MedSci 205: Lecture 18 - Mechanics of Breathing Part I

Question 1

How is Gas Exchange Brought About?

Answer 1

Only simple organisms can rely solely on diffusive exchange of O2 and CO2 with the environments.

The problem of diffusive distance is overcome in larger organisms by coupling diffusive processes with convectional processes.

Question 2

NOTE

Answer 2

Make a flashcard of picture with all the circles to do with previous flashcard.

Also do one for the schematic of the respiratory system.

And one of the spirometer graph.

Question 3

Structures that Form the Conducting Portion of the Respiratory System

Answer 3

NOTE: the tracheo-brachial tree has VOLUME but NO GASEOUS EXCHANGE takes place.

Nasal cavity.

Tongue.

Pharynx.

Vocal cords.

Eosophagus.

Larynx.

Trachea.

Left and right lungs.

Left and right primary bronchi.

Diaphragm.

Question 4

Lung Volumes

Answer 4

Anatomical dead space (VD) = ~0.15L

Total alveolar gas volume = ~3L

Pulmonary blood volume = ~0.07L

Tidal volume (VT) = 0.5L

Functional residual capacity = ~3L

Expiratory reserve volume = ~2L

Inspiratory reserve volume = ~2.5L

Inspiratory capacity = ~3L

Vital Capacity = ~4.5L

Residual volume = ~1.5L

Total lung capacity = ~6L

Question 5

Changes in Lung Volumes with Exercise

Answer 5

Exercise = increased ventilation

See increased VT, f (clarify what this is)

Question 6

Static Mechanisms of the Respiratory System

Answer 6

Chest wall exerts an outward force on the lungs.

Lungs exert an inwards force (try to collapse).

P(A) and P(ip)/P(pl) are the pressures that determine lung volume.

Lungs are highly elastic and tend to collapse, reducing P(ip) - pressure in sealed space between pleural membranes - to below P(atm) and pulls the chest wall inward.

With open airways and NO air flow: P(A) = P(B)

Question 7

Static Mechanisms of the Respiratory System II

Answer 7

To change lung volume and generate air flow it is necessary to create a pressure gradient.

P(total) = P(elastic) +P(resistive)
= V/C + V(dot)R (compliance)

Thoracic movements are transmitted to the lungs by pressure changes in the intra-pleural space.

Question 8

Pleural Membranes

Answer 8

Cover the outer surface of the lungs (visceral) and the thoracic cavities (parietal).

Are coupled together by a very thin layer of liquid (20micrometers).

This liquid layer acts as a lubricant and allows the lungs to move relative to the chest wall.

Question 9

Functional Residual Capacity and Compliance of the Lung and Chest Wall

Answer 9

Compliance = change in Volume / change in Pressure

FRC is determined by compliance of the lung and chest wall.

Compliance is the inverse of elastance and is a measure of how easily the lungs inflate.

Reduced outward mobility of the chest wall leads to decreased compliance. This occurs in the case of sever obesity and constrictive bandages.

Similarly, reduced lung volumes produce a decreased compliance. This could in occur in the case of pulmonary oedema, fibrosis, increased intra-abdominal pressure (late pregnancy).

Question 10

Relaxation Pressure-Volume Relationship Graph

Answer 10

At high lung volumes, recoil of both lungs and chest is positive.

When flow is zero: P(total) = P(elastic) = V/C

Question 11

Ventilation

Answer 11

The movement of gas into and out of the lungs.

Question 12

Mechanism of Ventilation

Answer 12

Mechanism is bulk flow (diffusion is too slow).

Respiratory muscles (including intercostals) are utilized to change thoracic volume and therefore create pressure gradients.

Air flows down the pressure gradient.

Question 13

Minute Ventilation

Answer 13

Volume of air shifted in and out of lungs per minute.

Vdot(E) = V(T) x f(R)

At rest: Vdot(E) = 0.5L x 12min(-1) = 6L/min

Exercise: Vdot(E) = 3L x 40min(-1) = 120L/min

Question 14

Alveolar Ventilation

Answer 14

The most important kind of ventilation.

Volume of air shifted in and out of the alveoli per minute.

Vdot(A) = f(R) x (V(T) - V(D))

Question 15

Effect of Surface Tension on the Alveoli

Answer 15

Surface tension calls for increased requirement of pressure during inflation (hysteresis)

Saline filled lungs have much larger compliance than air filled lungs.

This is because air filled lungs show the effects of elastic elements and surface tension.

NOTE: for surfactant and water, small areas have the surface tension reduced much more than large ares.

Question 16

Surfactant and its effect on Surface Tension in Alveoli of Different Diameter

Answer 16

For spheres of different radius, Pressure is different (P=2T/r), thus when the 2 spheres are connected, a pressure gradient is formed causing air to flow from one alveolus into the other.

Surfactant changes T of the smaller radius more by reducing it and thus increases compliance. This has the effect of equalising transpulmonary pressure (P(A) - P(ip)) across alveoli