D’YOUVILLE COLLEGE
BIOLOGY 108/508 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 10
RESPIRATORY SYSTEM II
GAS EXCHANGE
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
- PO2air = 150 mm. Hg (air contains approx. 20% oxygen)
- PO2alveolar = 104 mm. Hg (outside air diluted with residual volume of
lungs; PO2venous = 40 mm. Hg; thus, oxygen concentration gradient favoring uptake
by blood (fig. 22 - 17)
- PCO2air is negligible
- PCO2alveolar = 40 mm. Hg (residual volume of lungs contains relatively
high carbon dioxide concentration)
- PCO2venous = 45 mm. Hg (blood returning from tissues); thus, carbon
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) + O2 <---> HbO2 (oxyhemoglobin) + H+
- hydrogen ion is buffered by bicarbonate ion in red cell cytoplasm:
H+ + HCO3- (bicarbonate ion) <--------------------------> H2CO3 (carbonic acid)
H2CO3 (carbonic acid)<-----------**-----------> CO2 (carbon dioxide) + H2O
- ** = enzyme carbonic anhydrase required for reaction
• 70% of carbon dioxide is transported in blood as bicarbonate ion; 30%:
carbaminohemoglobin & dissolved CO2
- uptake of oxygen provides impetus (H+) for carbon dioxide release (in
lungs where PO2 is high and PCO2 is low - fig. 22 - 22)
- process reverses in tissues (where PO2 is low and PCO2 is high)
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 PO2 values (fig. 22 - 20)
- arterial portion: (high PO2 as in lungs) hemoglobin retains oxygen
- venous portion: (lower PO2 as in tissues) hemoglobin displays sensitive
response to dropping PO2 by releasing oxygen
• Bohr effect: high/low PCO2 or low/high pH cause shift in hemoglobin’s
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 O2
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 (PCO2), hydrogen ions (pH) and oxygen (PO2) of blood and/or
cerebrospinal fluid; alterations in resp. rate or depth usually are mediated by PCO2
and pH in negative feedback loop (fig. 22 - 25)