Understanding Gases

The gases of the atmosphere have a mass and a weight (5 x 1018 kg, most within 11 km of the surface).– Consequently, the atmosphere exerts a

significant force on every object on the planet (recall that pressure is measured as force applied per unit area, P = F/A.)

– We are “accustomed” to the tremendous force pressing down on every square inch of our body.

Understanding Gases

A barometer is an instrument that measures atmospheric pressure.– Baro = pressure or

weight– Meter = measure

Air pressure varies greatly depending on the altitude and the temperature.

Understanding Gases

There are many different units used to measure atmospheric pressure. At sea level, the air pressure is:– 14.7 lb/in2 = 1 atmosphere– 760 mmHg = 1 atmosphere– 76 cmHg = 1 atmosphere– 29.9 inHg = 1 atmosphere

At high altitudes, the atmospheric pressure is less; descending to sea level, atmospheric pressure is greater.

Understanding Gases

Gases obey laws of physics called the gas laws.– These laws apply equally to the gases of the

atmosphere, the gases in our lungs, the gases dissolved in the blood, and the gases diffusing into and out of the cells of our body.

– To understand the mechanics of ventilation and respiration, we need to have a basic understanding of 3 of the 5 common gas laws.

Understanding Gases

• Boyle’s law applies to containers with flexible

walls – like our thoracic cage.

– It says that volume and pressure are inversely

related.

• If there is a decrease in volume – there will be an

increase in pressure.

• V 1/P∝

Understanding Gases

Dalton’s law applies to a mixture of gases.– It says that the pressure of each gas is directly

proportional to the percentage of that gas in the total mixture: PTotal = P1 + P2 + P3 …

– Since O2 = 21% of atmosphere, the partial pressure exerted by the contribution of just O2 (written pO2 or PAO2) = 0.21 x 760 mmHg = 159.6 mmHg at sea level.

Gas Exchange

Gas Exchange

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Understanding GasesHenry’s law deals with gases and solutions.– It says that increasing the partial pressure of a gas

“over” (in contact with) a solution will result in more of the gas dissolving into the solution.

– The patient in this picture is getting more O2 in contact with his

blood - consequently, more oxygen goes

into his blood.Medicimage/Phototake

Understanding Gases

Gas will always move from a region of high pressure to a region of low pressure. Applying Boyle's law: If the volume inside the thoracic cavity , the pressure .

Ventilation and RespirationPulmonary ventilation is the movement of air between the atmosphere and the alveoli, and consists of inhalation and exhalation. – Ventilation, or

breathing, is made possible by changes in the intrathoracic volume.

Ventilation and RespirationIn contrast to ventilation, respiration is the exchange of gases.– External respiration

(pulmonary) is gas exchange between the alveoli and the blood.

– Internal respiration (tissue)is gas exchange between the systemic capillaries and the tissues of the body.

Ventilation and Respiration

External respiration in the lungs is possible because of the implications of Boyle’s law: The volume of the thoracic cavity can be increased or decreased by the action of the diaphragm, and other muscles of the chest wall. – By changing the volume of the thoracic cavity (and

the lungs – remember the mechanical coupling of the chest wall, pleura, and lungs), the pressure in the lungs will also change.

Ventilation and Respiration

Changes in air pressure result in movement of the air.– Contraction of the diaphragm and external

intercostal (rib) muscles increases the size of the thorax. This decreases the intrapleural pressure so air can flow in from the atmosphere (inspiration).

– Relaxation of the diaphragm, with/without contraction of the internal intercostals, decreases the size of the thorax, increases the air pressure, and results in exhalation.

Ventilation and Respiration

Certain thoracic muscles participate in inhalation; others aid exhalation.– The diaphragm is

the primary muscle of respiration – all the others are accessory.

Ventilation and RespirationThe recruitment of accessory muscles greatly depends on whether the respiratory movements are quiet (normal), or forced (labored).

Ventilation and Respiration(Interactions Animation)

In the following animation, the mechanical coupling mechanism can be understood by relating the concepts of the gas laws to the unique anatomical features of the airways, pleural cavities, and muscles of respiration.

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• Pulmonary Ventilation

Airflow and Work of Breathing

Differences in air pressure drive airflow, but 3 other factors also affect the ease with which we ventilate:1. The surface tension of alveolar fluid causes the

alveoli to assume the smallest possible diameter and accounts for 2/3 of lung elastic recoil. Normally the alveoli would close with each expiration and make our “Work of Breathing” insupportable.

• Surfactant prevents the complete collapse of alveoli at exhalation, facilitating reasonable levels of work.