Respiratory System: Structure and Function

Question 1

Respiratory System Function

Answer 1

1. Respiratory Function: Gas transport for metabolism
2. Non-Respiratory Function: Flitering and Metabolism

Question 2

Gas Transport

Answer 2

1. Ventilation (atmosphere <-> respiratory zone)
2. Lung diffusion: (respiratory zone <->erythrocyte/plasma)
3. Circulation: (blood flow carries erthyrocyte/plasma <->tissue)
4. Tissue diffusion: (erythrocyte/plasma <-> tissue cells)
5. Internal respiration: (cellular metabolism using O2 & producingCO2)

Question 3

1. Ventilation: (atmosphere <-> respiratory zone)

Answer 3

Ventilation is the movement of bulk airflow from the surrounding atmosphere into our lungs and the bulk airflow out of our lungs back into the room. Ventilation acts to bring air from the atmosphere to an area called the respiratory zone of the lung. In the respiratory zone the gases can diffuse into and out of the blood. Failure of ventilation can occur due to both diseases of the lung such as asthma and emphysema, but also diseases of the muscular system, which is pumping the lung. Duchenne muscular dystrophy would be one of the most classic examples of this.

Question 4

2. Lung diffusion: (respiratory zone <->erythrocyte/plasma)

Answer 4

The respiratory zone is where oxygen moves across the alveolar membrane into our bloodstream and then into our red blood cells where it is carried by hemoglobin. It is also where carbon dioxide diffuses out of blood back into the alveolar gas so that it can be then ventilated out into the room. Lung diffusion can be disrupted by pulmonary edema following a heart attack or an autoimmune disease called idiopathic pulmonary fibrosis where the thin alveolar walls of the lung become scarred. This blocks gas exchange and leads to decreased oxygen in the blood.

Question 5

3. Circulation: (blood flow carries erthyrocyte/plasma <->tissue)

Answer 5

Circulation is obviously critical for carrying oxygen away from the lung to the tissues and then returning carbon dioxide to the lung for clearance. Failure of perfusion of the lung can be as fatal as failure of perfusion of the body. Examples of failure of lung perfusion include cardiovascular collapse (shock) and pulmonary embolus (a clot in the lung).

Question 6

4. Tissue diffusion: (erythrocyte/plasma <-> tissue cells)

Answer 6

Tissue diffusion is the passive movement of oxygen from the blood supply into the cells and then the return of carbon dioxide. This can be disrupted in cases of severe tissue edema or infection.

Question 7

5. Internal respiration: (cellular metabolism using O2 & producing CO2)

Answer 7

Clearly the delivery of oxygen and clearance of carbon dioxide would be irrelevant were it not for the body’s capability to metabolize oxygen and fuel substrates such as glucose to make energy. This mechanism can be damaged through poisoning such as cyanide or in cases of overwhelming infection called sepsis.

Question 8

Gas Conduction

Answer 8

1. Upper respiratory tract: Gas humidification, filtration, and warming
2. Conducting airways: Gas distribution to respiratory zone

Question 9

1. Upper respiratory tract: Gas humidification, filtration, andwarming

Answer 9

• The upper respiratory tract is the “swamp cooler” and “air filtering” system of the respiratory tract.
• Our nasal passages and upper airway effectively act to humidify dry air so that it is not damaging to the lower airways. Importantly this also warms the air further conditioning it so that there is less loss of body heat.
• Finally, the nose is very effective at filtering out large particulate matter from the air. This filtering, humidification, and warming all depend upon the turbulence or spinning of air in the nose. In fact, our nose is designed to be partly obstructed by the turbinates so that they can cause this spinning. This high resistance can be a problem when one gets a cold or has nasal allergies, but it does play an important role in clearing the air.

