How we breathe

Function Overview

• Negative intrathoracic pressure

• Inspiration

• Expiration

• Respiratory volumes

• Exchange of gases in alveoli

• Partial pressures of gases

• Control of breathing

Negative Intrathoracic Pressure

• Partial vacuum exists within thorax

  • Pulls lungs tightly out against thoracic wall

• Allows lungs to follow movements of thoracic wall and diaphragm

  • Inspiration and expiration

• Aids in return of blood to the heart

  • Veins have no muscular pump to facilitate process

Inspiration = Inhalation

• Process of drawing air into the lungs

• Results from enlargement of volume of thorax

  • Lungs follow enlargement passively

  • Air is drawn into lungs

• Main inspiratory muscles:

  • Diaphragm and external intercostal muscles

Expiration = Exhalation

• Process of pushing air out of the lungs

• Opposite of inspiration:

  • Size of thorax is decreased

  • Lungs are compressed

  • Air is pushed out through respiratory passageways

• Main expiratory muscles:

  • Internal intercostal muscles and abdominal muscles

Respiratory Volumes

Standardized terms for describing quantity of air involved in respiration:

• Tidal volume — volume inspired and expired in one breath

• Minute volume — tidal volume in a minute's time

• Residual volume — amount left after exhalation

Alveolar Gas Exchange

Inspiration

• Inhaled air: high O2, low CO2

• Blood entering capillary: low O2, high CO2

Gas Exchange

• High level of O2 in air diffuses into blood where level is lower

• High level of CO2 in blood diffuses into air where level is lower

Expiration

• Exhaled air: higher CO2, lower O2

Respiratory Center

• Breathing controlled by area in medulla oblongata of the brain stem

  • Directs timing and strength of respiratory muscle contraction

• Individual control centers for inspiration, expiration, and breath-holding

• Breathing can be consciously controlled for brief periods of time

• Controlled by two control systems:

  • Mechanical

  • Chemical

Mechanical Control System for Breathing

• Operates through stretch receptors in lungs

• A preset and automatic system:

  • Nerve impulses sent to respiratory center indicate when lungs inflate to certain point

  • Muscle contractions that produce inspiration are stopped

  • Muscle contractions to produce expiration are initiated

  • Another set of nerve impulses is sent when lungs deflate to a certain point

  • Expiration is stopped; process of inspiration is begun

• Net effect is normal, rhythmic, resting breathing baseline pattern

Chemical Control System for Breathing

• System affects breathing pattern only when something is out of balance

  • Makes adjustments to maintain homeostasis

• Chemical receptors monitor blood

  • Located in carotid artery and aorta, and the brain stem

• Characteristics monitored:

  • CO2 content

  • The pH

  • O2 content

CO2 Variations in the Blood

• Blood level of CO2 and blood pH usually linked

• Increased CO2 in blood:

  • Decreases blood pH

  • Triggers respiratory center to increase rate and depth of respiration

• Decreased CO2 in blood:

  • Increases blood pH

  • Triggers respiratory center to decrease rate and depth of respiration

Oxygen Variations in the Blood

• Effects of variations not as clear-cut as with CO2

• Slight hypoxia:

  • Respiratory center signaled to increase rate and depth of breathing

• Severe hypoxia:

  • Neurons of respiratory system become so depressed that impulses cannot be sent to respiratory muscles

  • Can cause breathing to decrease or stop completely