Comprehensive Study Notes on Body Fluids and Circulation

General Mechanisms of Substance Transport

  • Necessity of Transport: All living cells require a continuous supply of nutrients, O2O_2, and other essential substances to function. Simultaneously, waste or harmful substances (CO2CO_2, nitrogenous wastes) produced by cellular metabolism must be removed continuously to maintain tissue health.

  • Evolutionary Adaptations:

    • Simple Organisms: Sponges and coelenterates utilize water from their surroundings, circulating it through their body cavities. This allows cells to directly exchange substances with the external environment.

    • Complex Organisms: These organisms utilize specialized internal fluids to transport materials efficiently.

    • Common Fluids: Blood is the primary body fluid used by higher organisms, including humans. Lymph (tissue fluid) is another fluid that assists in the transport of specific substances.

15.1 Blood

  • Definition: Blood is a specialized connective tissue comprised of a fluid matrix called plasma and various formed elements.

15.1.1 Plasma

  • Physical Characteristics: Plasma is a straw-colored, viscous fluid that accounts for nearly 55%55\% of the total blood volume.

  • Composition:

    • Water: Constitutes 90−92%90-92\% of the plasma.

    • Proteins: Contribute 6−8%6-8\% of the plasma. The three major proteins are:

      • Fibrinogen: Essential for blood clotting or coagulation.

      • Globulins: Primarily involved in the body's defense mechanisms (immunity).

      • Albumins: Play a crucial role in maintaining osmotic balance.

    • Minerals: Small amounts of electrolytes such as Na+Na^+, Ca++Ca^{++}, Mg++Mg^{++}, HCO3−HCO_3^-, and Cl−Cl^- are present.

    • Nutrients and Other Substances: Glucose, amino acids, and lipids are present as they are constantly in transit throughout the body.

    • Coagulation Factors: Factors required for blood clotting are present in plasma in an inactive form.

  • Serum: Defined as plasma from which the clotting factors have been removed.

15.1.2 Formed Elements

  • Overview: Erythrocytes, leucocytes, and platelets collectively make up the formed elements, constituting approximately 45%45\% of the blood.

  • Erythrocytes (Red Blood Cells - RBC):

    • Abundance: The most numerous cells in the blood. A healthy adult male averages 5×1065 \times 10^6 to 5.5×1065.5 \times 10^6 RBCs per mm−3mm^{-3} of blood.

    • Formation: Produced in the red bone marrow in adults.

    • Structure: Devoid of a nucleus in most mammals and possess a biconcave shape.

    • Haemoglobin: Contains a red-colored, iron-containing complex protein called haemoglobin. A healthy individual contains 12−16 gms12-16\,gms of haemoglobin in every 100 ml100\,ml of blood. It is vital for transporting respiratory gases (O2O_2 and CO2CO_2).

    • Life Span and Destruction: The average life span is 120 days120\,days. They are destroyed in the spleen, often referred to as the "graveyard of RBCs."

  • Leucocytes (White Blood Cells - WBC):

    • Characteristics: Colourless due to the lack of haemoglobin, nucleated, and fewer in number compared to RBCs, averaging 6000−8000 mm−36000-8000\,mm^{-3} of blood. They are generally short-lived.

    • Categories:

      1. Granulocytes: Includes Neutrophils, Eosinophils, and Basophils.

      2. Agranulocytes: Includes Lymphocytes and Monocytes.

    • Types and Functions:

      • Neutrophils: Most abundant WBCs (60−65%60-65\%). They are phagocytic, destroying foreign organisms.

      • Basophils: Least abundant (0.5−1%0.5-1\%). They secrete histamine, serotonin, and heparin and are involved in inflammatory reactions.

      • Monocytes: Make up 6−8%6-8\% of WBCs. Like neutrophils, they are phagocytic.

      • Eosinophils: Make up 2−3%2-3\% of WBCs. They resist infections and are associated with allergic reactions.

