By Bryan Cunningham,2014-12-03 16:55
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    LECTURE # 10



5. Alveolar Diffusion:

     • pulmonary ventilation objective: maintain steady-state gradients favoring diffusion of oxygen into blood and diffusion of carbon dioxide out of blood

     - atmospheric air pressure = 760 mm. Hg

     - PO = 150 mm. Hg (air contains approx. 20% oxygen) 2air

     - PO = 104 mm. Hg (outside air diluted with residual volume of 2alveolar

    lungs; PO = 40 mm. Hg; thus, oxygen concentration gradient favoring uptake 2venous

    by blood (fig. 22 - 17)

     - PCO is negligible 2air

     - PCO = 40 mm. Hg (residual volume of lungs contains relatively 2alveolar

    high carbon dioxide concentration)

     - PCO = 45 mm. Hg (blood returning from tissues); thus, carbon 2venous

    dioxide concentration gradient favoring carbon dioxide release by blood (fig. 22 - 17)

     - pressure gradient regulated (ventilation/perfusion coupling - fig. 22 - 19)

6. Blood Gas Transport:

     • carried out by red blood cells (erythrocytes) containing hemoglobin

     - hemoglobin contains 4 chains of amino acids (2 alpha and 2 beta

    polypeptides); each polypeptide chain has a non-amino acid grouping, heme

    (contains iron) (fig. 17 - 4)

     - iron of the heme loosely binds molecular oxygen in an easily reversible

    reaction called hemoglobin oxygenation

    + HHb (reduced hemoglobin) + O <---> HbO (oxyhemoglobin) + H 22

     - hydrogen ion is buffered by bicarbonate ion in red cell cytoplasm:

    +- H + HCO (bicarbonate ion) <--------------------------> HCO (carbonic acid) 323

     HCO (carbonic acid)<-----------**-----------> CO (carbon dioxide) + HO 2322

     - ** = enzyme carbonic anhydrase required for reaction

     70% of carbon dioxide is transported in blood as bicarbonate ion; 30%:

    carbaminohemoglobin & dissolved CO 2

    + - uptake of oxygen provides impetus (H) for carbon dioxide release (in lungs where PO is high and PCO is low - fig. 22 - 22) 22

     - process reverses in tissues (where PO is low and PCO is high) 22

Bio 108/508 lec. 10 - p. 2

     chloride shift: in lungs, bicarbonate, used up to buffer acidity from hemoglobin oxygenation, is replaced by uptake from plasma; acquired negative

    -charge is balanced by chloride (Cl) movement from red cell to plasma; reverse occurs in tissues

Bio 108/508 lec. 10 - p. 3

7. Oxyhemoglobin Dissociation Curve:

     • records hemoglobin’s binding of oxygen at different POvalues (fig. 22 - 20) 2

     - arterial portion: (high PO as in lungs) hemoglobin retains oxygen 2

     - venous portion: (lower PO as in tissues) hemoglobin displays sensitive 2

    response to dropping PO by releasing oxygen 2

     Bohr effect: high/low PCO or low/high pH cause shift in hemoglobin’s 2

    response to oxygen levels (fine tuning Hb binding behavior - fig. 22 - 21)

     - high/low temperatures, other conditions have similar effect

     - pharmacological or pathological effects of carbon monoxide, oxidation, etc. compromise hemoglobin ability to bind O 2

8. Regulation of Ventilation:

     a. Respiratory Centers: pontine center (pons) modifies activities of centers in

    medulla oblongata: dorsal respiratory group (DRG) receives sensory input & higher brain input and signals activity of ventral respiratory group (VRG) that sends output to breathing muscles (mostly diaphragm via phrenic nerve - fig. 22 - 23)

     - sensory inputs modify outputs of respiratory centers:

     b. Stretch Receptors: signals cause inhibition of inspiration when they are stretched by inflation (Hering-Breuer reflex) - protection against overinflation

     c. Chemoreceptors: found in carotid and aortic bodies of circulation and

    chemosensitive areas of medulla (figs. 22 - 24 & 22 - 26); respond to levels of carbon dioxide (PCO), hydrogen ions (pH) and oxygen (PO) of blood and/or 22

    cerebrospinal fluid; alterations in resp. rate or depth usually are mediated by PCO2

    and pH in negative feedback loop (fig. 22 - 25)

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