Erythroblast (smaller, making hemoglobin) → Normoblast (smaller, nucleus ejected, still making hemoglobin) → Reticulocyte (few organelles, still producing hemoglobin) → Erythrocyte (mature cell)

  • Entire process takes about 6-7 days from stem cell to mature RBC in circulation

Blood Types

  • Red blood cells may have surface antigens, known as “name tags”, which are the basis for blood group systems

  • ABO System: Consists of two antigens, A and B

    • Blood Types:

    • Type A: Has A antigen

    • Type B: Has B antigen

    • Type AB: Has both A and B antigens

    • Type O: Has neither antigen

  • Rh System: Contains one antigen, Rh (antigen D)

    • Blood type is determined by the presence or absence of these antigens

    • Genetically determined

Immune Response to Blood Types

  • Immune system produces antibodies against foreign antigens

    • Mechanism: Antibodies target specific antigens

  • Tolerance Mechanism: Body will not produce antibodies against similar antigens that it recognizes as self

  • If one receives mismatched blood, agglutination can occur due to antibody-antigen reaction leading to RBC destruction

Rh System Details

  • Rh Positive: RBCs have Rh antigen

  • Rh Negative: RBCs do not have Rh antigen

  • Anti-Rh antibodies are not spontaneously produced; only produced after exposure to Rh antigen, such as through blood transfusions

  • Once produced, the immune system has memory of the antigen and can attack if there is any subsequent exposure

  • In pregnancy, an Rh-negative mother may develop antibodies against an Rh-positive fetus, posing a risk in subsequent pregnancies (erythroblastosis fetalis)

  • Preventive Treatment: Injection of RhoGam, an immune globulin, is given to Rh-negative mothers to prevent their immune system from forming antibodies against Rh-positive fetal blood

Blood Transfusion

  • Must match donor and recipient blood types to avoid transfusion reactions

  • Mismatched blood causes agglutination and subsequent death of RBCs

  • Person with Type A blood (anti-B antibodies) cannot receive Type B or AB blood

Blood Clotting Process

  • Hemostasis: Damage to blood vessels activates hemostatic response

  • Steps involved:

    • Vasoconstriction: Narrowing of blood vessel

    • Platelet Plug Formation: Platelets adhere to the site of injury and aggregate to form a plug

    • Production of Fibrin Web: Fibrin mesh surrounds and stabilizes the platelet plug

Platelet Activation and Aggregation

  • In intact blood vessels, endothelial cells secrete factors (PGI2 and NO) that promote vasodilation and prevent platelet aggregation

  • CD39: An enzyme that breaks down ADP to AMP and Pi, reducing platelet activation

Platelet Activity in Damaged Vessels

  • In cases of damage, platelets bind to exposed collagen fibers

  • Platelet Release Reaction: Platelets release secretory granules containing ADP, thromboxane A2, and other factors to recruit additional platelets and promote aggregation

  • Formation of Platelet Plug: Sequential platelet release reactions lead to the formation of a stable plug

Fibrin Clot Formation

  • Clots consist of platelets, fibrin (insoluble polymer), and red blood cells

  • In Vivo Clot Formation: Amplification of the clotting cascade occurs, leading to a stable clot

Clot Dissolution

  • Activated factor XII promotes conversion of inactive prekallikrein to kallikrein

  • Kallikrein produces plasmin, an enzyme that digests fibrin and dissolves the clot

Cardiovascular System Overview

  • Composed of:

    • Heart: serves as a pump

    • Blood vessels: arterial and venous systems regulate blood circulation

    • Blood: cells and plasma

  • System of Valves: Ensures unidirectional blood flow throughout the system

  • Circulatory Components:

    • Arteries: Carry blood away from the heart.

    • Veins: Return blood to the heart.

    • Capillaries: Involved in exchange of materials at the tissue level.

Pulmonary and Systemic Circulation

  • Pulmonary Circulation: Blood travels from the right ventricle to the lungs, returns to the left atrium after gas exchange

  • Systemic Circulation: Blood carries oxygen from the left ventricle to body tissues and returns deoxygenated blood to the right atrium

  • Coronary Circulation: Specific blood supply to the heart muscle itself through coronary arteries and veins

Heart Structure

  • Pericardium: Tough membrane encasing the heart, with pericardial fluid for lubrication

  • Heart Chambers:

    • Divided into functional halves: completely independent pumps

    • Right side receives deoxygenated blood from the body and sends it to lungs

    • Left side receives oxygenated blood from the lungs and pumps it throughout the body

Heart Anatomy Details

  • Atria: Chambers receiving blood returning to the heart

  • Ventricles: Chambers pumping blood out to lungs and body

  • Heart skeleton consists of connective tissue surrounding valves, providing structural support and electrical insulation

Heart Valves Overview

  • Atrioventricular (AV) Valves:

    • Tricuspid Valve: Between right atrium and ventricle

    • Bicuspid (Mitral) Valve: Between left atrium and ventricle

  • Semilunar Valves: Between ventricles and major arteries

    • Aortic Valve: Between left ventricle and aorta

    • Pulmonary Valve: Between right ventricle and pulmonary trunk

  • Function: Prevent backflow and ensure one-way flow through the heart

Cardiac Conduction System

  • Comprised of specialized cardiac cells:

    • SA Node: Sets the rhythm as the main pacemaker of the heart

    • AV Node: Relays conduction signals to ventricles and delays them for synchronized contraction

  • Conduction Pathway: Signals travel from SA node → AV node → Bundle of His → Bundle branches → Purkinje fibers

Electrocardiogram (ECG)

  • Displays electrical activity of the heart through waves (P wave, QRS complex, T wave) and segments

  • Waves:

    • P Wave: Atrial depolarization

    • QRS Complex: Ventricular depolarization

    • T Wave: Ventricular repolarization

  • Intervals: Combination of waves and baseline between them, indicating various phases of electrical activity

Cardiac Cycle Overview

  • Cardiac cycle consists of two main phases:

    • Diastole: Muscle relaxation and chamber filling

    • Systole: Muscle contraction and blood ejection

  • Mechanical Events: Valves open/close at specific phases of the cardiac cycle, generating heart sounds (S1, S2)

Phases of Cardiac Cycle

  • Early Diastole: Both chambers relaxed, AV valves open

  • Atrial Systole: Atria contract, completing ventricular filling

  • Isovolumic Contraction: Ventricles contract without volume change (AV valves close)

  • Ventricular Ejection: Blood ejected as semilunar valves open

  • Isovolumic Relaxation: Ventricles relax and semilunar valves close

Blood Vessel Structure

  • Three Layers (Tunics):

    • Tunica Externa (outer layer)

    • Tunica Media (middle layer, smooth muscle + elastic fibers)

    • Tunica Intima (inner layer, endothelium)

  • Functionality: Varies between arteries (thicker walls) and veins (thinner walls)

Types of Blood Vessels

  • Elastic Arteries: Thick elastic layers, accommodate surge of blood during ventricular systole (e.g., aorta)

  • Muscular Arteries: Smaller than elastic arteries, regulate blood distribution to organs

  • Arterioles: Smallest arteries, controlling blood flow into capillary beds

  • Capillaries: Consist of endothelial cells only, facilitating exchange of substances

  • Veins: Larger lumens, thinner walls than arteries, serve as blood reservoirs

Lymphatic System Function

  • Transport: Interstitial fluid and absorbed fats from the digestive tract (lacteals)

  • Immune Function: Houses lymphocytes to aid in immune responses