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22 . The Respiratory System: Part B. Breathing or pulmonary ventilation. A mechanical process that depends on volume changes in the thoracic cavity Volume changes lead to pressure changes , which lead to the flow of gases to equalize pressure Consists of two phases

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The Respiratory System: Part B

Breathing or pulmonary ventilation

  • A mechanical process that depends on volume changes in the thoracic cavity

  • Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure

  • Consists of two phases

    • Inspiration – air flows into the lungs

    • Expiration – gases exit the lungs

Pressure Relationships in the Thoracic Cavity

  • Respiratory pressure is always described relative to atmospheric pressure

  • Atmospheric pressure (Patm)

    • Pressure exerted by the air surrounding the body

    • Negative respiratory pressure is less than Patm

    • Positive respiratory pressure is greater than Patm

  • Intrapleural pressure (Pip) – pressure within the pleural cavity

    • always less than intrapulmonary pressure and atmospheric pressure

  • Intrapulmonary pressure (Ppul) – pressure within the alveoli

    • eventually equalizes itself with atmospheric pressure

Intrapleural Pressure and Pressure Relationships

  • Negative Pip is caused by opposing forces

    • Two inward forces promote lung collapse

      • Elastic recoil of lungs decreases lung size

      • Surface tension of alveolar fluid reduces alveolar size

    • One outward force tends to enlarge the lungs

      • Elasticity of the chest wall pulls the thorax outward

  • If Pip = Ppul the lungs collapse

  • (Ppul – Pip) = transpulmonary pressure

    • Keeps the airways open

Intrapleural pressure

  • As a result of the relationship between the lungs and the pleurae (lungs pull the visceral pleura in), the intrapleural pressure is below atmospheric pressure – average of -4 mmHg

  • This pressure (or the fluid bond between the pleurae) prevents the collapsing of the lungs due to there elasticity

Pulmonary ventilation – inspiration and expiration

  • Pulmonary ventilation depends on volume changes in the thoracic cavity

    • Volume changes lead to pressure changes

    • Pressure changes lead to flow of gases

  • Boyle’s Law- The volume of a fixed amount of gas is inversely proportional to the total amount of pressure applied.

    • If the pressure doubles, the volume shrinks to half.

  • In the lungs – if lungs volume increase the pressure decreases (intrapulmonary pressure)

Modes of breathing

  • Quiet breathing – eupnea

    • Inhalation is active and exhalation is passive (relaxation of muscles)

  • Forced breathing – hyperpnea

    • Both inhalation and exhalation involve muscle contraction – both active

    • Inhalation involve muscles like the pec. minor, sternocleidomastoid and more

    • Exhalation involves the internal intercostals and abdominal muscles among others

Physical Factors Influencing Ventilation

  • 3 factors influence pulmonary ventilation

    • Airway Resistance

    • Alveolar Surface Tension

    • Lung Compliance

Airway Resistance

  • As airway resistance rises, breathing movements become more difficult

  • Resistance is usually insignificant because of

    • Large airway diameters in the first part of the conducting zone

    • Progressive branching of airways as they get smaller, increasing the total cross-sectional area

  • Severely constricted or obstructed bronchioles:

    • Can prevent life-sustaining ventilation

    • Can occur during acute asthma attacks which stops ventilation

  • Epinephrine release via the sympathetic nervous system dilates bronchioles and reduces air resistance

Alveolar Surface Tension

  • Surface tension – the attraction of liquid molecules to one another at a liquid-gas interface

  • The liquid coating the alveolar surface is always acting to reduce the alveoli to the smallest possible size

  • Surfactant, a detergent-like complex, reduces surface tension and helps keep the alveoli from collapsing

    • Normally, surfactant synthesis starts at about the 25th week of fetal development and production reaches optimal levels at 34th week

    • Premature babies with insufficient surfactant can be treat with aerosol administration with artificial surfactant until lungs mature

Lung Compliance

  • Compliance is the indication of the lungs expandability

    • The ease with which lungs can be expanded

  • Factors that diminish lung compliance

    • Scar tissue or fibrosis that reduces the natural elasticity of the lungs

    • Blockage of the smaller respiratory passages with mucus or fluid

    • Reduced production of surfactant

    • The mobility of the thoracic cage – changes cause to the articulations of the ribs or to the muscles involved.

