The Respiratory SystemPart I

Dr. Adelina Vlad

The Respiratory Process

Breathing automatic, rhythmic and centrally-regulated mechanical process by which the atmospheric gas moves into and out of the lungs

Respiration the overall process of metabolites oxidation for the production of energy by living organisms; it includes breathing

Respiratory process:

External respiration + Internal (mitochondrial) respiration

The Respiratory Process

External respiration - the exchange of O2 and CO2 between the atmosphere and the mitochondria

A dual process of transporting

- O2 from the atmosphere to the mitochondria

- CO2 from the mitochondria to the atmosphere

Internal respiration (oxidative phosphorylation) - the oxidation of carbon-containing compounds to form CO2

The lectures on respiratory physiology focus on external respiration

External Respiration

Can be divided into four major functions:

1) Pulmonary ventilation

2) Diffusion of O2 and CO2 between the alveoli and the blood

3) Transport of O2 and CO2 in the blood and body fluids, to and from the bodys tissue cells

4) Regulation of ventilation

Components of the Respiratory System

Air pump

delivers air to and removes air from the alveolar air spaces = alveolar ventilation

consist of the lungs and the airways, the rib cage and the thoracic cavity with the muscles of respiration

Surface for gas exchange represented by the alveoli

Circulatory system internal convective system that delivers O2 to and removes CO2 from the tissues

Mechanisms for carrying O2 and CO2 in the blood red blood cells are the main players

Mechanisms for locally regulating the distribution of ventilation and perfusion complex feedback loops to regulate air flow and blood flow in the lungs

Mechanisms for centrally regulating ventilation

The respiratory apparatus in humans

Nonrespiratory Roles of the Lungs

Olfaction

Ventilation is essential for delivering odorants to the olfactory epithelium

Sniffing allows one to sample the chemicals in the air without the risk of bringing potential noxious agents deep into the lungs

Left-ventricular reservoir

The blood contained in the highly compliant pulmonary vessels is an important buffer for filling the left ventricle

The left heart can sustain cardiac output for about two beats only with blood from pulmonary circulation

Filtering small emboli from the blood

The pulmonary vasculature can trap microscopic emboli present in the venous blood (e.g., blood clots, fat, air bubbles) before they reach the left heart

If the emboli are few and small, the affected alveoli can recover their function; if pulmonary emboli are large or frequent, they can cause serious symptoms or death

Emboli made up of cancer cells may find a breeding ground for supporting metastatic disease

Biochemical reactions

The pulmonary capillary endothelium plays an important role in converting angiotensin I to angiotensin II, a reaction that is catalyzed by angiotensin converting enzyme (ACE),

and selectively removes agents from the circulation (PGE1, PGE2, PGF2a, leukotrienes, serotonin, bradykinin)

Pulmonary VentilationPulmonary mechanics - is the physics of the lungs, airways and chest wall- explains how the body moves air in (inspiration) and out (expiration) of the lungs

The functional chest wall includes the rib cage, diaphragm, and abdomen

Inspiration

Is an active process that implies contaction of:

primary muscles of inspiration during aquiet breathing:

diaphragm external intercostals

secondary muscles of inspiration during a forced breathing:

sternocleidomastoids anterior serrati scalenes

Expiration

Is passive durig a quiet breathing

Is active during a forced breathing and occurs by contracting the accessory muscle of expiration:

internal intercostals rectus abdominis external obliques

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

During inspiration and expiration the air moves in and out of the lungs due to variations of the:

1. Intrapleural pressure

2. Alveolar pressure

3. Transpulmonary pressure

Chest Wall Lung Interaction

The lungs are elastic structures, kept distended inside the thoracic cavity due to their interaction with the chest wall

This interaction occurs via the intrapleural space, which is a potentialcavity between the visceral and parietal pleural membranes

Chest Wall Lung Interaction

Elastic recoil of the lungs their tendency to collapse

Elastic recoil of the chest wall its tendency to pull the thoracic cage outward

The chest wall and the lungs pull away from each other relative vacuum between them that makes the pressure inside the intrapleural space lower than the barometric pressure (the intrapleural pressure is negative)

Intrapleural Pressure, PIP

Upright subject

Is the pressure of the uid inside the intrapleural space and has negative values

Gravity and the respiratory movements influence PIP values:

By pulling the lungs downward gravity makes PIP more negative to the apex compared to the base of the thoracic cavity of an upright subject

At the beginning of inspiration, PIP is about 5 cm H2O; at the end of a quiet inspiration, PIP decreases to an average of about 7.5 cm H2O

Pleural fluid - transudat with mucoid characteristic, favoring slippage of the lungs during ventilation

The pumping of the uid from the intrapleural space by the lymphatics into the mediastinum, the superior surface of the diaphragm, and the lateral surfaces of the parietal pleura maintains a negative PIP

PIP and the Pleural Fluid

Alveolar Pressure, PA

Is the pressure of the air inside the alveoli

When the glottis is open and no air flows into or out of the lungs, the pressures in all parts of the respiratory tree, including the alveoli (PA), are equal to atmospheric pressure (PB)

PA PB governs the gas exchange between the lungs and the atmosphere; the alveolar pressure, PA, is a dynamic element, directly involved in producing air flow

Transpulmonary Pressure (PTP) Is the force responsible for keeping

the alveoli open, expressed as the pressure gradient across the alveolar wall:

PTP = PA PIP PA should be always > PIP (PTP > 0) in

order to maintain the lungs expanded in the thoracic cavity

PTP is a static parameter which does not cause airflow, but determines lung volume (VL)

PIP has a static component (-PTP) that determines lung volume and a dynamic component (PA) that determines air flow

PIP, PA and PTP During Quiet Breathing

PIP is the pressure directly controlled by the activity of the respiratory muscles; PA and PTPflow from PIP

The negative shift in PIP occurring during inspiration has two effects: PA becomes more negative and PTP is made more positive

Static Compliance of the Lungs

Is the extent to which the lungs will expand for each unit increase in transpulmonary pressure (a measure of how easy it is to inflate the lungs):

C=DVL/DPTP Static compliance - determined at a steady state, when the glottis was

open and the breathing movements were stopped, allowing no airflow

Elastance the compliance reciprocal, a measure of the elastic recoil of the lungs:

E=1/C The characteristics of static compliance are determined by the elastic

forces of the lungs, represented by

(1) elastic forces of the lung tissue itself and

(2) elastic forces caused by surface tension of the uid that lines the inside walls of the alveoli and other lung air spaces

Static Pressure-Volume Curves for Lungs in Health and Disease

The compliance decreases at high lung volumes due to anatomic and viscous limitations

Fibrosis: stiff lungs due to fibrous tissue deposition, with decreased compliance (elastic recoil is much greater)

Emphysema: floppy lungs as a result of elastin destruction, with increased compliance (much less elastic recoil)

Compliance Diagram in a Healthy Person

This diagram shows compliance of

the lungs alone

1. Stable VL at low lung volumes it is difficult to pop open an almost completely collapsed airway; rising PTP has little effect on VL

2. Opening of airways the first increases in VL reflect the popping open of the proximal airways, followed by their expansion and recruitment of others

3. Linear expansion of open airways when all the airways are open, making PIP more negative by chest wall expansion inflates the lungs and increases VL in a linear fashion

4. Limit of airway inflation at high VLlungs compliance decreases

Hysteresis

Defines the difference between the inflation and deflation compliance paths

It exists because a greater pressure difference is required to open a previously closed (or narrowed) airway than to keep an open airway from closing

Surface Tension

When water forms a surface with air, the water molecules on the surface of the water have a strong attraction for one another the water surface is always attempting to contract

On the inner surfaces of the alveoli the water surface is also attempting to contract (surface water molecules tend to dive into the bulk, decreasing the area of the air-water interface), causing an elastic contractile force of the entire lungs, called the surface tension elastic force the surface tension contributes to the elastic recoil

Effect of Surface Tension on Compliance

Lungs inflated with saline solution (= no air-water interface and null surface tension) have an up to three times higher compliance, proving that the surface tension at the air-water interface accounts for about two thirds of the elastic recoil of the lungs (von Neergaard, 1929) surface tension decreases lung compliance

In lungs inflated with saline solution hysteresis is much smaller as well

The pressure generated as a result of surface tension in occluded alveoli is inversely affected by the radius of the alveolus (Laplaces equation):

the smaller the alveolus radius, the higher the pressure needed to keep it open as alveoli have different sizes and are interconnected, smaller alveoli would tend to collapse in bigger ones, decreasing the total alveolar surface area (gas exchange area)