Question 10

2. Conducting airways: Gas distribution to respiratory zone

Answer 10

• The airways have irregular dichotomous branching with 20-28 branches of which first 16-17 are conducting only (the conducting zone).
• Note: The conducting zone only distributes and collects gas. There is no gas diffusion into or out of the body in the airways above the respiratory bronchioles.
• patency

-airways maintain their patency depending upon their structure

•cross sectional area

-increases dramatically as we move from the trachea to the respiratory zone

•airway clearance

-endogenous peptides “defensins”

Question 11

The Venitilatory Pump

Answer 11

1. Rib Cage and Spine
2. Diaphragm
3. Intercostal Muscles
4. Abdominal Muscles
5. Accessory Muscles (neck and shouldre girdle)
6. Visceral and Parietal Pleurae and Pleural Fluid

Question 12

Rib Cage and Spine

Answer 12

The rib cage and spine are the walls of the pump. Their stability is critical. Also, they help to increase the volume of the chest cage during inspiration thereby pulling gas from the room into our lungs. The lower ribs move upward like the handle on a bucket. The upper ribs seem to act like a handle on a pump and probable play their most important role in stabilizing the chest wall. Finally, the ribs and intercostal muscles also play an important role in body posture.

Question 13

Diaphragm

Answer 13

The diaphragm is a strong and efficient muscle. Newborn babies are capable of generating negative pressures in their chests of up to 70 centimeters of water on their first breath. Moreover the diaphragm is one of the last muscles to fail in cases of muscular dystrophy or electrolyte imbalance. It works day in and day out without complaining and generates most of the pressure change that pulls gas into the lung. In some young babies, the diaphragm may have a hernia in it and this outpouching of abdominal content into the lung can prevent normal development of the lung in the fetus and cause respiratory failure at birth.

Question 14

Intercostal Muscles

Answer 14

There are two layers in the intercostal muscles. The outer layer (external intercostal) is arranged so that it helps the chest wall expand. The inner layer (internal intercostals) appears to help with exhalation. It turns out that the most important role of these muscles maybe actually be in maintaining our posture. Indeed, the intercostals have reflexes that similar to the patellar reflex in their ability to unconsciously maintain posture.

Question 15

Abdominal Muscles

Answer 15

The abdominal muscles are muscles of expiration. Although expiration is normally passive; i.e. at the end of normal inspiration we simply relax and the lung recoils just like a rubber balloon does. This gives energy to the alveolar gas by compressing it, which then moves the gas out of the body. Normally, we do not actually use muscles to expire. In cases of significant lung disease or with vigorous exercise we may, however, use active expiration to help push gas out of the lung during tidal breathing. We also use abdominal muscles to talk loudly, to shout, or to sing. Finally, the abdominal muscles play a critical role in coughing. Contraction of the abdominal muscles compresses the gas in the chest and gives the alveolar gas the energy needed to cough. Individuals whose abdominal muscles are paralyzed or babies who are born without strong abdominal muscles can have a difficult time coughing and clearing their lungs.

Question 16

Accessory muscles (Neck and shoulder girdle)

Answer 16

The diaphragm is normally more than adequate to maintain respiration in healthy lungs. However, in the case of lung disease or vigorous exercise there may be a need to add to the inspiratory force developed by the diaphragm. One of the classic findings of accessory muscle use is that of ‘tripod sitting’. In this case the patient takes his elbows and hands and leans forward onto a table or desk to stabilize the shoulder girdle. This allows neck and shoulder muscles to act on the chest wall. In this case, many of the shoulder and back muscles can be used to help inspiration. It is a classic finding of severe respiratory distress or significant chronic respiratory disease. In milder disease, patients may simply use their sternocleidomastoid muscles to help stabilize the upper chest wall.

Question 17

Visceral and parietal pleurae and pleural fluid

Answer 17

The lung is not normally attached to the inside of the chest wall. Instead, the lung sits inside the chest cavity and is covered in a shiny, smooth material called pleura. The visceral pleura (covering the lung) does not have pain fibers. Normally the lung and the visceral pleura cannot feel pain. In contrast, the parietal pleura (covering the inside of the rib cage) is abundantly rich in pain fibers. This is the area in which pain develops when a patient has pleurisy (an infection in the pleural space). The pleural space is normally closed and the two pleurae are in apposition. However, the pleural space can become full of air if there is a hole in the lung or the chest wall (pneumothorax). The pleural space may also become full of fluid in the event of hemorrhage (hemothorax) or when there is an infection with the development of pus (empyema). Lastly, there normally is about 15 ml of pleural fluid acting as a lubricant between the two surfaces. This helps them slide on each other during respiration and aids in the transmission of the energy from the inspiratory muscles to the lung. (It should be noted that parietal pleura is the “shiny stuff” that one sees on the inside of ribs when visiting a BBQ Rib Joint.)

