BLOOD: A SPECIAL CONNECTIVE TISSUE

Plasma:

• Definition: Straw-colored, viscous fluid comprising 55% of blood.
• Composition:
  • Water: 90-92%.
  • Proteins: 6-8% (Fibrinogen, Globulins, Albumins).
    • Fibrinogens: Essential for blood clotting.
    • Globulins: Involved in defense mechanisms.
    • Albumins: Maintain osmotic balance.
  • Minerals: Na⁺, Ca⁺⁺, Mg⁺⁺, HCO^3–, Cl^–.
  • Other constituents: Glucose, amino acids, lipids, etc.
• Clotting Factors: Present in an inactive form; absence forms serum.

Formed Elements:

• Constituents: Erythrocytes (RBCs), Leucocytes (WBCs), Platelets.
• Composition: Constitute 45% of blood.

Erythrocytes (Red Blood Cells – RBCs):

• Abundance: Most abundant blood cells.
• Origin: Formed in red bone marrow in adults.
• Characteristics:
  • Biconcave shape.
  • Lack nuclei in mammals.
  • Contain hemoglobin.
• Hemoglobin: Iron-containing complex protein; crucial for gas transport.
• Lifespan: Approximately 120 days; destroyed in the spleen.

Leucocytes (White Blood Cells – WBCs):

• Color: Colorless (lacking hemoglobin), nucleated.
• Number: Lesser in number (6000-8000 mm–3).
• Categories: Granulocytes (Neutrophils, Eosinophils, Basophils), Agranulocytes (Lymphocytes, Monocytes).
• Functions:
  • Neutrophils and monocytes: Phagocytic, destroy foreign organisms.
  • Basophils: Secrete substances involved in inflammatory reactions.
  • Eosinophils: Resist infections, associated with allergic reactions.
  • Lymphocytes: Responsible for immune responses (B and T forms).

Platelets (Thrombocytes):

• Source: Fragments produced from megakaryocytes in the bone marrow.
• Normal Count: 1,500,00-3,500,00 platelets mm–3.
• Functions: Release substances involved in blood clotting.
• Importance: Reduction leads to clotting disorders, risking excessive blood loss.

Blood Groups and Grouping

ABO Grouping:

• Basis: Presence or absence of surface antigens (A and B) on RBCs.
• Natural Antibodies: Plasma contains natural antibodies corresponding to absent antigens.
• Groups:
  • A: Antigen A, Anti-B antibodies.
  • B: Antigen B, Anti-A antibodies.
  • AB: Both Antigens (A and B), No antibodies.
  • O: No Antigens, Anti-A, and Anti-B antibodies.
• Significance: Essential for blood transfusion compatibility, prevents clumping.

Rh Grouping:

• Rh Antigen: Present in nearly 80% of humans (Rh+ve), absent in others (Rh-ve).
• Rh Incompatibility:
  • Rh+ve to Rh-ve: Generally safe.
  • Rh-ve to Rh+ve: Forms antibodies against Rh antigens, causing issues.
• Special Case: Rh incompatibility during pregnancy (erythroblastosis foetalis).
• Erythroblastosis Foetalis:
  • Mother (Rh-ve) exposed to Rh+ve fetal blood during delivery.
  • The mother develops Rh antibodies.
  • Subsequent pregnancies may lead to antibodies affecting fetal RBCs.
  • Condition: Erythroblastosis foetalis.
• Prevention: Administering anti-Rh antibodies to Rh-ve mothers post-first delivery.

Coagulation of Blood

• Purpose: Prevent excessive blood loss after injury or trauma.
• Process:
  • Formation of Clot (Coagulam):
    • A network of fibrin threads traps dead and damaged formed elements.
    • Fibrins formed from inactive fibrinogens by the enzyme thrombin.
    • Thrombins derived from inactive prothrombin.
    • Enzyme complex thrombokinase, formed by the cascade process, is crucial.
  • Platelet Activation: Injury stimulates platelets, releasing factors.
  • Tissue Factors: Released by injured tissues also initiate coagulation.
  • Calcium Ions: Essential for the clotting process.
• Cascade Process:
  • Series of linked enzymic reactions.
  • Involves various factors present in plasma in an inactive state.
• Result: Dark reddish-brown scum was observed at the injury site.
• Role of Calcium Ions: Crucial in the coagulation mechanism.

