Respiratory physiology


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Respiratory physiology

  1. 1. 1Kalsoom Muhammad SaleemRespiratory PhysiologyI. Answer to the following questions.Q1. Explain briefly the mechanics of ventilation.Ans. Ventilation:Air is alternatively moved into and out of lungs so that air can be exchanged between theatmosphere (external environment) and air sacs (alveoli) of lungs. This exchange is accomplished bymechanical act of Breathing or Ventilation. This is achieved by body’s metabolic need for oxygenuptake and carbondioxide removal.1Explanation:Ventilation involves lungs hence also known as Pulmonary Ventilation. It includes twoprocesses which function consecutively referred as inhalation and exhalation.Mechanics of Pulmonary Ventilation:The lungs can be expanded and contracted in two ways,1. By downward and upward movement of diaphragm to lengthen or shorten the chestcavity and2. By elevation and depression of ribs to increase and decrease the anteroposteriordiameter of chest cavity.Inhalation (inspiration):Stages involved during inhalation are:1. External intercostal muscles contract2. Internal intercostal muscles relax3. Rib cage moves upward and forward4. Diaphragm contracts and flattens5. Intrapulmonary pressure decreases6. Air pushes in1. External intercostal muscles contract:The external intercostals muscles are responsible for ~25% of air entrance during normal quitebreathing. The external intercostals assist in deep inspiration by increasing the anterioposteriordiameter of the chest.2They do it so by contracting, as they contract they elevate the ribs andsternum which in turn increases the front-to-back dimension of thoracic cavity and air moves in.1
  2. 2. 2Kalsoom Muhammad SaleemMuscles other than external intercostals do cause elevation of ribcage such as sternocledomastoid,anterior serrate and scalene.22. Internal intercostals muscles relax:The relaxed form of internal intercostal muscles does not have any effect on inspiration asthey don’t change their posture to cause inspiration.13. Ribcage moves upward and forward:Aided by movements of ribs and diaphragm the ribs are moved upward as well as forward.24. Diaphragm contracts and flattens:Diaphragm is an important structure for ventilation. During normal quite breathing ~75%of air enters lungs by movement of diaphragm1. It contracts during inhalation and enlarges thethoracic cavity and creating suction that draws air into lungs.25. Intra-pulmonary pressure decreases:The pressure inside lungs is decreased by two means:i. Contraction of external intercostals muscles causes expansion of lungs hence decreasingthe pulmonary pressureii. Contraction of diaphragm also decreases the pulmonary pressure by expansion of lungs,therefore, air moves in.36. Air moves in:All theses stages of inhalation aid the passage of air form atmosphere into the lungs downthe concentration gradient. Pressure of air in atmosphere is greater as compared to pressureinside the lungs, therefore, air moves from higher concentration gradient to lower concentrationgradient.3Exhalation (expiration):Stages involved during exhalation are:1. External intercostal muscles relax2. Internal intercostal muscles contract3. Rib cage moves downward and backward4. Diaphragm relaxes5. Intrapulmonary pressure increases6. Air moves out1. External intercostals muscles relax:As these muscles contract, they return the diaphragm and ribs to resting position.2
  3. 3. 3Kalsoom Muhammad Saleem2. Internal intercostals muscles contract:These muscles contract and assist in expiration by pulling the ribcage down4. Depression ofribs decreases the transverse dimension of thoracic cavity.53. Ribcage moves downward and backward:Aided by the movement of intercostals muscles and diaphragm, the ribcage movesdownward and backward that restores the thoracic cavity to preinspiratory volume.4. Diaphragm relaxes:During expiration it simply relaxes that brings the elastic recoil of lungs, chest wall andabdominal structures that compresses the lungs and expels the air.25. Intrapulmonary pressure increases:Intrapulmonary pressure is decreased by two