THE RESPIRATORY SYSTEM

 

OUTLINE

 

I. ANATOMY

            a. upper respiratory system

            b. lower respiratory system

II. PHYSIOLOGY

            a. pulmonary ventilation

                        a1. inspiration

                        a2. expiration

                        a3. air volume

            b. gas exchange

            c. gas transport

III. REGULATION

            1-neural

            2-other factors

 

I. ANATOMY

            Role of the respiratory system: to bring in O2 to the tissues and dispose of CO2

 

            a- Upper air way

Formed by the nose and pharynx. The nose projects away from the face, has two openings, the nares.  The septum is the wall separating the two nares.  The bony wall of the nose is covered with a moist ciliated epithelium.

Roles of the nose:

            -Warm, humidify, and filter air

            -Resonance box for voice

            -Smell

The pharynx is located behind the nasal and oral cavities.  The nasopharynx (behind the nasal cavity) is a passage way for air.  The oropharynx (behind the oral cavity) is a passage way for air and food.

            b- Lower airway

b1- Larynx-box formed by three pairs of large cartilages, the thyroid, epiglottic and cricoid cartilages, and three pairs of small cartilages, the arytenoid, corniculate and cuneiform cartilages.  The thyroid cartilage is the largest one, located in the front of the neck.  It is more developed in man where it forms the Adam's apple.  The thyroid gland is located in front of it.  The cricoid cartilage is a thick cartilaginous ring located below the thyroid cartilage and used as a landmark in the neck for performing tracheotomy.  The epiglottic cartilage has a flap shape, located at the entrance of the larynx.  During swallowing, this cartilage rises and closes the entrance, thus preventing food from entering the airway.  The 3 smaller pairs of cartilage are located in the back, where they close the larynx and also apply tension on the vocal cords.

 

   The vocal cords are folds of the membrane within the larynx.  There are 2 pairs: the upper folds are the false vocal cords, they can prevent foreign objects from entering the airway by closing.  The true vocal cords are located just below the false ones and produce the sounds of the voice when vibrating.  The space between the vocal cord is the glottis (or rima glottis).  The larynx is a passage way for air as well as the voice box.

 

b2-Trachea - formed with 20 C-rings of cartilage.  The back of the C-ring is closed by an elastic membrane which is against the esophagus.  The cartilage, once again, maintains the airway open. The mucosa inside the trachea is formed by a pseudostratified ciliated epithelium with mucous cells.  The mucus traps dusts with the cilia bringing it back at the airway entrance where it will be cough out or swallowed.  The lower trachea ends at the carina where it splits into the right and left primary bronchi.

b3- Bronchi- The right bronchus is wider and more vertical than the left one. When an old person aspirates food, it tends to go down into the right bronchus, and causes right lobe (aspiration) pneumonia.  The primary bronchi split into the secondary bronchi, 3 on the right sides (because of 3 lobes) and 2 on the left side (2 lobes).  They, in turn, split into smaller bronchi, the tertiary bronchi.  They eventually become the bronchioles which leads to the alveolar sac.  The airway from the trachea to bronchioles is referred as the bronchial tree. Cartilage and pseudostratified epithelium are present up to the beginning of the bronchioles.  Smooth muscles replace the cartilage in the bronchioles.  When these muscles contract inappropriately, they prevent expiration of air, thus asthma.  The pseudostratified epithelium is replaced by cuboidal epithelium. 

           

b4- Alveoli- Bronchioles ends at the alveolar sac, a grape like structure formed by a cluster of alveoli.  The wall of the alveoli is formed by a single cell layer, a simple squamous epithelium.  The cells of this epithelium (type I) allow gas exchange with the blood circulating in capillaries located against the alveoli.  A second type of cells (type II) is found in the alveolar wall. They secrete a substance, surfactant (phospholipids and lipoproteins), preventing alveolar wall collapse during expiration. Surfactant is secreted during pregnancy toward the 7 th month.  A baby born too prematurely is at risk for lung disease.  Surfactant can now be artificially synthesized and is given to premature babies.  This therapy has greatly increased the chances of survival of these babies.

b5- Lungs- Each lung is surrounded by a two fold membrane, the visceral pleura, attached to the lung, and the parietal pleura, attached to the ribs.  The space in between is the pleural space, containing a thin layer of fluid.  This fluid diminishes friction during breathing.  The negative pressure existing between these layers helps maintain the lungs expanded.

