The lungs can be expanded and contracted in two way

• by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity, and

• by elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity.

inspiration -contraction of the diaphragm pulls the lower surfaces of the lungs downward.

all the muscles that elevate the chest cage are classified as muscles of inspiration

• external intercostals - The most important muscles that raise the rib cage

• sternocleidomastoid muscles - lift upward on the sternum

• anterior serrati - which lift many of the ribs

• scaleni - which lift the first two ribs.

expiration - the diaphragm simply relaxes, and the elastic recoil of the lungs, chest wall, and abdominal structures compresses the lungs and expels the air. During heavy breathing

• Contraction of the abdominal muscles, which pushes the abdominal contents upward against the bottom of the diaphragm, thereby compressing the lungs.

• Abdominal Recti - powerful effect of pulling downward on the lower ribs

• internal intercostal - pull down on the rib cage

Pressures That Cause the Movement of Air In and Out of the Lungs

Pleural pressure - is the pressure of the fluid in the thin space between the lung pleura and the chest wall pleura

• The normal pleural pressure at the beginning of inspiration is about −5 centimeters of water, during normal inspiration, expansion of the chest cage pulls outward on the lungs with greater force and creates more negative pressure, to an average of about −7.5 centimeters of water.

Alveolar pressure - the pressure of the air inside the lung alveoli

• To cause inward flow of air into the alveoli during inspiration, the pressure in the alveoli must fall to a value slightly below atmospheric pressure (below 0)

• During normal inspiration, alveolar pressure decreases to about −1 centimeters of water. This slight negative pressure is enough to pull 0.5 liter of air into the lungs in the 2 seconds required for normal quiet inspiration.

• During expiration, opposite pressures occur: The alveolar pressure rises to about +1 centimeter of water, and this forces the 0.5 liter of inspired air out of the lungs during the 2 to 3 seconds of expiration.

Transpulmonary Pressure - the difference between the alveolar pressure and the pleural pressure. it is a measure of the elastic forces in the lungs that tend to collapse the lungs at each instant of respiration, called the recoil pressure.

Compliance of the Lungs

Compliance - extent to which the lungs will expand for each unit increase in transpulmonary pressure

The lungs have two main forces that affect how stretchy (or compliant) they are: tissue elasticity and surface tension

• Tissue elasticity is like the natural "stretchiness" of the lung tissues, kind of like how a rubber band stretches.

• Surface tension happens at the boundary between the air inside the lungs and the liquid lining the alveoli (tiny air sacs). Surface tension makes the lungs less stretchy because it pulls the walls of the alveoli inward, making them harder to expand.

Air-filled lungs: When filled with air, the surface tension from the air-liquid interface adds an extra force that resists lung expansion, making the lungs less compliant (harder to stretch).

Saline-filled lungs: With saline solution, there's no air, so there's no air-liquid interface and no surface tension. This leaves only the tissue elasticity, making the lungs more compliant (easier to stretch).

the tissue elastic forces tending to cause collapse of the air-filled lung represent only about one third of the total lung elasticity, whereas the fluid-air surface tension forces in the alveoli represent about two thirds.

Surfactant, Surface Tension, and Collapse of the Alveoli

Principle of Surface Tension - When water forms a surface with air, the water molecules on the surface of the water have an especially strong attraction for one another. As a result, the water surface is always attempting to contract.

Surfactant - surface active agent in water, which means that it greatly reduces the surface tension of water

• type II alveolar epithelial cells - produces surfactant

Pulmonary Volumes and Capacities

Spirometry - a method called Pulmonary ventilation can be studied by recording the volume movement of air into and out of the lungs

• TIDAL VOLUME = 500 ML

• INSPIRATORY RESERVE VOLUME = 3000 ML

• EXPIRATORY RESERVER VOLUME = 1100 ML

• RESIDUAL VOLUME = 1200 ML

• DEAD SPACE = 150 ML

PULMONARY CAPACITIES

• INSPIRATORY CAPACITY = TV + IV

• FUNCTIONAL RESIDUAL CAPACITY = ERV + RV

• VITAL CAPACITY = TV + IRV + ERV

• TOTAL LUNG CAPACITY = VT + RV

Minute Respiratory Volume Equals Respiratory Rate Times Tidal Volume

The minute respiratory volume - is the total amount of new air moved into the respiratory passages each minute; this is equal to the tidal volume times the respiratory rate per minute.

• The normal tidal volume is about 500 milliliters,

• normal respiratory rate is about 12 breaths per minute

Alveolar Ventilation

• the ultimate importance of pulmonary ventilation is to continually renew the air in the gas exchange areas of the lungs, where air is in proximity to pulmonary blood

Alveolar ventilation - the rate at which new air reaches the area

Dead space - the air a person breathes that never reaches the gas exchange area but simply fills respiratory passages/ areas that do not participate in gas exchange

• on expiration air in the dead space is expired first

Anatomical dead space - areas where gas exchange does not occur

Alveolar dead space - areas where gas exchange is suppose to occur but does not

Physiological dead space - incorporates both the anatomical dead space and the dead space resulting from alveoli that does not participate in gas exchange due to poor blood flow in capillaries

• Physiological dead space is the alveolar dead space and anatomical dead space

To measure the dead space volume, a patient must take a deep breath of pure oxygen which fills the entire dead space then the person expires through a nitrogen meter

VOLUME OF DEAD SPACE = GRAY AREA X TOTAL VOLUME OF EXPIRED AIR/PINK AREA + GRAY AREA

Alveolar ventilation per munite - total volume of new air entering the alveloi and adjacent gas exchange areas each munite

VA = RR X ( TIDAL VOLUME - PHYSIOLOGIC DEAD SPACE VOLUME )