      • Lymphocytes: Make up 20−25%20-25\% of WBCs. They exist in two forms, ‘B’ and ‘T’ lymphocytes, both responsible for the body's immune responses.

  • Platelets (Thrombocytes):

    • Origin: Cell fragments produced from megakaryocytes (specialized cells in the bone marrow).

    • Count: Normal blood contains 150,000−350,000150,000-350,000 platelets per mm−3mm^{-3}.

    • Function: They release substances involved in blood coagulation. A reduction in platelet count can lead to clotting disorders and excessive blood loss.

15.1.3 Blood Groups

  • ABO Grouping: Based on the presence or absence of two surface antigens on RBCs: A and B. Plasma contains two natural antibodies: anti-A and anti-B.

    • Blood Group A: Antigen A on RBCs; anti-B antibodies in plasma. Can receive from House A and O.

    • Blood Group B: Antigen B on RBCs; anti-A antibodies in plasma. Can receive from B and O.

    • Blood Group AB: Both antigens A and B on RBCs; no antibodies in plasma. Universal Recipient (can receive from A, B, AB, O).

    • Blood Group O: No antigens on RBCs; both anti-A and anti-B antibodies in plasma. Universal Donor (can be donated to any group).

  • Blood Transfusion: Careful matching of donor and recipient blood is required to avoid clumping (destruction of RBCs).

  • Rh Grouping: Based on the Rh antigen (similar to the one in Rhesus monkeys).

    • Rh Positive (Rh+veRh+ve): Present in nearly 80%80\% of humans who have the Rh antigen on their RBC surface.

    • Rh Negative (Rh−veRh-ve): Antigen is absent. If an Rh−veRh-ve person is exposed to Rh+veRh+ve blood, they will develop specific antibodies against Rh antigens.

    • Erythroblastosis Foetalis: A condition of Rh incompatibility during pregnancy. If an Rh−veRh-ve mother carries an Rh+veRh+ve foetus, maternal blood may be exposed to foetal blood during the first delivery. In subsequent pregnancies, maternal Rh antibodies can cross the placenta and destroy foetal RBCs, leading to severe anaemia, jaundice, or death. This is prevented by administering anti-Rh antibodies to the mother immediately after the first delivery.

15.1.4 Coagulation of Blood

  • Mechanism: Prevents excessive blood loss following injury. It involves a "cascade process" of linked enzymatic reactions.

  • Steps in Clotting:

    1. Fibrin Formation: Inactive fibrinogen in plasma is converted to fibrins by the enzyme thrombin.

    2. Thrombin Formation: Thrombin is formed from inactive prothrombin in plasma.

    3. Enzyme Complex: This conversion requires thrombokinase, an enzyme complex formed by a series of reactions involving various plasma factors.

  • Involvement: An injury stimulates platelets and tissues to release specific factors that initiate this mechanism. Calcium ions (Ca++Ca^{++}) play a critical role in the clotting process.

15.2 Lymph (Tissue Fluid)

  • Formation: As blood passes through tissue capillaries, water and small water-soluble substances filter out into interstitial spaces, leaving behind large proteins and formed elements.

  • Function: It serves as the medium for the exchange of nutrients and gases between blood and cells.

  • Lymphatic System: An elaborate network of vessels that collects interstitial fluid (now called lymph) and drains it back into major veins.

  • Characteristics of Lymph: A colourless fluid containing specialized lymphocytes. It is responsible for immune responses and acts as a carrier for nutrients and hormones. Fats are absorbed via lymph in lacteals within intestinal villi.

15.3 Circulatory Pathways

  • Open Circulatory System: Found in arthropods and molluscs. Blood is pumped into open spaces or body cavities (sinuses).

  • Closed Circulatory System: Found in annelids and chordates. Blood is circulated through a closed network of vessels, allowing for more precise regulation of flow.

  • Vertebrate Heart Evolution:

    • Fishes: 2-chambered (one atrium, one ventricle). Single circulation (heart pumps deoxygenated blood to gills for oxygenation).