meansi. The contraction of internal intercostals muscles causes compression of lungs that decreasesthe volume of lungs. In turn, the pressure within the lungs increases relative to atmosphericpressure and, hence, air is expelled out.ii. The relaxation of diaphragm further causes compression of lungs that increases the pressurewithin the lungs and air is expelled out.6. Air moves out:All theses stages of exhalation aid the passage of air form lungs into the atmosphere downthe concentration gradient. Pressure of air in lungs is greater as compared to pressure ofatmosphere, therefore, air moves from higher concentration gradient to lower concentrationgradient i.e. out in atmosphere.3Q2. Write the composition of O2 and CO2 in atmosphere, alveoli and blood.Ans. Table 1.11, 2Composition Atmospheric Air Alveolar Air BloodO2 21% 13.6% Physicallydissolved1.5%(1)Bound toHb98.5%(1)CO2 0.3% 5.3% Physicallydissolved10%(1)Bound toHb30% (1)Asbicarbonate60% (1)There are nearly four reasons four changed concentration of O2 and CO2 in atmosphere, alveoli andblood. First, the alveolar air is only partially replaced by atmospheric air in each breath. Secondly,
  4. 4. 4Kalsoom Muhammad Saleemfrom alveoli oxygen is constantly being absorbed into blood. Thirdly, carbondioxide diffuses fromblood into alveoli. And fourth, before dry air reaching the alveoli it is being humidified.2Q3. Draw oxygen dissociation curve and explain Bohr’s curve.Ans.Bohr’s curve:The influence of CO2 and acid on release of O2 is known as Bohr’s effect. (1). Two types ofshifts are observed in Bohr’s effect i) Right shift and ii) Left shift.Right shift:As blood passes through the tissues, CO2 diffuses from tissue cells into blood henceincreasing partial pressure of CO2. More the concentration of CO2 in blood, more formation ofH2CO3 and hydrogen ions. Thos effect shifts the oxygen-hemoglobin dissociation curve downwardand right. This means now hemoglobin has less affinity for O2 , therefore O2 is forced away fromhemoglobin as a result increased amount of O2 is delivered to tissues.Left shift:The effect occurring antagonistic to right shift causes left shift. CO2 diffuses from blood intothe alveoli in lungs. This reduces the partial pressure of CO2 and decreases the hydrogenconcentration therefore shifting the curve to left and upward. This means now hemoglobin hasgreater affinity for O2, therefore more oxygen can bind to hemoglobin which allows greater O2transport to tissues.2Q4. Write short note on O2 and CO2 transport.Ans. Transport of O2:Oxygen is transported mostly i) in physically dissolved form and ii) bound to hemoglobin.
  5. 5. 5Kalsoom Muhammad Saleemi. Physically dissolved form:Very little O2 physically dissolves in plasma water because o2 is poorly soluble in body fluids.The amount dissolved is directly proportional to the PO2 of blood. The higher the PO2, the more O2dissolved. At normal arterial PO2 of 100 mm Hg, only 3 ml of O2 can dissolve in 1 liter of blood. Thus,only 15 ml of O2/min can dissolve in the normal pulmonary blood flow of 5 liters. Physically dissolvedform of O2 contributes to only 1.5% of O2 transportation.1ii. O2 bound to hemoglobin:The normal blood contains 15 grams of hemoglobin in each 100 milliliters of blood and eachgram of hemoglobin can combine with 1.34 milliliters of O2.Hb + O2 → HbO2Therefore, on average, 15 grams of hemoglobin can combine with 20.1 milliliters of O2. Onpassing through the tissue capillaries, this amount is reduced to 14.4 milliliters. Thus, under normalconditions about 5 milliliters of O2 are transported from the lungs to the tissues by each 100milliliters of blood.2About 98.5% of O2 is transported bound to hemoglobin.1Transport of CO2:CO2 is transported mainly in three form i) Physically dissolved, ii) Bound to hemoglobin andiii) As bicarbonate ions.i. Physically dissolved form:Only about 3 milliliters of CO2 is transported in dissolved form by each 100 milliliters ofblood flow.2This is about 10% of all the CO2 transported to venous blood.ii. Bound to hemoglobin:Another 30% of CO2 combines with hemoglobin to form carbaminohemoglobin.Hb + CO2 → HbCO2This is the