The Respiratory System

The Respiratory System

• Cells produce energy:– for maintenance, growth, defense,

and division– through mechanisms that use oxygen

and produce carbon dioxide

Oxygen

• Is obtained from the air by diffusion across delicate exchange surfaces of lungs

• Is carried to cells by the cardiovascular system which also returns carbon dioxide to the lungs

5 Functions of the Respiratory System

1. Provides extensive gas exchange surface area between air and circulating blood

2. Moves air to and from exchange surfaces of lungs

3. Protects respiratory surfaces from outside environment

4. Produces sounds5. Participates in olfactory sense

External & Internal Respiration

External Respiration• Mechanics of

breathing• The movement of

gases into & out of body

• Gas transfer from lungs to tissues of body

• Maintain body & cellular homeostasis

Internal Respiration• Intracellular oxygen

metabolism• Cellular

transformation• Krebs cycle –

aerobic ATP generation

• Mitochondria & O2 utilization

Organization of Respiratory System

• Nose• Nasal cavities• Paranasal sinuses• Pharynx• Larynx• Trachea• Bronchi and lungs

– Bronchioles– Alveoli

Airway BranchingAirway Branching

Trachea 0

Main Bronchi 1

Lobar Bronchus 2

Segmental Bronchus 3-4

Bronchioles 5-15

Terminal Bronchioles 16

Resp. Bronchioles 17-19

Alveolar Ducts 20-22

Alveolas Sacs 23

Source: SEER Training Website (training.seer.cancer.gov)

Alveoli

• ~ 300 million air sacs (alveoli).– Large surface area

(60–80 m2).– Each alveolus is 1 cell

layer thick.

• 2 types of cells:– Alveolar type I:

• Structural cells.

– Alveolar type II:• Secrete surfactant.

Alveolar Organization

Respiratory bronchioles are connected to alveoli along alveolar ductsAlveolar ducts end at alveolar sacs:

common chambers connected to many individual alveoli

Respiratory Mechanics

Multiple factors required to alter lung volumes

• Respiratory muscles generate force to inflate & deflate the lungs

• Tissue elastance & resistance impedes ventilation

• Distribution of air movement within the lung, resistance within the airway

• Overcoming surface tension within alveoli

The Breathing Cycle• Airflow requires a pressure gradient• Air flow from higher to lower pressures• During inspiration alveolar pressure is sub-

atmospheric allowing airflow into lungs• Higher pressure in alveoli during expiration

than atmosphere allows airflow out of lung• Changes in alveolar pressure are generated

by changes in pleural pressure

Muscles of inpiration Muscles of expiration

• diaphragm-most important

• External intercostals• Accessory muscles :

– Sternocleidomastoid– Serratus anterior – scaleni

• Abdominal recti• Internal intercostal

muscles

The Respiratory Muscles

Most important are:the diaphragm external intracostal muscles of the ribsaccessory respiratory muscles:

activated when respiration increases significantly

The Respiratory Muscles

Figure 23–16c, d

The Mechanics of Breathing

• Inspiration:– always active

• Expiration:– active or passive

3 Muscle Groups of Inspiration

1. Diaphragm:– contraction draws air into lungs– 75% of normal air movement

2. External intracostal muscles:– assist inhalation– 25% of normal air movement

3. Accessory muscles assist in elevating ribs:

– sternocleidomastoid– serratus anterior– pectoralis minor– scalene muscles

Muscles of Active Expiration

1. Internal intercostal and transversus thoracis muscles:

– depress the ribs

2. Abdominal muscles:– compress the abdomen– force diaphragm upward

Movement of Thorax During Breathing Cycle

Movement of Diaphragm

Pleura and Pleural Cavities

• The outer surface of each lung and the adjacent internal thoracic wall are lined by a serous membrane called pleura.

• The outer surface of each lung is tightly covered by the visceral pleura.

• while the internal thoracic walls, the lateral surfaces of the mediastinum, and the superior surface of the diaphragm are lined by the parietal pleura.

• The parietal and visceral pleural layers are continuous at the hilus of each lung.

Pleural Cavities

The potential space between the serous membrane layers is a pleural cavity.

• The pleural membranes produce a thin, serous pleural fluid that circulates in the pleural cavity and acts as a lubricant, ensuring minimal friction during breathing.

• Pleural effusion – pleuritis with too much fluid

Intrapleural Pressure

• Pressure in space between parietal and visceral pleura

• Averages —4 mm Hg• Maximum of —18 mm Hg• Remains below Patm throughout

respiratory cycle

Intrapulmonary Pressure

• Also called intra-alveolar pressure• Is relative to Patm

• In relaxed breathing, the difference between Patm and intrapulmonary pressure is small:– about —1 mm Hg on inspiration or +1

mm Hg on expiration

Transpulmonary Pressure

• The pressure difference between the alveolar pressure & pleural pressure on outside of lungs

• The alveoli tend to collapse together while the pleural pressure attempts to pull outward

• The elastic forces which tend to collapse the lung during respiration is Recoil Pressure

Physical Properties of the Lungs

• Ventilation occurs as a result of changes in lung volume of given pressure difference

• Physical properties that affect lung function:– Compliance.– Elasticity. – Surface tension.