Lymph (Tissue Fluid): Exchange and Transport

1. Capillary Exchange:
   • Blood passing through capillaries releases water and small water-soluble substances into tissue spaces.
   • Larger proteins and formed elements remain in the blood vessels.
2. Interstitial Fluid:
   • Fluid released into tissue spaces is known as interstitial fluid or tissue fluid.
   • Composition mirrors that of plasma, including mineral distribution.
3. Role in Nutrient Exchange:
   • Acts as a medium for the exchange of nutrients, gases, and other substances between blood and cells.
4. Lymphatic System:
   • An elaborate network of vessels, the lymphatic system, collects interstitial fluid.
   • Drains fluid back into major veins.
5. Formation of Lymph:
   • Fluid collected in the lymphatic system is referred to as lymph.
   • Lymph is colorless and carries lymphocytes, key components of immune responses.
6. Immune Responses:
   • Lymphs contain specialized lymphocytes responsible for immune responses in the body.
7. Nutrient and Hormone Transport:
   • Serves as a carrier for nutrients, hormones, and other essential substances.
8. Fat Absorption:
   • Lymph plays a crucial role in fat absorption.
   • Lacteals in intestinal villi absorb fats through the lymphatic system.

Circulatory Pathways: Open vs. Closed

1. Open Circulatory System:
   • Present in arthropods and molluscs.
   • The heart pumps blood into sinuses or open spaces in the body cavities.
   • Blood flows through large vessels and open areas.
2. Closed Circulatory System:
   • Found in annelids and chordates, including vertebrates.
   • The heart pumps blood into a closed network of blood vessels.
   • Offers more precise regulation of fluid flow.

Vertebrate Heart Structure:

1. Fish:
   • 2-chambered heart (atrium and ventricle).
   • Deoxygenated blood is pumped out, oxygenated by gills, and supplied to the body (single circulation).
2. Amphibians and Reptiles (except crocodiles):
   • 3-chambered heart (two atria, one ventricle).
   • The left atrium receives oxygenated blood, right atrium receives deoxygenated blood.
   • Incomplete double circulation.
3. Crocodiles, Birds, and Mammals:
   • 4-chambered heart (two atria, two ventricles).
   • Oxygenated and deoxygenated blood remain separate.
   • Complete double circulation.

Human Circulatory System: Structure and Function

1. Heart Structure:

• Location: In the thoracic cavity, between the lungs, slightly tilted to the left.
• Protection: Double-walled membranous bag called pericardium, enclosing pericardial fluid.
• Size: Approximately the size of a clenched fist.

2. Chambers of the Heart:

• Atria:
  • Two relatively small upper chambers.
  • Separated by the interatrial septum.
• Ventricles:
  • Two larger lower chambers.
  • Separated by the inter-ventricular septum.
  • Thicker walls than atria.

3. Valves in the Heart:

• Right Atrium to Right Ventricle: Tricuspid valve (three cusps).
• Left Atrium to Left Ventricle: Bicuspid or Mitral valve.
• Ventricles to Pulmonary Artery or Aorta: Semilunar valves.
• Function: Allow one-directional blood flow, preventing backward flow.

4. Cardiac Musculature:

• The entire heart is made of cardiac muscles.
• Ventricular walls are thicker than atrial walls.