    • Amphibians/Reptiles (except crocodiles): 3-chambered (two atria, one ventricle). Incomplete double circulation (oxygenated and deoxygenated blood mix in the single ventricle).

    • Crocodiles, Birds, Mammals: 4-chambered (two atria, two ventricles). Double circulation (oxygenated and deoxygenated blood remain separate).

15.3.1 Human Circulatory System

  • Anatomy of the Heart:

    • Origin and Location: Mesodermally derived; located in the thoracic cavity between the lungs, tilted to the left. Approximately the size of a clenched fist.

    • Protection: Enclosed in a double-walled membranous bag called the pericardium, containing pericardial fluid.

    • Chambers: Two upper small atria and two lower large ventricles.

    • Septa:

      • Inter-atrial septum: Thin muscular wall separating the two atria.

      • Inter-ventricular septum: Thick wall separating the two ventricles.

      • Atrio-ventricular septum: Thick fibrous tissue separating the atrium and ventricle of the same side, containing an opening.

    • Valves:

      • Tricuspid Valve: Guards the opening between the right atrium and right ventricle (three flaps).

      • Bicuspid or Mitral Valve: Guards the opening between the left atrium and left ventricle.

      • Semilunar Valves: Guard the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta.

    • Function of Valves: Ensure unidirectional blood flow (atria $\rightarrow$ ventricles $\rightarrow$ arteries) and prevent backflow.

  • Nodal Tissue (Specialized Musculature):

    • Myogenic Heart: The heart is autoexcitable, meaning it generates action potentials without external stimuli.

    • Sino-atrial Node (SAN): Located in the right upper corner of the right atrium. Acts as the Pacemaker by generating the highest rate of action potentials (70−75 min−170-75\,min^{-1}).

    • Atrio-ventricular Node (AVN): Located in the lower left corner of the right atrium.

    • AV Bundle and Purkinje Fibres: The AV bundle emerges from the AVN, passes through the interventricular septum, and divides into right and left branches that give rise to Purkinje fibres throughout the ventricular walls.

15.3.2 Cardiac Cycle

  • Phases of the Cycle:

    1. Joint Diastole: All four chambers are relaxed. Tricuspid and bicuspid valves are open; blood flows from vena cava and pulmonary veins into ventricles.

    2. Atrial Systole: SAN generates an action potential, causing atria to contract. This increases ventricular filling by approximately 30%30\%.

    3. Ventricular Systole: Action potential reaches ventricles via AVN and Bundle of His. Ventricles contract; atria relax (diastole). Increased pressure closes tricuspid/bicuspid valves (producing the 'lub' sound) and opens semilunar valves.

    4. Ventricular Diastole: Ventricles relax; pressure falls. Semilunar valves close (producing the 'dub' sound) to prevent backflow from arteries. Tricuspid and bicuspid valves open as atrial pressure exceeds ventricular pressure.

  • Duration: With a heart rate of 72 beats/min72\,beats/min, one cardiac cycle lasts 0.8 seconds0.8\,seconds.

  • Stroke Volume: Approximately 70 mL70\,mL of blood pumped by each ventricle per beat.

  • Cardiac Output: The volume of blood pumped by each ventricle per minute.

    • Cardiac Output=Stroke Volume×Heart Rate\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}

    • Average Output=70 ml×72≈5000 mL\text{Average Output} = 70\,ml \times 72 \approx 5000\,mL (5 litres5\,litres). Output can increase, such as in athletes.

15.3.3 Electrocardiogram (ECG)

  • Definition: A graphical representation of the electrical activity of the heart during a cardiac cycle, obtained using an electro-cardiograph.

  • Lead Placement: Standard ECG uses three leads (each wrist and left ankle). Multiple chest leads are used for detailed analysis.

  • Waves:

    • P-wave: Represents atrial depolarisation (excitation), leading to atrial contraction.