The right lung has 3 lobes, the left lungs 2 lobes.  The left lung is smaller due to the presence of the heart on the left side. The entrance of the bronchi, blood vessels and nerve into the lungs is called the hilus. The thoracic cavity (thus the lungs) are expanded by the diaphragm, intercostal muscles and others accessory muscles during breathing.  The lung tissue between the alveoli contains blood vessels, connective tissue rich in elastic fibers which help in lung recoil.

Interesting web site:  http://www.pennhealth.com/health_info/Surgery/main.html

 

         

 

     When the pleura is ruptured because of broken ribs, stabbing, gun shot wound, air rushes between the 2 pleural folds, the lung tissue recoils because of the elastic fibers.  The lungs collapse.  The presence of air in the pleural cavity is called a pneumothorax, the hemothorax being the presence of blood in the thorax, and a  pyothorax, the presence of pus in the pleural cavity (pus in the lung is lung abscess).  Pleurisy is an inflammation of the pleura. Atelectasis refers to alveoli collapse.

    Anatomic Relationship of Heart and Lungs

 

Great web site:  http://www.gonzaga.k12.nf.ca/academics/science/sci_page/biology/biology.html

 

 

       

II. PHYSIOLOGY

a-Ventilation

Boyle's Law: The pressure of a gas in a closed container is inversely proportional to the volume of the container.

http://www.grc.nasa.gov/WWW/K-12/airplane/aglussac.html

http://www.grc.nasa.gov/WWW/K-12/airplane/boyle.html

http://bengu-pc2.njit.edu/trp-chem/chemistry/Gases/gas.html

 

                

                 Boyle's law                                                                   Charle's law

 

Charles Law: The volume of a gas is directly proportional to its absolute temperature.

http://physiol.umin.jp/resp/index_e.html     (detailed explanations)

 

            a1. inspiration - When the diaphragm contracts, the volume of chest increases, thus inducing negative pressure in lungs (Boyle's law)- air rushes through the nose into the lungs to equilibrate the pressure, thus inspiration.  Additionally, the inspired air arrived in warm lungs, thus expands (Charle's law).

 

            a2. expiration- The muscles relax, so the chest cavity returns to its initial smaller shape.  The air within it has less space, thus the pressure increases (Boyle's law).  The air, moving from high to low pressure (as for inspiration) will move out, thus expiration.  In addition, elastic tissue within the lung helps in the recoil of the lungs and return to the original shape.

  

            a3. air volume

Tidal volume - normal breathing depth

Inspiratory reserve volume - breathing in as much air as you can, above normal inspiration

Expiratory reserve volume - exhale as much as you can, after normal expiration

Vital capacity - total volume of air in lungs without the residual volume added in

Residual volume - Volume of air remaining in the lungs after forced expiration

Total lung capacity - total air in lungs

Anatomical dead space - Volume of air present in the airway, from the nose to the beginning of the alveoli.

 

 

Diseases like COPD(Chronic Obstructive Lung Disease), emphysema, asthma destroy the lung tissue and decrease the vital capacity and/or increase the residual volume.

 

Henry's Law:  The quantity of a gas that will dissolve in a liquid is  proportional to the partial pressure of the gas and to its solubility.

     This law usually does not apply to persons always living at the same altitude.  Person diving in water are under greater than normal pressure.  O2 itself becomes toxic at great depth (so it must be mixed in lower than usual percentages).  Nitrogen also is toxic and triggers in  the diver a state of "drunkenness", nitrogen narcosis.  So it is replaced by helium at great depth.  In addition, if a diver comes back to the surface too quickly, the pressure decreases too quickly, nitrogen bubbles  form into the tissue and blood.  These bubbles block the capillaries and create numerous mini emboli, leading to decompression sickness (or the bends).

 

       b. Gas exchanges

            b1- external respiration - in alveoli

            internal respiration - in tissues - gas transports

 

Dalton's Law:  Each gas exerts its own pressure independently of other gases.

            The total pressure of the air in the atmosphere is equal to 760 mm Hg (Hg= mercury).  Air

             is composed of 79% of nitrogen, 21% of oxygen and 0.04% carbon dioxide.  Thus, the

      partial pressure of O2 will be pO2= 160 and CO2 pCO2= 0.3 mm Hg.

