Human Physiology, 14th edition (2016)

Respiratory Physiology 553

16.5 REGULATION OF BREATHING

The motor neurons that stimulate the respiratory muscles

are controlled by two major descending pathways: one

that controls voluntary breathing and another that controls

involuntary breathing. The unconscious rhythmic control of

breathing is influenced by sensory feedback from recep-

tors sensitive to the P^ CO 2 , pH, and P^ O 2 of arterial blood.

Decompression Sickness

As the diver ascends to sea level, the amount of nitrogen dis-
solved in the plasma decreases as a result of the progressive
decrease in the P^ N 2. If the diver surfaces slowly, a large amount
of nitrogen can diffuse through the alveoli and be eliminated in
the expired breath. If decompression occurs too rapidly, how-
ever, bubbles of nitrogen gas (N 2 ) can form in the tissue fluids
and enter the blood. This process is analogous to the formation
of carbon dioxide bubbles in a champagne bottle when the cork
is removed. The bubbles of N 2 gas in the blood can block small
blood channels, producing muscle and joint pain as well as more
serious damage. These effects are known as decompression
sickness, commonly called “the bends.”
Airplanes that fly long distances at high altitudes (30,000 to
40,000 ft) have pressurized cabins so that the passengers and crew
do not experience the very low atmospheric pressures of these
altitudes. If a cabin were to become rapidly depressurized at high
altitude, much less nitrogen could remain dissolved at the greatly
lowered pressure. People in this situation, like the divers that
ascend too rapidly, would thus experience decompression sickness.

CLINICAL APPLICATION

Hyperbaric oxygen therapy ( HBOT ), in which a patient

breathes 100% oxygen at 2 to 3 atmospheres pressure,

does not increase the amount of oxygen carried by hemo-

globin, because this is already nearly maximum (with 97%

oxyhemoglobin saturation) when breathing sea level air.

However, it does significantly increase the oxygen carried

by plasma—from 0.3 ml O 2 /100 ml blood at sea level to up

to 6 ml O 2 /100 ml blood for 100% oxygen at 3 atmospheres

pressure. This is useful in the treatment of decompression

sickness from scuba diving that results when air bubbles

form in the tissues and travel in the blood as air emboli,

because (from Boyle’s law) raising the pressure decreases

the size of the bubbles. HBOT also helps to kill anaerobic

bacteria and is used to treat gas gangrene, which is caused

by certain bacteria that produce gas and tissue necrosis

that may require amputation of the affected limb. Addition-

ally, hyperbaric oxygen helps to treat carbon monoxide poi-

soning, severe traumatic injuries (such as crush injuries),

certain inflammations, and other conditions.

Clinical Investigation CLUES

Peter collapsed in his closed garage with his car running

and was treated with hyperbaric oxygen.

• What caused Peter’s collapse, and how did it make
  him lose consciousness?


• What is hyperbaric oxygen therapy, and how did it
  work to save Peter?

| CHECKPOINT

7a. Explain how the P^ O 2 of air is calculated and how this

value is affected by altitude, diving, and water vapor

pressure.

7b. Explain how blood P^ O 2 measurements are taken, and

discuss the physiological and clinical significance of

these measurements.

8a. Explain how the arterial PO 2 and the oxygen content

of whole blood are affected by (a) hyperventilation,

(b) breathing from a tank containing 100% oxygen,

(c) anemia (low red blood cell count and hemoglobin

concentration), and (d) high altitude.

8b. Describe the ventilation/perfusion ratios of the lungs,

and explain why systemic arterial blood has a slightly

lower PO 2 than alveolar air.

8c. Explain how decompression sickness is produced in

divers who ascend too rapidly.

LEARNING OUTCOMES

After studying this section, you should be able to:

9. Explain how ventilation is regulated by the CNS.


10. Explain how blood gases and pH influence
    ventilation.

Inspiration and expiration are produced by the contraction and

relaxation of skeletal muscles in response to activity in somatic

motor neurons in the spinal cord. The activity of these motor

neurons is controlled, in turn, by descending tracts from neu-

rons in the respiratory control centers in the medulla oblongata

and from neurons in the cerebral cortex.

Brain Stem Respiratory Centers

The somatic motor neurons that stimulate the respiratory mus-

cles (see fig. 16.13 ) have their cell bodies in the gray matter of

the spinal cord. The motoneurons of the phrenic nerve, stimu-

lating the diaphragm, have cell bodies in the cervical level of

the spinal cord; those that innervate the respiratory muscles of

the rib cage and abdomen have cell bodies in the thoracolumbar