Effect of Surface Tension on Total Alveolar Surface Area

Pulmonary Surfactant

Surfactant is a surface active agent in water = reduces the surface tension of water (e.g. pressure generated in occluded alveoli of identical size is 18 cm H2O without and 4 cm H2O with surfactant)

Due to its components with both hydrophobic and hydrophilic properties, the surfactant gets into the surface of the air-water interface and decreases here the density of water molecules

The most important components of the pulmonary surfactant are the phospholipid dipalmitoyl-phosphatidylcholine, surfactant apoproteins, and calcium ions

It is secreted by type II alveolar epithelial cells starting the 6th

and 7th month of gestation respiratory distress syndrome of the newborn in underdeveloped infants due to insufficient secretion of surfactant

By reducing alveolar surface tension, the surfactant reduces the elastic forces of the lung and increases compliance

Chest Wall Compliance

The thoracic cage has its own elastic and viscous characteristics

The compliance of the entire pulmonary system (the lungs and thoracic cage together) is measured while expanding the lungs of a totally relaxed person

The compliance of the combined lung-thorax system is almost one half that of the lungs alone

When the lungs are expanded to high volumes or compressed to low volumes the limitations of the chest wall become extreme and the compliance of the combined lung-thorax system can be less than one fth that of the lungs alone

Work of Breathing

Under resting conditions, the respiratory muscles perform work to cause inspiration, and not expiration

The work of inspiration can be divided into three fractions:

(1) Work required to expand the lungs against the lung and chest

elastic forces, called compliance work or elastic work

(2) Work required to overcome the viscosity of the lung and

chest wall structures, called tissue resistance work

(3) Work required to overcome airway resistance to movement of

air into the lungs, called airway resistance work

VC = IRV + TV + ERVVC = IC + ERVTLC = VC + RVTLC = IC + FRCFRC = ERV + RV

Lung Volumes and Capacities

IRV = Inspiratory reserve volume 1.9 2.5 L TV = Tidal volume 0.4 0.5 L

ERV = Expiratory reserve volume 1.1 1.5 LRV = Residual volume 1.5 1.9 L

TLC = Total lung capacity 4.9 6.4 LIC = Inspiratory capacity 2.3 3 L

FRC = Functional residual capacity 2.6 3.4 LVC = Vital Capacity 3.4 4.5 L

The magnitude of IRV depends on

Lung compliance any disorder causing a decrease in compliance is decreasing IRV

Muscle strength IRV decreases if the respiratory muscles are weak or if their innervation is compromised

Comfort pain limits the ability to perform a maximal inspiration

Flexibility of skeleton IRV is decreased by joint stiffness (arthritis, kyphoscoliosis)

Posture IRV is lower in a recumbent position because the movement of the diaphragm downward is more difficult without the help of the gravity

The magnitude of ERV depends on the same factors, plus the strength of the accessory expiratory muscles

Functional Residual Capacity

FRC the volume of air that remains in the lungs at the end of each normal expiration

To measure FRC the spirometer is used in an indirect manner, e.g. by determining the degree of dilution of the helium after the subject has breathed, starting from the end of a normal expiration, air mixed with He at a known initial concentration:

Knowing FRC, one can calculate RV and TLC as well:

Alveolar Ventilation

The scope of pulmonary ventilation is the renewal of the air in the gas exchange areas: the alveoli, alveolar sacs, alveolar ducts, and respiratory bronchioles

Alveolar ventilation is the rate at which new air reaches gas exchange areas

Alveolar ventilation is one of the major factors determining the concentrations of oxygen and carbon dioxide in the alveoli

Anatomic dead space respiratory passages where gas exchange does not occur; the air from the dead space just fills the proximal (conducting) airways, and never reaches exchange areas it is not used for refreshing the alveolar air

Physiologic dead space - on occasion, some of the alveoli themselves are nonfunctional or only partially functional because of absent or poor blood ow through the adjacent pulmonary capillaries = alveolar dead space

When the alveolar dead space is included in the total measurement of dead space, this is called the physiologic dead space

The rate of alveolar ventilation:

where VT is the tidal volume, and VD is the physiologic dead space volume

Normally, alveolar ventilation would equal 12 x (500 150) = 4200 ml/min