Compliance

• Compliance describes the dispensability of the system – Ease with which the lungs can expand

• Thus the lung compliance describes how volume changes for the given change in pressure

• Change in lung volume per change in transpulmonary pressure.

V/P

Compliance of the lungs

• Measurement of lung compliance requires simultaneous measurement of lung pressure & volume .

Compliance of the lungs(continued)

• The characteristics of the compliance diagram are determined by the elastic forces of the lungs .these can be divided into two parts– Elastic forces of lung tissue itself– Elastic forces caused by surface

tension .

Surface Tension

• Force exerted by fluid in alveoli to resist distension.

• Lungs secrete and absorb fluid, leaving a very thin film of fluid.

– This film of fluid causes surface tension.– The attractive forces between adjacent

molecules of liquid are stronger than forces between molecule of liquid and a molecule of gas in the alveoli

• H20 molecules at the surface are attracted to other H20 molecules by attractive forces.– Force is directed inward, that tends to

collapse the alveoli

Surface Tension (continued)

• Law of Laplace:– Pressure in alveoli is

directly proportional to surface tension; and inversely proportional to radius of alveoli.

– Pressure in smaller alveolus would be greater than in larger alveolus, if surface tension were the same in both.

Insert fig. 16.11

Surfactant

• Surfactant is surface active agent in water that greatly reduces the surface tension .

• Secreted by type II alveolar epithelial cells , it lines the alveoli & reduces their surface tension.

• Complex mixture of phospholipids, dipalmotylphophotidylcholine & surfactant apoprotien

Role of surfactant

Role of surfactant

• Surfactant provides two functions – Reduces the surface tension thereby

reducing the collapsing forces in alveoli– It increases the lung compliance .

In neonatal distress syndrome, surfactant is lacking

Surfactant not begin to secret normally before gestational ages of 24 to 28 week

Dead Space• The volume of the airways that does not

participate in gas exchange• Anatomical dead space – volume of the

conducting respiratory passages (150 ml)• Functional dead space – alveoli that cease to

act in gas exchange due to collapse or obstruction

• Physiological dead space – sum of alveolar and anatomical dead spaces

Alveolar Ventilation

• Amount of air reaching alveoli each minute

• Calculated as:tidal volume — anatomic dead space

respiratory rate

• Alveoli contain less O2, more CO2 than atmospheric air:– because air mixes with expiration air

Alveolar Ventilation Rate

• Determined by respiratory rate and tidal volume:– for a given respiratory rate:

• increasing tidal volume increases alveolar ventilation rate

– for a given tidal volume:• increasing respiratory rate increases

alveolar ventilation

4 Calculated Respiratory Capacities

1. Inspiratory capacity: tidal volume + inspiratory reserve volume

2. Functional residual capacity (FRC): expiratory reserve volume + residual

volume

3. Vital capacity: expiratory reserve volume + tidal volume +

inspiratory reserve volume

4. Total lung capacity: vital capacity + residual volume

Diffusion of GasesDiffusion of Gases

Gas Movement due to Gas Movement due to DiffusionDiffusion

• Diffusion - movement of gas due to molecular motion, rather than flow.

– Akin to the spread of a scent in a room, rather than wind.

– Random motion leads to distribution of gas molecules in alveolus.

Gas Movement due to DiffusionGas Movement due to Diffusion

Source: Jkrieger (wikimedia.org)

DiffusionDiffusion

• Driven by concentration gradients:– differences in partial pressure of the

individual gases.

• Movement of O2 and CO2 between the level of the respiratory bronchiole and that of the alveolar space depends only on diffusion.

• The distances are small, so diffusion here is fast.

Pathway of diffusion

Diffusion of Gas Through Diffusion of Gas Through the the

Alveolar WallAlveolar WallAlveolar airspace

Source: Undetermined

Diffusion of Oxygen Diffusion of Oxygen Across the Alveolar WallAcross the Alveolar Wall

Pulmonary SurfactantPulmonary Surfactant

Alveolar EpitheliumAlveolar Epithelium

Alveolar InterstitiumAlveolar Interstitium

Capillary EndotheliumCapillary Endothelium

PlasmaPlasma

Red Blood CellRed Blood Cell

HemoglobinHemoglobin

Diffuses/Dissolves

Diffuses/Dissolves

Diffuses/Dissolves

Diffuses/Dissolves

Diffuses/Dissolves

Binds

Fick’s Law for DiffusionFick’s Law for Diffusion

VVgasgas = = A x D x (PA x D x (P11 – P – P22))

