Body Fluids and Circulation - Detailed Study Notes

Body Fluids and Circulation

All living cells require nutrients, O2O_2, and essential substances. Waste removal is crucial for healthy tissue function. Efficient mechanisms are needed for substance transport to and from cells. Different animals have evolved various transport methods. Sponges and coelenterates circulate water through body cavities. Complex organisms use special body fluids like blood and lymph.

15.1 Blood

Blood is a specialized connective tissue composed of:

  • Fluid matrix: Plasma
  • Formed elements: Cells and cell fragments
15.1.1 Plasma
  • Straw-colored, viscous fluid.
  • Constitutes about 55% of blood volume.
  • Composition:
    • 90-92% water
    • 6-8% proteins
      • Fibrinogen: Involved in blood clotting or coagulation.
      • Globulins: Primarily involved in defense mechanisms.
      • Albumins: Help maintain osmotic balance.
    • Small amounts of minerals:
      • Na+Na^+
      • Ca++Ca^{++}
      • Mg++Mg^{++}
      • HCO3−HCO_3^−
      • Cl−Cl^−
    • Other substances in transit:
      • Glucose
      • Amino acids
      • Lipids
    • Inactive clotting factors
  • Serum: Plasma without clotting factors.
15.1.2 Formed Elements
  • Constitute about 45% of blood volume.
  • Types:
    • Erythrocytes (Red Blood Cells - RBCs):
      • Most abundant blood cells.
      • Healthy adult man: 5 to 5.5 million RBCs per mm3mm^3 of blood.
      • Formed in the red bone marrow in adults.
      • Devoid of nucleus in most mammals; biconcave shape.
      • Contain hemoglobin: An iron-containing complex protein that transports respiratory gases.
      • Healthy individual: 12-16 gms of hemoglobin per 100 ml of blood.
      • Life span: Average of 120 days, then destroyed in the spleen (graveyard of RBCs).
    • Leucocytes (White Blood Cells - WBCs):
      • Colorless due to lack of hemoglobin.
      • Nucleated, fewer in number: 6000-8000 per mm3mm^3 of blood.
      • Generally short-lived.
      • Two main categories:
        • Granulocytes:
          • Neutrophils: Most abundant (60-65% of total WBCs).
          • Eosinophils: (2-3%) Resist infections, associated with allergic reactions.
          • Basophils: Least common (0.5-1%); secrete histamine, serotonin, heparin (involved in inflammatory reactions).
        • Agranulocytes:
          • Lymphocytes: (20-25%) 'B' and 'T' forms; responsible for immune responses.
          • Monocytes: (6-8%) Phagocytic; destroy foreign organisms.
      • Neutrophils and monocytes are phagocytic.
    • Platelets (Thrombocytes):
      • Cell fragments produced from megakaryocytes in bone marrow.
      • Normal count: 150,000 - 350,000 per mm3mm^3.
      • Release substances involved in blood coagulation or clotting.
      • Reduction in number leads to clotting disorders and excessive blood loss.
15.1.3 Blood Groups
  • Human blood varies in certain aspects.
  • Two main groupings:
    • ABO
    • Rh
15.1.3.1 ABO Grouping
  • Based on presence or absence of two surface antigens (A, B) on RBCs.
  • Plasma contains natural antibodies (anti-A, anti-B).
  • Four blood groups: A, B, AB, O.
  • Blood transfusion requires careful matching.
    • Group O: Universal donors (can donate to all blood types).
    • Group AB: Universal recipients (can receive blood from all blood types).
15.1.3.2 Rh Grouping
  • Rh antigen (similar to that in Rhesus monkeys) present on RBCs of ~80% of humans.
    • Rh positive (Rh+veRh^{+ve}): Have the antigen.
    • Rh negative (Rh−veRh^{-ve}): Lack the antigen.
  • Rh−veRh^{-ve} individuals exposed to Rh+veRh^{+ve} blood form antibodies against Rh antigens.
  • Rh group should be matched before transfusions.
  • Rh incompatibility in pregnancy:
    • If a Rh−veRh^{-ve} mother carries a Rh+veRh^{+ve} fetus, Rh antigens from the fetus may enter the mother's blood during delivery.
    • The mother produces anti-Rh antibodies.
    • In subsequent pregnancies, these antibodies can cross the placenta and destroy fetal RBCs (erythroblastosis fetalis).
    • Can be prevented by administering anti-Rh antibodies to the mother immediately after the first delivery.
15.1.4 Coagulation of Blood
  • Blood clots in response to injury or trauma to prevent excessive blood loss.
  • Formation of a clot (coagulum):
    • Network of threads called fibrins.
    • Trapped dead and damaged formed elements.
  • Process:
    • Fibrins are formed from inactive fibrinogens by the enzyme thrombin.
    • Thrombin is formed from prothrombin.
    • Thrombokinase (enzyme complex) is required for the above reaction.
    • This complex is formed by a series of linked enzymatic reactions (cascade process) involving several factors in an inactive state.
    • Injury stimulates platelets to release factors that activate coagulation.
    • Tissues at the injury site also release factors to initiate coagulation.
    • Calcium ions play a crucial role in clotting.