Question 18

Quiet breathing: Inspiration active and expiration passive

Answer 18

Under normal conditions, the diaphragm contracts and expands the volume of the thorax. This then decreases the pressure in the pleural space below atmospheric (negative pressure). The lung then expands and as the alveoli increase their volume they decompress the alveolar gas thereby dropping its pressure below atmospheric. This negative pressure in the alveoli sets up a pressure gradient across the airways and flow begins down the airways into the alveoli. At the end of the inspiration, the inspiratory muscles relax. This allows the lung to spontaneously recoil. The alveoli decrease their volume and compress the alveolar gas thus raising its pressure. Now the alveolar gas has a higher pressure than the atmosphere (positive). This creates a pressure gradient across the airways and flow begins up the airways to the atmosphere. This exhalation is passive and expiratory muscles are not used. The lung recoil is the source of energy that pushes the gas out of the chest.

Question 19

Exercise/Lung disease: Expiration may become active

Answer 19

If there is a need to move air out of the lungs rapidly as in exercise, coughing and loud vocalization then the expiratory muscles will be used to provide a boost to the lung’s recoil. The abdominal muscles and the internal intercostals will contract. This will further compress the gas in the alveoli, thus raising its pressure even higher and causing even higher expiratory flows. Expiration may also become active in obstructive lung disease where the airways are narrowed and offer a great deal of resistance to flow. In this case the lung’s recoil may be insufficient to complete exhalation in a timely manner. The expiratory muscles will then be used to provide the expiratory boost needed to overcome the increased airway resistance.

Question 20

Diffusion

Answer 20

1. Gas diffusion is PASSIVE from high partial pressure to low partial pressure.
2. Alveolar PO2 > Pulmonarycapillary PO2: Thus O2 diffuses to capillary.
3. Pulmonary capillary PCO2 >Alveolar PCO2: Thus CO2 diffuses to alveolar gas.
4. FICK’S LAW OF DIFFUSION applied to gases in the lung:

Flow of gas = Area x 1/(mol wt)0.5 x Solubility x Driving Pressure /Thickness of membrane

The lung has evolved very efficiently to maximize gas diffusion according to Fick’s Law of Diffusion. Although we cannot control the molecular weight or solubility of a given gas our lungs are designed to maintain the pressure gradients for oxygen and carbon dioxide. We also have a large diffusive area and a very thin alveolar membrane.

Question 21

The Respiratory Zone

Answer 21

1. Respiratory bronchioles, alveolar ducts and alveoli.
2. Designed to maximaize efficient gas exchange by diffusion.

Question 22

The Respiratory Zone: Respiratory Bronchioles, Alveolar Ducts and Alveoli

Answer 22

These are the structures in the lung that make up the respiratory zone. The respiratory zone is defined as the zone of the lung that is able to conduct gas exchange. Respiratory bronchioles are very small airways that have alveoli in their walls and thus they can conduct gas exchange. Remember the conducting airways do not exchange gas to any significant degree.

Question 23

The Respiratory Zone: The zone is designed to maximize efficient gas exchange by diffusion

Answer 23

•Large surface area:

-70-85 m2 (300 millionalveoli) It is quite remarkable to imagine that we have a surface area for gas exchange inside our chest roughly equal to the surface area of a tennis court. This is in a unique honeycomb-like structure made up of small air sacs called alveoli. Any given alveolus is only 0.3 mm in diameter and there are over 300 million of them in the adult.