5. Nodal Tissue:

• Sino-Atrial Node (SAN):
  • Located in the right upper corner of the right atrium.
  • Initiates and maintains rhythmic contractile activity.
  • Acts as the pacemaker, generating 70-75 action potentials per minute.
• Atrio-Ventricular Node (AVN):
  • Located in the lower left corner of the right atrium.
  • Connected to the AV bundle.
• Atrio-Ventricular Bundle (AV Bundle) and Purkinje Fibres:
  • AV bundle continues from AVN.
  • Divides into right and left bundles on the top of the inter-ventricular septum.
  • Purkinje fibers spread throughout ventricular musculature.

6. Action Potentials:

• Nodal musculature is auto-excitable, generating action potentials without external stimuli.
• SAN acts as the pacemaker, regulating the heart’s rhythmic activity.
• Normal heart rate: 70-75 beats per minute.

Cardiac Cycle: Understanding Heart Function

1. Initiation of the Cardiac Cycle:

• All four heart chambers are in a relaxed state (joint diastole).
• Tricuspid and bicuspid valves open, allowing blood from pulmonary veins and vena cava into the ventricles.
• Semilunar valves closed.

2. Atrial Systole:

• SAN generates an action potential, stimulating simultaneous contraction of both atria (atrial systole).
• Increased blood flow into ventricles by about 30%.

3. Ventricular Systole:

• The action potential is conducted to ventricles through AVN and AV bundle.
• Ventricular muscles contract (ventricular systole).
• Atria undergo relaxation (diastole) during ventricular systole.
• Increased ventricular pressure closes tricuspid and bicuspid valves.
• Semilunar valves guarding the pulmonary artery and aorta are forced open, allowing blood flow into circulatory pathways.

4. Ventricular Diastole:

• Ventricles relax (ventricular diastole).
• Ventricular pressure falls, closing semilunar valves to prevent backflow.
• Tricuspid and bicuspid valves are pushed open as atrial pressure increases due to blood inflow.
• Blood moves freely into the ventricles.

5. Repeat Cycle:

• SAN generates a new action potential, initiating a new cardiac cycle.
• Sequential events of systole and diastole repeated cyclically.

6. Cardiac Output:

• Stroke volume: Approximately 70 mL of blood is pumped out by each ventricle during a cardiac cycle.
• Heart rate: 72 beats per minute.
• Cardiac output: Stroke volume × Heart rate.
• Averages 5000 mL or 5 liters per minute in a healthy individual.

7. Sounds of the Cardiac Cycle:

• First heart sound (lub): Associated with closure of tricuspid and bicuspid valves.
• Second heart sound (dub): Associated with the closure of semilunar valves.
• Clinically significant for diagnostic purposes.

Electrocardiograph (ECG): Understanding Heart Electrical Activity

1. ECG Basics:

• Definition: Electrocardiogram (ECG) is a graphical representation of the heart’s electrical activity during a cardiac cycle.
• Monitoring Machine: Typically shown in hospitals, the ECG machine displays voltage traces on a screen and produces characteristic sounds.

2. ECG Procedure:

• Patient Connection:
  • Connected to the machine with three electrical leads (wrist and left ankle).
  • Multiple leads for detailed evaluation are attached to the chest region.

3. ECG Peaks and Letters:

• Identification:
  • Peaks in the ECG are labeled with letters from P to T, each corresponding to a specific electrical activity.
• P-Wave:
  • Represents the electrical excitation (depolarization) of the atria.
  • Leads to the contraction of both atria.
• QRS Complex:
  • Represents the depolarization of the ventricles.
  • Initiates ventricular contraction (systole).
  • Contraction starts shortly after Q in the QRS complex.
• T-Wave:
  • Represents the repolarization of the ventricles.
  • Marks the return of ventricles from an excited to a normal state.
  • The end of T-wave signifies the end of systole.

4. Heart Beat Rate Determination:

• QRS Complex Count:
  • By counting the number of QRS complexes in a given time period, the heart beat rate of an individual can be determined.

5. Clinical Significance:

• Shape Consistency:
  • ECGs from different individuals have a similar shape for a given lead configuration.
  • Deviations from this shape indicate possible abnormalities or diseases.
  • Significant for clinical diagnosis.