TT

Vgas = volume of gas diffusing through the tissue barrier per time, in ml/min

A = surface area available for diffusionD = diffusion coefficient of the gas (diffusivity)T = thickness of the barrierP1 – P2 = partial pressure difference of the gas

Gas Exchange

• Occurs between blood and alveolar air

• Across the respiratory membrane• Depends on:

– partial pressures of the gases– diffusion of molecules between gas

and liquid

Oxygen Transport

• Due to low solubility, only 1.5 % of oxygen is dissolved in plasma

• 98.5 % of oxygen combines with hemoglobin

• Each Hb consists of a globin portion composed of 4 polypeptide chains

• Each Hb also contains 4 iron containing pigments called heme groups

• Up to 4 molecules of O2 can bind one Hb molecule because each iron atom can bind one oxygen molecule

• There are about 250 million Hb hemoglobin molecules in one Red Blood Cell

• When 4 oxygen molecules are bound to Hb, it is 100% saturated, with fewer, it is partially saturated

• Oxygen binding occurs in response to high partial pressure of Oxygen in the lungs

• Oxygen + Hb Oxyhemoglobin (Reversible)

• Cooperative binding Hb’s affinity for O2 increases as its saturation increases (similarly its affinity decreases when saturation decreases)

• In the lungs where the partial pressure of oxygen is high, the rxn proceeds to the right forming Oxyhemoglobin

• In the tissues where the partial pressure of oxygen is low, the rxn reverses. OxyHb will release oxygen, forming again Hb (or properly said deoxyhemoglobin)

Hemoglobin Saturation Curve

BOHR EFFECT

Bohr Effect• Bohr Effect refers to the changes in

the affinity of Hemoglobin for oxygen • It is represented by shifts in the Hb-O2

dissociation curve• Three curves are shown with

progressively decreasing oxygen affinity indicated by increasing P(50)

• SHIFT to the RIGHT Decreased affinity of Hb for Oxygen Increased delivery of Oxygen to tissues

• It is brought about by

1. Increased partial pressure of Carbon Dioxide

2. Lower pH (high [H+])

3. Increased temperature

4. Increased levels of 2,3 DPGA

• Ex: increased physical activity, high body temperature (hot weather as well), tissue hypoxia (lack of O2 in tissues)

• SHIFT to the LEFT Increased affinity of Hb for Oxygen Decreased delivery of Oxygen to tissues

• It is brought about by

1. Decreased partial pressure of Carbon Dioxide

2. Higher pH (low [H+])

3. Decreased temperature

4. Decreased levels of 2,3 DPGA

• Ex: decreased physical activity, low body temperature (cold weather as well), satisfactory tissue oxygenation

The Effect of pH and Temperature on Hemoglobin

Saturation

A Functional Comparison of Fetal and Adult Hemoglobin

Carbon Dioxide Transport

• Produced by cells thru-out the body• CO2 diffuses from tissue cells and into

the capillaries• 7% dissolves in plasma• 93% diffuses into the Red Blood Cells• Within the RBC ~23% combines with

Hb (to form carbamino hemoglobin) and ~ 70% is converted to Bicarbonate Ions which are then transported in the plasma

• In the lungs, which have low Carbon Dioxide partial pressure, CO2 dissociates from CarbaminoHemoglobin, diffuses back into lungs and is exhaled

• Within the RBC, CO2 combines with water and in the presence of carbonic anhydrase it transforms into Carbonic acid

• Carbonic acid then dissociate into H+ and HCO3-

• In the lungs CO2 diffuses out into the alveoli. This lowers the partial press. Of Co2 in blood, causing the chemical reactions to reverse

Summary: Gas Transport

Figure 23–24

Control of Respiration

•Medullary centers– Respiratory rhythmicity centers

set pace• Dorsal respiratory group (DRG)–

inspiration

• Ventral respiratory group (VRG)– forced breathing

Respiratory centers of the brain

•Pons– Apneustic and pneumotaxic centers: ● regulate the respiratory rate and

the depth of respiration in response to sensory stimuli or input from other centers in the brain

Respiratory centers of the brain

Respiratory Centers and Reflex Controls

Mechanism of rhythmic breathing

Respiratory reflexes

• Hering-breuer reflexes – Hering-breuer inflation reflex– Hering-breuer deflation reflex

• Reflex from lung irritant receptors• Reflex from J receptors•

Chemical regulation of respiration

Chemoreceptors• Chemoreceptors are located

throughout the body (in brain and arteries).

– chemoreceptors are more sensitive to changes in PCO2

(as

sensed through changes in pH).• Ventilation is adjusted to

maintain arterial PC02 of 40 mm Hg.

• Central chempreceptors

• Peripheral chemoreceptors

• Presence of hypoxia together with rise in pCO2

• Hypoxia

Medullary Respiratory Centers