amount of CO2 that can be carried form the peripheral tissues to the lungs in formof HbCO2 i.e. 1.5milliliters of CO2 in each 100 milliliters of blood.2iii. As bicarbonate ions:By far the most important means of CO2 transport is as bicarbonate ions.1The dissolvedform of CO2 in the blood reacts with water to form carbonic acid. The carbonic acid formed in thered blood cells dissociates into hydrogen and bicarbonate ions.2CO2 + H2O → H2CO3 → H++ HCO3-In turn, many of bicarbonate ions diffuse from the red blood cells into the plasma. Thisaccounts to the 70% of CO2 transported into the lungs from the tissues.1Q5. Explain briefly the factors affecting diffusion of respiratory gases.Ans. Factors that affect the rate of transportation of gases through respiratory membranes are
  6. 6. 6Kalsoom Muhammad Saleemi. The thickness of membrane ii. The surface area of membraneiii. The diffusion coefficient of gases in iv. The partial pressure difference of gasessubstance of membranei. The thickness of membrane:Rate of diffusion is inversely proportional to respiratory membrane i.e. rate of transferdecreases as thickness increases. Thickness usually remains normal but certain pathological factorsdo increase the thickness of membrane more than two –three times normal which interferesignificantly with normal respiratory exchange of gases (table 1.2).ii. The surface area of membrane:Rate of transfer increase as surface area increases. Increasing of surface area is observedduring exercise, as more pulmonary capillaries open up when the cardiac output increases whichexpands alveoli to increase breathing rate.2When total surface area is decreased to about one-thirdto one-fourth normal, gas exchange through the membrane is decreased (table 1.2).iii. The diffusion coefficient:Rate of transfer increases as diffusion coefficient increases.1Diffusion coefficient is directlyproportional to gas’s solubility and inversely proportional to square root of gas’s molecular weight.Therefore, CO2 diffuses 20 times rapidly as O2 and O2 diffuses twice as rapidly as N2 (table 1.2).2iv. The partial pressure difference of gases:Pressure difference across the respiratory membrane is the difference between the partialpressure of gas in alveoli and blood. When the pressure of gas in alveoli is greater than pressure ofgas in blood, the gas moves down the gradient of higher pressure to lower pressure i.e. from alveoliinto blood as in case of O2. The partial pressure of gas is greater in blood than of gas in alveoli, thegas moves from blood into the alveoli as in case of CO2.2Table 1.21Factors Affect on rate of gas transport CommentsThickness of barrier separatingthe air and blood across thealveolar membraneRate of transfer as thickness Thickness normally remainsconstant.Thickness with pathologicalconditions such as pulmonaryedema (fluid accumulation ininterstitial spaces), pulmonaryfibrosis (lungs tissues replacesby scar-forming tissues) andpneumonia (fluid and bloodaccumulation in alveoli)Surface area of alveolarmembraneRate of transfer as surface area Surface area remains normalunder resting conditions.It during exercise leading rapiddiffusion.
  7. 7. 7Kalsoom Muhammad SaleemIt with pathological conditionssuch as emphysema(breakdownof alveolar walls) and lungcollapse.Diffusion coefficient Rate of transfer as diffusioncoefficientDiffusion constant for CO2 is 20times that of O2 offsettingsmaller partial pressuregradient for CO2; thereforeequal amount of CO2 and O2 aretransported.Partial pressure of gradients ofO2 and CO2Rate of transfer as partialpressure gradientMajor determinant of rate oftransfer.References1. Sherwood, Lauralee. (2010): The Respiratory System, Human Physiology from Cells toSystem, 7thed. Pg 461-497.2. Guyton, Arthur.C, Hall, John.E. (2011): Respiration, Textbook of Medical Physiology, 12thed. Pg 465-518.3. BarrettKE, Barman SM, Boitano S, Brroks H: Pulmonary Function, Review of MedicalPhysiology, 23rded.4. Philip Tate: Internal Intercostal Muscles, Seeley’s Principles of Anatomy and Physiology.5. Saladin, Kenneth: The Unity of form and function of Intercostal Muscles, Anatomy andPhysiology, 5th ed.