Respiratory Volumes

  • Used to assess a person’s respiratory status

    • Tidal volume (TV)

    • Inspiratory reserve volume (IRV)

    • Expiratory reserve volume (ERV)

    • Residual volume (RV)

Respiratory Capacities

  • Inspiratory capacity (IC) equals TV plus IRV

    • Maximum amount of air (about 3.5 liters) a person can breath in

  • Functional residual capacity (FRC) equals the ERV plus RV

    • Amount of air remains in the lungs at the end of normal expiration

  • Vital capacity (VC) equals IRV+ERV+TV

    • Maximum amount of air a person can expel from the lungs after filling with inspiratory capacity

  • Total lung capacity (TLC) equals VC+RV

    • Maximum volume to which the lungs can be expanded

Dead Space

  • Some of the inspired air does not contribute to the gas exchange in the alveoli

    • Anatomical dead space – volume of the conducting respiratory passages (150 ml)

    • Alveolar dead space – alveoli that cease to act in gas exchange due to collapse or obstruction

    • Total dead space – sum of alveolar and anatomical dead spaces

  • On expiration, the air in the anatomical dead space is expired first

Pulmonary Function Tests

  • Spirometer – an instrument used to evaluate respiratory function

  • Spirometry can distinguish between:

  • Obstructive pulmonary disease – increased airway resistance by narrowing or blocking airways (ex. Asthma)

  • Restrictive disorders – reduction of pulmonary compliance thus limiting inflation of lungs.

    • Caused by any disease that produces pulmonary fibrosis

Nonrespiratory Air Movements

  • Most result from reflex action

  • Examples include: coughing, sneezing, crying, laughing, hiccupping, and yawning

Gas exchange and transport

Respiratory physiology is a series of integrated processes

  • External respiration

    • Exchange of gases between interstitial fluid and the external environment

  • Internal respiration

    • Exchange of gases between interstitial fluid and cells

  • Transport of oxygen and carbon dioxide

  • To understand the above processes, first consider

    • Physical properties of gases

    • Composition of alveolar gas

Basic properties of gases

  • Dalton’s Law of Partial Pressures

    • Total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture (as if no other gases were present)

    • The separate contribution of each gas in a mixture is called partial pressure (symbolized with P)

      • The partial pressure of each gas is directly proportional to its percentage in the mixture

Composition of air in alveoli

  • The composition of air we breath is not the composition in the alveoli:

    • The air is humidified by the contact with the mucus membrane – so PH2O is >10 times higher than the inhaled air

    • Freshly inspired air is mixed with residual air left from previous breathing cycle

      • That causes the oxygen to be diluted and CO2 to be higher

  • Alveolar gas exchanges O2 and CO2 with blood

  • As a result, PO2 of alveolar air is about 65% of that of the inhaled air and PCO2 is >130 higher

  • Alveolar gas exchange Diffusion between liquid and gases (Henry’s law)

    • When a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure

    • The greater the concentration of a particular gas, the more and the faster that it will go into a solution

    • The amount of gas that will dissolve in a liquid also depends upon its solubility:

      • Carbon dioxide is the most soluble

      • Oxygen is 1/20th as soluble as carbon dioxide

      • Nitrogen is practically insoluble in plasma

    External Respiration: Pulmonary Gas Exchange

    • Factors influencing the movement of oxygen and carbon dioxide across the respiratory membrane (what is the respiratory membrane?)

      • Partial pressure gradients and gas solubility

      • Matching of alveolar ventilation and pulmonary blood perfusion

      • Structural characteristics of the respiratory membrane

    Partial Pressure Gradients and Gas Solubility

    • The partial pressure oxygen (PO2) of venous blood is 40 mm Hg; the partial pressure in the alveoli is 104 mm Hg

      • This steep gradient allows oxygen partial pressures to rapidly reach equilibrium (in 0.25 seconds)

      • this is one third of the time a RBC is in the pulmonary capillary (0.75 seconds)

    • Although carbon dioxide has a lower partial pressure gradient (45 mm Hg in the blood and 40 mm Hg in the alveoli; a gradient of 5 mm Hg):

