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.

You must be connected to the internet to run this animation Gas Exchange Gas Exchange Dalton’s law is understood as the sum of the partial pressures of all the individual respiratory gasses You must be connected to the internet to run this animation

Understanding Gases Henry’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 . http://edugen.wiley.com/edugen/courses/crs2056/simulations/interactions/Respiration/content/html/pages/main2.htm

Ventilation and Respiration Pulmonary 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 Respiration In 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 Respiration The 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. Pulmonary Ventilation You must be connected to the internet to run this animation

Airflow and Work of Breathing Differences in air pressure drive airflow, but 3 other factors also affect the ease with which we ventilate: 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.