    • QRS complex: Represents ventricular depolarisation, initiate ventricular contraction (starts shortly after Q).

    • T-wave: Represents ventricular repolarization (return to normal state). The end of the T-wave marks the end of systole.

  • Clinical Significance: Deviations in the shape or number of QRS complexes indicate heart abnormalities.

15.4 Double Circulation

  • Blood Vessel Structure:

    1. Tunica intima: Inner lining of squamous endothelium.

    2. Tunica media: Middle layer of smooth muscle and elastic fibres (thinner in veins).

    3. Tunica externa: External layer of fibrous connective tissue with collagen.

  • Circulatory Pathways:

    • Pulmonary Circulation: Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary veins $\rightarrow$ Left atrium.

    • Systemic Circulation: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Artery system $\rightarrow$ Tissues $\rightarrow$ Vein system/Vena cava $\rightarrow$ Right atrium.

  • Special Circulations:

    • Hepatic Portal System: A vascular connection where the hepatic portal vein carries blood from the intestine to the liver before reaching systemic circulation.

    • Coronary System: Dedicated vessels for blood supply to and from the cardiac musculature.

15.5 Regulation of Cardiac Activity

  • Intrinsic Regulation: The heart is myogenic (regulated by nodal tissue).

  • Neural Regulation: The medulla oblongata modulates function via the Autonomic Nervous System (ANS):

    • Sympathetic Nerves: Increase heart rate, strength of contraction, and cardiac output.

    • Parasympathetic Nerves: Decrease heart rate, speed of conduction, and cardiac output.

  • Hormonal Regulation: Adrenal medullary hormones (adrenaline/noradrenaline) increase cardiac output.

15.6 Disorders of Circulatory System

  • Hypertension (High Blood Pressure): Blood pressure higher than the normal 120/80 mm Hg120/80\,mm\,Hg. A reading of 140/90 mm Hg140/90\,mm\,Hg or higher indicates hypertension, which can damage the brain, kidneys, and heart.

  • Coronary Artery Disease (CAD): Also called atherosclerosis. Caused by deposits of calcium, fat, cholesterol, and fibrous tissue in the arteries supplying the heart, narrowing the lumen.

  • Angina: Or angina pectoris. Acute chest pain resulting from insufficient oxygen reaching the heart muscle. Common in middle-aged and elderly individuals.

  • Heart Failure: The heart fails to pump blood effectively to meet body needs. Known as congestive heart failure due to lung congestion. It differs from cardiac arrest (heart stops) and heart attack (muscle damage).

Questions & Discussion

  • Q: What are the components of formed elements?

    • A: Erythrocytes (gas transport), Leucocytes (defense/immunity), and Platelets (blood clotting).

  • Q: What is the importance of plasma proteins?

    • A: Fibrinogen for clotting, globulins for defense, and albumins for osmotic balance.

  • Q: Why is blood considered a connective tissue?

    • A: It has a fluid matrix (plasma) and links various parts of the body through circulation, originating from the mesoderm.

  • Q: What is the difference between lymph and blood?

    • A: Blood contains RBCs and large proteins; lymph is colourless, lacks RBCs, and has fewer proteins but is rich in lymphocytes.

  • Q: Explain the significance of double circulation.

    • A: It ensures that oxygenated and deoxygenated blood do not mix, allowing for high efficiency in oxygen delivery to tissues.

  • Q: Why is the heart called myogenic?

    • A: Because the impulse for the heartbeat originates within the specialized nodal musculature (SAN) of the heart itself, not from external nerves.

  • Q: What is the role of the SAN as the pacemaker?

    • A: It generates the maximum number of action potentials per minute (70−75 min−170-75\,min^{-1}), setting the rhythmic pace for heart contraction.

  • Q: What are heart sounds?

    • A: 'Lub' marks the closure of AV valves at the start of ventricular systole; 'Dub' marks the closure of semilunar valves at the start of ventricular diastole.