      • It diffuses in equal amounts with oxygen because it is 20 times more soluble in plasma than oxygen

    Surface Area and Thickness of the Respiratory Membrane

    • The amount of gas that moves across a tissue is

      • proportional to the area of the sheet

      • inversely proportional to its thickness

    • Respiratory membranes:

      • Are only 0.5 to 1 m thick, allowing for efficient gas exchange

      • Have a total surface area of about 60 m2 (40 times that of one’s skin)

    Internal Respiration

    • The factors promoting gas exchange between systemic capillaries and tissue cells are the same as those acting in the lungs

      • The partial pressures and diffusion gradients are reversed

      • PO2 in tissue is always lower than in systemic arterial blood

      • PO2 of venous blood draining tissues is 40 mm Hg and PCO2 is 45 mm Hg

    Oxygen Transport: Role of Hemoglobin

    • Molecular oxygen is carried in the blood:

      • Bound to hemoglobin (Hb) within red blood cells

      • Dissolved in plasma (O2 has low solubility in water and only 1.5% is dissolved in plasma)

    • Each Hb molecule binds four oxygen atoms in a rapid and reversible process

    • The hemoglobin-oxygen combination is called oxyhemoglobin (HbO2)

    • Hemoglobin that has released oxygen is called reduced hemoglobin/deoxyhemoglobin (HHb)

    • Saturated hemoglobin – when all four hemes of the molecule are bound to oxygen

    • Partially saturated hemoglobin – when one to three hemes are bound to oxygen

    The Oxygen-Hemoglobin Saturation Curve

    • PO2 of 40 mm Hg – (average in the tissues) Hb is 75% saturated

    • only 25% of the O2 is unload from Hb in resting conditions

    • PO2 of 60-70 mm Hg – Hb is 90% saturated

    • PO2 of 20 mm Hg – Hb is only 30% saturated

    • Ex. – active muscle; relatively high percentage of O2 released with small decrease in Po2

    Factors That Increase Release of Oxygen by Hemoglobin

    • As cells metabolize glucose, carbon dioxide is released into the blood causing:

      • Increases in PCO2 and H+ concentration in capillary blood

      • Declining pH (acidosis), which weakens the hemoglobin-oxygen bond (Bohr effect)

    • Metabolizing cells have heat as a byproduct and the rise in temperature increases BPG synthesis

    • All these factors ensure oxygen unloading in the vicinity of working tissue cells

    Carbon Dioxide Transport

    • Carbon dioxide is transported in the blood in three forms

      • Dissolved in plasma – 7 to 10%

      • Chemically bound to hemoglobin – 20% is carried in RBCs as carbaminohemoglobin

      • Bicarbonate ion in plasma – 70% is transported as bicarbonate (HCO3–)

    Carbon Dioxide Transport

    • In areas with high PCO2, carbon dioxide leaves the cell, diffuses through the interstitial fluid and enters a capillary.

    • Most of it enters an erythrocyte that contains an enzyme, carbonic anhydrase which catalyses the following reaction:

      • CO2  + H20  -----> H2CO3 -----> H+  + HCO3-- .

    • The bicarbonate ion leaves the red blood cell (against concentration gradient) and travels to the lungs in the plasma of the blood.

    • In exchange, Cl- moves from plasma into RBCs to maintain the electrical balance between plasma and RBC (chloride shift)

    • It often combines with Na+ present in the plasma to form sodium bicarbonate which plays a role in maintaining the homeostasis of blood pH.

    Haldane Effect

    • The amount of CO2 that can be transported in the blood is influenced by Hb saturation with O2.

    • The lower the amount of Hb-O2 the higher the CO2 carrying capacity (Haldane effect):

      • Deoxyhemoglobin has higher affinity to CO2

      • Deoxyhemoglobin buffers more H+ thus promoting conversion of CO2 to HCO3-

    • At the tissues, as more carbon dioxide enters the blood:

      • More oxygen dissociates from hemoglobin (acidosis - Bohr effect)

      • More carbon dioxide combines with hemoglobin, and more bicarbonate ions are formed

    • This situation is reversed in pulmonary circulation

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