15.2 Lymph (Tissue Fluid)

  • As blood flows through capillaries, some water and small water-soluble substances move into spaces between tissue cells.
  • Larger proteins and most formed elements remain in the blood vessels.
  • The fluid released is called interstitial fluid or tissue fluid.
  • It has a similar mineral distribution to plasma.
  • Nutrient and gas exchange between blood and cells occurs through this fluid.
  • The lymphatic system collects this fluid and drains it back into major veins.
  • Lymph: The fluid within the lymphatic system.
    • Colorless fluid containing specialized lymphocytes for immune responses.
    • Important carrier for nutrients and hormones.
    • Fats are absorbed through lymph in lacteals present in intestinal villi.

15.3 Circulatory Pathways

  • Two types:
    • Open circulatory system:
      • Present in arthropods and mollusks.
      • Blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.
    • Closed circulatory system:
      • Present in annelids and chordates.
      • Blood is always circulated through a closed network of blood vessels.
      • More advantageous as fluid flow can be precisely regulated.
  • Vertebrates possess a muscular chambered heart.
    • Fishes: 2-chambered heart (atrium and ventricle); single circulation.
    • Amphibians and reptiles (except crocodiles): 3-chambered heart (two atria and single ventricle); incomplete double circulation, mixing of oxygenated and deoxygenated blood occurs in the single ventricle.
    • Crocodiles, birds, and mammals: 4-chambered heart (two atria and two ventricles); double circulation.
  • Single circulation (e.g., fish):
    • Heart pumps deoxygenated blood to gills for oxygenation; oxygenated blood supplied to body parts; deoxygenated blood returns to heart.
  • Incomplete double circulation (e.g., amphibians and reptiles):
    • Left atrium receives oxygenated blood from gills/lungs/skin; right atrium receives deoxygenated blood from body parts; mixing occurs in the single ventricle.
  • Double circulation (birds and mammals):
    • Oxygenated and deoxygenated blood are kept separate.
    • Two separate circulatory pathways.
15.3.1 Human Circulatory System

  • Also called the blood vascular system.

  • Consists of:

    • Muscular chambered heart.
    • Network of closed branching blood vessels.
    • Blood (the circulating fluid).

  • Heart:

    • Mesodermally derived organ.
    • Located in the thoracic cavity, between the two lungs, slightly tilted to the left.
    • Size of a clenched fist.
    • Protected by a double-walled membranous bag, the pericardium, enclosing the pericardial fluid.
    • Four chambers:
      • Two relatively small upper chambers: Atria.
      • Two larger lower chambers: Ventricles.
    • Inter-atrial septum: Separates the right and left atria.
    • Inter-ventricular septum: Separates the left and right ventricles.
    • Atrio-ventricular septum: Separates the atria and ventricles on the same side; contains openings.
    • Tricuspid valve: Guards the opening between the right atrium and right ventricle (three muscular flaps or cusps).
    • Bicuspid (mitral) valve: Guards the opening between the left atrium and left ventricle.
    • Semilunar valves: Present at the openings of the right and left ventricles into the pulmonary artery and aorta, respectively.
    • Valves allow blood flow in only one direction: from atria to ventricles, and from ventricles to pulmonary artery/aorta; prevent backward flow.
    • The walls of ventricles are much thicker than that of the atria.
    • Entire heart made of cardiac muscles.

  • Nodal Tissue:

    • Specialized cardiac musculature distributed in the heart.
    • Sino-atrial node (SAN): Located in the right upper corner of the right atrium.
    • Atrio-ventricular node (AVN): Located in the lower left corner of the right atrium, close to the atrio-ventricular septum.
    • Atrio-ventricular bundle (AV bundle): Bundle of nodal fibers that continues from the AVN, passes through the atrio-ventricular septa, emerges on top of the inter-ventricular septum, and immediately divides into a right and left bundle.
    • Purkinje fibers: Minute fibers that arise from these bundles and spread throughout the ventricular musculature.
    • Nodal musculature is autoexcitable (can generate action potentials without external stimuli).
    • SAN generates the maximum number of action potentials (70-75 per minute) and initiates/maintains rhythmic contractile activity; called the pacemaker.
    • Normal Heart Rate: 70-75 times per minute (average 72 beats/min).
15.3.2 Cardiac Cycle
  • Sequence of events in the heart, cyclically repeated.
  • Consists of systole (contraction) and diastole (relaxation) of both atria and ventricles.
  • Steps:
    • Joint Diastole: All four chambers are relaxed. Tricuspid and bicuspid valves are open. Blood flows from pulmonary veins and vena cava into the left and right ventricles, respectively, through the atria. Semilunar valves are closed.
    • Atrial Systole: SAN generates an action potential that stimulates both atria to contract simultaneously. Increases blood flow into ventricles by ~30%.
    • Ventricular Systole: Action potential is conducted to the ventricular side by the AVN and AV bundle, then transmitted through the ventricular musculature. Ventricular muscles contract, atria relax (diastole). Ventricular pressure increases, closing tricuspid and bicuspid valves. Blood attempts to backflow into the atria.
    • Blood Ejection: As ventricular pressure increases further, semilunar valves guarding the pulmonary artery and aorta are forced open. Blood flows into the circulatory pathways.
    • Ventricular Diastole: Ventricles relax. Ventricular pressure falls, causing semilunar valves to close (prevents backflow).
    • Valves Open: As ventricular pressure declines, tricuspid and bicuspid valves are pushed open by the pressure in the atria (due to blood emptying into them by veins). Blood moves freely into the ventricles.
    • Repeat: Ventricles and atria are again in joint diastole. SAN generates a new action potential, and the cycle repeats.
  • Duration: 0.8 seconds per cardiac cycle (at a heart rate of 72 beats/min).
  • Stroke Volume: Each ventricle pumps out approximately 70 mL of blood per cycle.
  • Cardiac Output: Volume of blood pumped out by each ventricle per minute.
    • CardiacOutput=StrokeVolume<br/>≠HeartRateCardiac Output = Stroke Volume <br />\neq Heart Rate
    • Averages 5000 mL or 5 liters in a healthy individual.
    • Body can alter stroke volume and heart rate to change cardiac output (e.g., athletes have higher cardiac output).
  • Heart Sounds: Two prominent sounds are produced during each cycle.
    • First heart sound (lub): Closure of the tricuspid and bicuspid valves.
    • Second heart sound (dub): Closure of the semilunar valves.
    • These sounds have clinical diagnostic significance.
15.3.3 Electrocardiogram (ECG)
  • Graphical representation of the heart's electrical activity during a cardiac cycle.
  • Obtained using an electrocardiograph.
  • Standard ECG: Patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle).
  • Multiple leads can be attached to the chest region for a detailed evaluation.
  • Each peak in the ECG is identified with a letter (P to T).
    • P-wave: Represents the electrical excitation (depolarization) of the atria, leading to atrial contraction.
    • QRS complex: Represents the depolarization of the ventricles, initiating ventricular contraction.
    • T-wave: Represents the return of the ventricles from excited to normal state (repolarization).
  • Heartbeat Rate: Determined by counting the number of QRS complexes in a given time period.
  • Clinical Significance: Deviations from the standard ECG shape indicate possible abnormalities or diseases.

15.4 Double Circulation

  • Blood flows through arteries and veins.
  • Vessel Structure (Arteries and Veins):
    • Tunica intima: Inner lining of squamous endothelium.
    • Tunica media: Middle layer of smooth muscle and elastic fibers (thinner in veins).
    • Tunica externa: External layer of fibrous connective tissue with collagen fibers.
  • Pulmonary Circulation:
    • Right ventricle pumps deoxygenated blood into the pulmonary artery.
    • Pulmonary artery carries blood to the lungs for oxygenation.
    • Pulmonary veins carry oxygenated blood back to the left atrium.
  • Systemic Circulation:
    • Left ventricle pumps oxygenated blood into the aorta.
    • Aorta carries blood to arteries, arterioles, and capillaries, which supply tissues with nutrients and O2O_2.
    • Deoxygenated blood is collected by venules, veins, and the vena cava, and returned to the right atrium.
    • Eliminates CO2CO_2 and harmful substances.
  • Hepatic Portal System:
    • Unique vascular connection between the digestive tract and liver.
    • Hepatic portal vein carries blood from the intestine to the liver before it enters the systemic circulation.
  • Coronary System:
    • Special system of blood vessels for circulation to and from the cardiac musculature.

15.5 Regulation of Cardiac Activity

  • Normal activities of the heart are intrinsically regulated by specialized muscles (nodal tissue).
  • Heart is myogenic.
  • Medulla oblongata can moderate cardiac function through the autonomic nervous system (ANS).
    • Sympathetic nerves (part of ANS) increase heart rate, ventricular contraction strength, and cardiac output.
    • Parasympathetic nerves (another component of ANS) decrease heart rate, conduction speed of action potential, and cardiac output.
  • Adrenal medullary hormones can also increase cardiac output.

15.6 Disorders of the Circulatory System

  • High Blood Pressure (Hypertension):
    • Blood pressure higher than normal (120/80 mm Hg).
    • Systolic pressure: Pumping pressure (120 mm Hg).
    • Diastolic pressure: Resting pressure (80 mm Hg).
    • Hypertension: Repeated blood pressure checks of 140/90 or higher.
    • Leads to heart disease and affects vital organs (brain, kidney).
  • Coronary Artery Disease (CAD):
    • Often referred to as atherosclerosis.
    • Affects vessels supplying blood to the heart muscle.
    • Caused by deposits of calcium, fat, cholesterol, and fibrous tissues, narrowing the artery lumen.
  • Angina (Angina Pectoris):
    • Symptom of acute chest pain when not enough oxygen reaches the heart muscle.
    • Can occur in men and women of any age, but more common in middle-aged and elderly.
    • Occurs due to conditions affecting blood flow.
  • Heart Failure:
    • The heart is not pumping blood effectively enough to meet the body's needs.
    • Sometimes called congestive heart failure because lung congestion is a main symptom.
    • Not the same as cardiac arrest (heart stops beating) or heart attack (heart muscle is suddenly damaged by inadequate blood supply).