•Large volume of gas to maintain diffusion pressure gradient (150 ml in the conduction zone vs 4,000 ml in the respiratory zone)

-we are able to deliver gas in roughly 150 mls or 5 ounces of volume to a surface area the size of a tennis court which encloses a volume of approximately 4000 ml or 4.2 quarts. This enables us to walk around with a large amount of oxygen at all times in our chest. Thus, we do not turn blue when we exhale and pink when we inhale, in fact we are able to maintain our oxygen pressures in our blood within 3% over a tidal breath. This large volume that is left in our lungs at the end of each exhalation is called the functional residual capacity (FRC). It acts as a reservoir of oxygen for continuous delivery to the body and a sump to continuously remove carbon dioxide from the body.

•Very thin membrane (0.5 micron)

-Having a large alveolar surface area would be of no use if the membrane separating the blood from the air was not very thin. In the case of the lung it is remarkably thin with a thickness of only 0.5-micron. To put this into perspective it is 1/14th the diameter of a red blood cell. Since the air in the alveoli is topologically speaking on the outside of the body, there is literally a tennis court folded up inside the chest and the only thing on that surface keeping the blood inside is a membrane that is so thin it would be invisible to the human eye.

Question 24

Alveolar Blood-Gas Barrier

Answer 24

1. Respiratory Epithelium
2. Interstitial Space
3. Capillary Endothelium
4. Plasma
5. Erythrocyte

Question 25

Pulmonary Blood Flow

Answer 25

•low pressure. 1/17 of that in systemic arterial circulation

1. All of cardiac output (≅ 95%) passes through the respiratoryzone.
2. Pulmonary arteries carry blood from the right heart (de-oxygenated)
3. Pulmonary arteries distribute blood to the pulmonary capillary system to maximize erythrocyte exposure to alveolar gastensions.
4. The churning/pumping action of the heart aids in gas diffusion in the respiratory zone.
5. The bronchial vessels arise from the aorta and carry nutrient blood tothe airways.

Question 26

Non-Respiratory Function of the Lung

Answer 26

1. Maintenance and defense
2. Filtering
3. Chemical processing

Question 27

Non-Respiratory Function of the Lung: Maintenance and Defense

Answer 27

The lung must keep itself clean and sterile and this requires both cleansing mechanisms as well as the innate and adaptive immune systems. At the same time, the lung is turning over its epithelia and remodeling itself relatively frequently. Indeed one of the striking characteristics of the lung is just how difficult it is to permanently scar. Pneumonia may cause a lobe of the lung to become entirely solid with the consistency of liver. Yet, in a few months, the lung will be completely aerated and normal. In other words it will have remodeled itself to return to its normal healthy state. Finally, the lung has to produce surfactant to maintain alveolar compliance.

Question 28

Non-Respiratory Function of the Lung: Filtering

Answer 28

The capillaries are so small that only one red cell can pass through at a time. Since all the pulmonary flow goes through these capillaries, they are effective at filtering out physical material such as blood clots, clumped white cells with bacteria, air, and foreign bodies. If this filtering function is bypassed due to congenital heart disease then there is great danger of these materials bypassing the lung and getting into the cerebral circulation where they can cause a stroke or infection.

Question 29

Non-Respiratory Function of the Lung: Chemical Processing

Answer 29

The lung is well situated to be metabolically active because it contacts virtually all the cardiac output before it returns to the left heart to be sent to the body. This is a brief summary of its metabolic activities:

•Production of hormones: ACTH, Prostaglandins, Vaso-active peptides, growth factors, serotonin and others

Arachidonic acid metabolites are very potent agents with marked effects upon smooth muscle and inflammatory cells:

i. Damage to the pulmonary endothelium releases ARACHIDONIC acid from the phospholipid cell membrane (Phospholipase A).
ii. Lipoxygenase pathway takes arachidonate to leukotrienes.
iii. Cyclooxygenase pathway takes arachidonate to endoperoxides.
iv. These substances have effects in the lung and elsewhere in the body.

• Clearance function: Serotonin, nor-epinephrine, bradykinin, prostaglandins.
• Transformation: Angiotensin I to Angiotensin II by angiotensin converting enzyme (ACE)