respiratory

Bohr Effect

Alterations in hemoglobin’s structure Shift to the right in the oxyhemoglobin

dissociation curve Loading of O2 is not affected

– the flat upper portion is not altered Unloading of O2 is enhanced

– along steep lower portion, more O2 is unloaded at a given PO2 with the shift

respiratory

respiratory

Myoglobin and Muscle Oxygen Storage

Skeletal & cardiac muscle contain compound myoglobin.

Each myoglobin contains only one heme in contrast to 4 in hemoglobin (Hb).

Myoglobin binds and retains O2 at low pressures.

Facilitates oxygen transfer to mitochondria at start of exercise and intense exercise when cellular PO2 ↓ greatly.

respiratory

Carbon Dioxide Transportin the Blood

Dissolved in plasma – CO2 is 20 times more soluble than O2

– 7% to 10% of CO2 is dissolved

Combined with amino compounds– hemoglobin is most common– Haldane effect: Hb’s de-oxygenation enables bind CO2

– about 20% of CO2 is carried as carbamino compounds

Bicarbonate – about 70% carried as bicarbonate

respiratory

Carbon Dioxide Transportin the Blood

respiratory

Formation of Bicarbonate at Tissue Level

CO2 diffuses into RBC Enzyme, carbonic anhydrase, absent in plasma

but present in RBC drives reaction of CO2 + H2O => H2CO3

H2CO3 dissociates a proton => HCO3- + H+

CO2 + H2O => H2CO3 => HCO3- + H+

HCO3- moves into plasma via HCO3

- / Cl- anion exchanger to prevent electrical imbalance

Hb acts as buffer and accepts the H+

respiratory

Bicarbonate in the Lungs

In lungs, carbon dioxide diffuses from plasma into alveoli; lowers plasma PCO2.

HCO3- + H+ recombine to form carbonic acid.

H2CO3 dissociates to H2O and CO2, allowing carbon dioxide to exit through the lungs.

CO2 + H2O <= H2CO3 <= HCO3- + H+

respiratory

Ventilatory Regulation

Two factors regulate pulmonary ventilation: Neural input from higher brain centers

provides primary drive to ventilate Gaseous and chemical state of blood: humoral

factors

respiratory

Pulmonary Ventilation Control

Clusters neurons in medulla oblongata referred to as respiratory center.

Inspiratory center activates diaphragm & intercostals.

Expiratory center inhibits inspiratory neurons.

Stretch receptors assist regulation of breathing

Pneumotaxic & apneustic centers contribute (depth).

respiratory

Humoral Factors Chemoreceptors are specialized neurons. Chemoreceptors monitor blood conditions, provide

feedback– Peripheral located in aortic arch and bifurcation of

common carotid respond to CO2, “temperature”-no, H+

– Central located in medulla affected by PCO2 & H+

Specialized receptors in lungs sensitive to stretch and irritants act to provide feedback

Interaction among factors controls ventilation– CO2 production is closely associated with

ventilation rate

respiratory

Receptor Location and Function

Central chemoreceptors located within the medulla– respond to changes in

PCO2 & H+ in cerebral spinal fluid

– ventilation increases with elevations of PCO2 or H+

respiratory

Receptor Location and Function

Peripheral chemoreceptors located in aortic arch and common carotid arteries– respond to changes in

PO2, PCO2 and H+

– at sea level changes in PO2 have little effect on VE

respiratory

Ventilatory Control at Rest

Carbon dioxide pressure in arterial plasma (PaCO2) provides the most important respiratory stimulus at rest.

Urge to breathe after 40 s breath-holding results mainly from increased arterial PCO2.

Hyperventilation decreases Alveolar PCO2 to 15 mm Hg, which decreases PaCO2 below normal, allows longer breath holding.

respiratory

Ventilatory Control in Exercise

Very rapid increase at start of exercise

Chemical stimuli cannot explain initial hyperpnea during exercise.

Nonchemical factors mediate the rapid response– Cortical: motor cortex– Peripheral:

mechanoreceptors in joints, tendons and muscles

respiratory

Integrated Response

Control of breathing is not result of a single factor but of combined result of several chemical and neural factors.

Composite of ventilatory response to exercise.

respiratory

References

Axen and Axen. 2001. Illustrated Principles of Exercise Physiology. Prentice Hall.

Kapit, Macey, Meisami. 1987. Physiology Coloring Book. Harper & Row.

McArdle, Katch, Katch. 2006. Image Collection Essentials of Exercise Physiology, 3rd ed. Lippincott William & Wilkens.