A Closer Look At the Lungs

Respiratory Membrane

• In lungs where gas exchange between air and blood occurs
• Formed by walls of alveoli and capillaries
• Very thin for diffusion of gases

Rhythmic Ventilation

• Normal respiration rate is 12-20 resp. per minute (adults)
• Controlled by neurons in medulla oblongata
• Rate is determined by number of times resp. muscles are stimulated

Nervous Control of Breathing

• Higher brain centers allow voluntary breathing
• Emotions and speech affect breathing
• Hering-Breuer Reflex: inhibits respiratory center when lungs are stretched during inspiration

Chemical Control of Breathing

• Chemoreceptors in medulla oblongata respond to changes in blood pH
• Blood pH are produced by changes in blood CO2 levels
• An increase in CO2 causes decreased pH, result is increased breathing
• Low blood levels of O2 stimulate chemoreceptors in carotid and aortic bodies, increased breathing

Pressure Changes and Air Flow

• When thoracic cavity volume increases pressure decreases.
• When thoracic cavity volume decreases pressure increases.
• Air flows from areas of high to low pressure.

Alveoli:

• Small air sacs
• Where gas exchange occurs
• Surrounded by capillaries
• 300 million in lungs

Pleural Membranes and Cavities

Air Passageways of Lungs

• Pleura: double-layered membrane around lungs
• Parietal pleura: membrane that lines thoracic cavity
• Visceral pleura: membrane that covers lung’s surface
• Pleural cavity: space around each lung

• Primary bronchi
• Bronchioles
• Terminal bronchioles
• Alveolar ducts
• Alveoli
• Structures become smaller and more numerous from primary bronchi to alveoli

Ventilation

• What is it- breathing
• Process of moving air in and out of lungs
• Uses diaphragm: skeletal muscle that separates thoracic and abdominal cavities
• Phases of Ventilation
• Inspiration:
• Breathe in
• Uses external intercostal muscles
• Diaphragm descends and rib cage expands
• Thoracic cavity volume increases, pressure decreases
• Atmospheric pressure is greater than (high) alveolar pressure (low)
• Air moves into alveoli (lungs)

Gas Exchange

Partial Pressure

Pleural Pressure

• Pressure exerted by a specific gas in a mixture of gases
• Ex.Total pressure of all gases is 760(mm Hg) and 21% of mixture is O2 then partial pressure for O2 is 160 mm Hg
• Symbol is P and gas (Po2)

• Respiratory membrane:
• Where gas exchange between blood and air occurs
• Primarily alveoli
• Influenced by thickness of membrane, total area of membrane, partial pressure of gases
• Increased thickness decreases rate of diffusion

Diffusion of Gases in Lungs

Carbon Dioxide Transport and Blood pH

• Cells in body use O2 and produce CO2.
• Blood returning from tissues and entering lungs has a decreased Po2 and increased Pco2
• O2 diffuses from alveoli into pulmonary capillaries (blood)
• CO2 diffuses from capillaries into alveoli

• CO2 diffuses from cells into capillaries
• CO2 enters blood and is transported in plasma, comb. with blood proteins, bicarbonate ions
• CO2 levels increase blood pH decreases

Diffusion of Gases in Tissues

• Pressure in pleural cavity
• Less than alveolar pressure
• Keep alveoli from collapsing

• Blood flow from lungs through left side of heart to tissue capillaries
• Oxygen diffuses from capillaries into interstitial fluid because Po2 in interstitial fluid is lower than capillary
• Oxygen diffuses from interstitial fluid into cells (Po2) is less

Expiration:

• Breathe out
• Uses internal intercostal muscles
• Diaphragm relaxes and rib cage recoils
• Thoracic cavity volume decreases, pressure increases
• Alveolar pressure is greater than (high) atmospheric pressure (low)
• Air moves out of lungs

A Closer Look At the Lungs