Respiratory Regulation During Exercise

4. Respiratory Regulation During Exercise

RESPIRATORY REGULATION DURING EXERCISE

A. Pulmonary Ventilation

Pulmonary Ventilation (breathing) is the process by which air is moved into and out of the lungs. It has two phases: inspiration & expiration.

1. Inspiration Inspiration is an active process in which the diaphragm and the external

intercostals muscles increase the dimension, and thus the volume, of the thoracic cage. This decreases the pressure in the lungs and draws air in.

During forced or labored breathing (such as during heavy exercise), inspiration is further assisted by the action of other muscles (neck & pectoral muscles). These muscles help raise the ribs even more than during regular breathing.

2. Expiration Expiration is usually a passive process. The inspiratory muscles relax and the

elastics tissue of the lung recoils, returning the thoracic cage to its smaller, normal dimensions. This increases the pressure in the lungs and forces air out.

During forced breathing, expiration becomes a more active process. The internal intercostals muscles can actively pull the ribs down. This action assisted by the latissimus dorsi & lumborum muscles. These muscles also pull the rib cage down.

B. Pulmonary Diffusion

Pulmonary diffusion is the process by which gases are exchanged across the respiratory membrane (alveolar-capillary membrane) in the alveoli (lungs).

The amount of gas exchange that occurs across the membrane depends primarily on the partial pressure of each gas, though gas solubility and temperature are also important.

Gases diffuse along a pressure gradient, moving from an area of higher pressure to one of lower pressure. Thus, oxygen enters the blood and carbon dioxide leaves it.

The greater the pressure gradient across the respiratory membrane, the more rapidly oxygen diffuses across it.

Oxygen diffusion capacity increases as the body move from rest to exercise. When the body needs more oxygen, oxygen exchange is facilitated.

The pressure gradient for carbon dioxide exchange is less than for oxygen exchange, but carbon dioxide’s membrane solubility is 20 times greater than of oxygen, so carbon dioxide crosses the membrane easily, even without a large pressure gradient.

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C. Transport of Oxygen and Carbon Dioxide

1. Oxygen Transport Oxygen is transported in the blood primarily bound to hemoglobin

(oxyhemoglobin), though a small part of it is dissolved in blood plasma.

Hemoglobin oxygen saturation decreases when :a) Partial pressure of oxygen (PO2) decreases,b) pH decreases, orc) temperature increases.Each of these conditions can reflect increased local oxygen demand. They increase oxygen unloading in the needy area.

Hemoglobin is usually about 98% saturated with oxygen. This is a much higher oxygen content than our bodies require, so the blood’s oxygen-carrying capacity seldom limits performance.

2. Carbon Dioxide Transport Carbon dioxide is transported in the blood primarily as bicarbonate ion. This

prevents the formation of carbonic acid, which can cause H+ to accumulate, decreasing the pH. Smaller amounts of carbon dioxide are carried either dissolved in the plasma or bound to hemoglobin (carbaminohemoglobin).

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D. Gas Exchange at the Muscle

1. The Arterial-Venous Oxygen Difference The a-vO2 diff is the difference in the oxygen content of arterial and venous

blood. This measure reflects the amount of oxygen taken up by the tissues.

2. Factors Influencing Oxygen Delivery and Uptake Oxygen delivery to the tissues depends on:

a) the oxygen content of the blood, b) the amount of blood flow to the tissues, andc) local conditions (e.g., tissue temperature & PO2)

3. Carbon Dioxide Removal Carbon dioxide exchange at the tissues is similar to oxygen exchange, except

that CO2 leaves the muscles, where it is formed, and enters the blood to be transported to the lungs for clearance.

E. The Regulation of Pulmonary Ventilation

1. The Mechanisms of Regulation

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The respiratory centers in the brain stem set the rate and depth of breathing.

Central chemoreceptors in the brain respond to changes in concentrations of CO2

and H+. When either of these rises, the inspiratory center increases respiration. Peripheral receptors in the arch of the aorta and the bifurcation of the common

carotid artery respond primarily to changes in blood O2 levels, but also to changes in CO2 and H+ levels. If O2 levels drop too low, or if the other levels rise, these chemoreceptors relay their information to the inspiratory center, which in turn increases respiration.