Double Circulation: Pulmonary and Systemic Circulation

1. Blood Vessel Structure:

• Arteries and Veins:
  • Consist of three layers:
    • Inner lining (tunica intima) of squamous endothelium.
    • Middle layer (tunica media) of smooth muscle and elastic fibers.
    • External layer (tunica externa) of fibrous connective tissue with collagen fibers.
  • Veins have a comparatively thinner tunica media.

2. Pulmonary Circulation:

• Route:
  • Blood from the right ventricle enters the pulmonary artery.
  • The pulmonary artery carries deoxygenated blood to the lungs.
  • Oxygenated blood returns to the left atrium via the pulmonary veins.
• Function:
  • Oxygenation of blood in the lungs.

3. Systemic Circulation:

• Route:
  • Oxygenated blood from the left ventricle is pumped into the aorta.
  • A network of arteries, arterioles, and capillaries carry oxygenated blood to the tissues.
  • Deoxygenated blood is collected by venules, veins, and vena cava, and emptied into the right atrium.
• Function:
  • Provides nutrients, oxygen, and essential substances to tissues.
  • Removes carbon dioxide and other harmful substances for elimination.

4. Hepatic Portal System:

• Special Connection:
  • The vascular connection between the digestive tract and the liver.
  • The hepatic portal vein carries blood from the intestine to the liver before reaching systemic circulation.

5. Coronary System:

• Exclusive Circulation:
  • Dedicated blood vessel system for circulation to and from the cardiac musculature.
  • Ensures the heart’s own nutrient and oxygen supply.

Regulation of Cardiac Activity

1. Intrinsic Regulation:

• Myogenic Control:
  • Heart activities are intrinsically regulated by specialized muscles (nodal tissue).
  • The heart is termed myogenic due to its ability for auto-regulation.

2. Extrinsic Regulation:

• Autonomic Nervous System (ANS):
  • Neural signals from a special neural center in the medulla oblongata.
  • Sympathetic Nerves:
    • Increase heart rate.
    • Enhance the strength of ventricular contraction.
    • Augment cardiac output.
  • Parasympathetic Nerves:
    • Decrease heart rate.
    • Reduce the speed of action potential conduction.
    • Decrease cardiac output.

3. Hormonal Regulation:

• Adrenal Medullary Hormones:
  • Increase cardiac output.
  • Influence heart rate and contraction strength.

4. Overall Impact:

• Synergistic Effect:
  • Coordination between intrinsic and extrinsic factors.
  • Ensures effective regulation of cardiac activity.
  • Maintains homeostasis in response to varying physiological demands.

Disorders of the Circulatory System

1. High Blood Pressure (Hypertension):

• Definition:
  • Blood pressure higher than normal (120/80 mm Hg).
• Measurements:
  • Systolic Pressure: 120 mm Hg (pumping pressure).
  • Diastolic Pressure: 80 mm Hg (resting pressure).
• Diagnostic Criteria:
  • Repeated readings of 140/90 mm Hg or higher.
• Consequences:
  • Increased risk of heart diseases.
  • Affects vital organs like the brain and kidneys.

2. Coronary Artery Disease (CAD):

• Atherosclerosis:
  • Affects vessels supplying blood to the heart muscle.
  • Caused by deposits of calcium, fat, cholesterol, and fibrous tissues.
  • Narrows the lumen of arteries.

3. Angina:

• Definition:
  • Also known as ‘angina pectoris.’
  • Presents as acute chest pain when insufficient oxygen reaches the heart muscle.
• Prevalence:
  • Common in middle-aged and elderly individuals.
• Causes:
  • Conditions affecting blood flow.

4. Heart Failure:

• Definition:
  • The inability of the heart to pump blood effectively for the body’s needs.
  • Also termed congestive heart failure.
• Symptoms:
  • Congestion of the lungs.
• Distinguishing Factors:
  • Not synonymous with cardiac arrest or heart attack.