Stretch receptors in the air passages and lungs can cause the expiratory center to shorten respirations to prevent overinflation of the lungs. In addition, the body can exert some voluntary control over respiration.

2. Pulmonary Ventilation During Exercise During exercise, ventilation shows an almost immediate increase, resulting from

increased inspiratory center stimulation caused by the muscle activity itself. This is followed by a more gradual increase that results from the rise in temperature and chemical changes in the arterial blood that are caused by the muscular activity.

3. Problems Associated With Breathing During Exercise Breathing problems associated with exercise include dyspnea (shortness of

breath), hyperventilation (overbreathing), and performance of the Valsalva maneuver.

Valsalva maneuver is referred to a respiratory procedure that is frequently performed in certain types of exercise and that can be potentially dangerous. This occurs when the individual:a) Closes the glottis (the opening between the vocal cords),b) Increases the intra-abdominal pressure by forcibly contracting the

diaphragm and the abdominal muscles, andc) Increases the intrathoracic pressure by forcibly contracting the respiratory

muscles.

F. Ventilation and Energy Metabolism

1. The Ventilatory Equivalent for Oxygen During mild, steady-state exercise, ventilation accurately reflects the rate of

energy metabolism. Ventilation parallels oxygen uptake. The ratio of air ventilated to O2 consumed is the ventilatory equivalent of oxygen (VE/VO2).

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2. The Ventilatory Breakpoint The Ventilatory Breakpoint is the point at which ventilation abruptly increases,

even though O2 consumption does not. This increase reflects the need to remove excess CO2.

3. Anaerobic Threshold The anaerobic threshold can be determined by identifying the point at which

VE/VO2 shows a sudden increase while VE/VO2 stays relatively stable. Anaerobic threshold has been used as a noninvasive estimate of lactate

threshold.

G. Respiratory Limitations to Performance

More than 15% of the body’s total O2 consumption during heavy exercise can occur in the respiratory muscles.

Pulmonary ventilation is usually not a limiting factor for performance, even during maximal effort, though it can limit performance in highly trained people.

The respiratory muscles seem to be better designed for avoiding fatigue during long-term activity than muscles of the extremities.

Airway resistance and gas diffusion usually do not limit performance in normal, healthy individuals.

The respiratory system can limit performance in people with restrictive or obstructive respiratory disorders.

H. Respiratory Regulation of Acid-Base Balance

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Excess H+ (decreased pH) impairs muscle contractility and ATP formation. The respiratory system plays an integral role in maintaining acid-base balance. Whenever H+ levels start to rise, the inspiratory center responds by increasing

respiration. Removing CO2 is an essential means for reducing the H+ concentrations. CO2 is transported primarily bound to bicarbonate. Once it reaches the lungs, CO2 is formed again and exhaled.

Whenever H+ levels begin to rise, whether from CO2 or lactate accumulation, bicarbonate ion can buffer the H+ to prevent acidosis.

SUMMARY

1. Pulmonary Ventilation (breathing) is the process by which air is moved into and out of the lungs. It has two phases: inspiration & expiration.

2. Inspiration is an active process in which the diaphragm and the external intercostals muscles increase the dimension, and thus the volume, of the thoracic cage. This decreases the pressure in the lungs and draws air in.

3. Expiration is usually a passive process. The inspiratory muscles relax and the elastics tissue of the lung recoils, returning the thoracic cage to its smaller, normal dimensions. This increases the pressure in the lungs and forces air out.

4. Forced inspiration and expiration are active processes, dependent on muscles actions.

5. Pulmonary diffusion is the process by which gases are exchanged across the respiratory membrane in the lungs.

6. The amount of gas exchange that occurs across the membrane depends primarily on the partial pressure of each gas.

7. The greater the pressure gradient across the respiratory membrane, the more rapidly oxygen diffuses across it.

8. Oxygen diffusion capacity increases as the body move from rest to exercise. When the body needs more oxygen, oxygen exchange is facilitated.

9. The pressure gradient for carbon dioxide exchange is less than for oxygen exchange, but carbon dioxide’s membrane solubility is 20 times greater than of oxygen, so carbon dioxide crosses the membrane easily, even without a large pressure gradient.

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Respiratory Regulation During Exercise

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