 

Gases cross the plasma membrane freely - diffusion affects the exchange since there is a difference in concentration (or pressure) between the 2 sides of the membrane.  The gases go from high to low concentration.

 

In alveoli, the air is a mixture of old and new air, so the pO2= 105 and pCO2 = 40.  Across the membrane, blood is circulating in capillaries.  The partial pressures in blood returning to the lungs is pO2= 40 and pCO2= 45.  Factors affecting gas exchange are:

                        1. The difference in partial pressure - This is driving diffusion across the

                             membranes. 

                        2. surface area for gas exchange - decreased surface area leads to decrease in gas

                            exchange.  Decreased surfaces occur in emphysema, COPD which leads to the

                            destruction of the alveolar sac.

                        3. diffusion distance - The greater the distance, the less gas exchange there is.

                            This occurs as a result of scar tissue due to COPD .., pulmonary edema.

                        4. rate and depth of breath - breathing rate is influenced by many factors

                            (high CO2, acidosis, alkalosis ...). Breathing depth must be greater than the

                             anatomical dead space; if not, the same air is moved up and down the airway

                             without being exchange.

      

 

                        External respiration (alveoli-blood)                                Internal respiration (blood-tissue)

            b2- Internal Respiration - in tissues - O2 is used by the tissues so its partial pressure is always less than in the capillaries bringing blood to the tissues. CO2 is a byproduct of metabolism so is always being formed.  Its partial pressure is always higher than in the capillaries.  Because of diffusion, O2 leaves the capillaries to go into the tissue and vice versa for the CO2.  The partial pressure in the venous blood is pO2= 40 and pCO2= 45.

 

     c- Gas transport

            c1- Oxygen     1. hemoglobin (oxyhemoglobin) - 98%

                                    2. dissolved in plasma - 2%

 

                Factors affecting O2 binding to hemoglobin:

                        1. pO2 - the higher the pressure the easier the binding to hemoglobin

                                    oxyhemoglobin - when O2 is being bound to hemoglobin: red - pink

                                    no oxygen on hemoglobin - bluish skin color - cyanosis

                        2. acidosis (or decreased pH) and  increased CO2 promotes O2 leaving

                                    hemoglobin (which is fortunate: pCO2 is high in the tissue when O2 is

                                    needed most --> Hb is induced to released its O2)

 

                        3. temperature - the higher the temperature, the easier it is for hemoglobin to release its oxygen.

                                                            (since temperatures are higher in tissue, O2 tends to leave Hb in tissue).

                        4. fetal hemoglobin - has a higher affinity for O2 than adult Hb.  So the fetus  "grabs" O2 from the mother.

   Carbon monoxide (CO) poisoning - CO binds to the oxygen binding site on Hb because  

                        hemoglobin has a greater affinity to CO (much higher).  When most of the O2 sites

                        are occupied by CO, the  person will be dying of CO poisoning, since no O2 can

                         be brought to the tissues.

 

Hypoxia - low level of O2 in blood. Several causes result in hypoxia:

            hypoxic hypoxia: Due to lack of available oxygen in the air ( or no air).  ex. high altitude, choking, drowning

            Anemic hypoxia:  There is enough oxygen in the air but it can not be picked up because there is not enough hemoglobin in the blood.  ex. anemia due to bleeding,

            Stagnant hypoxia: Air, hemoglobin are normal, but blood circulation is inadequate and is not enough to bring O2 to the tissue. ex: shock

            Histotoxic hypoxia: Air, hemoglobin, and circulation are normal, but an enzyme using O2 in the tissue is blocked by poison (ex: cyanide).

 

Cyanosis: lack of O2 in the blood à blood color.  Chronic cyanosis results in clubbing of the finger nails.

      

 

                c2- CO2  1. dissolved in plasma - 7%.  After CO2 leaves the tissue and enters the blood

                                     it dissolves into the plasma.

                            2. hemoglobin (carbaminohemoglobin) - 23% Some of the CO2 from the

                                     plasma enters the red blood cells where it binds to hemoglobin, on a

                                     different site than oxygen.

                            3. bicarbonate ions - 70%  Some of the CO2 from the plasma reacts with

                                     water to form carbonic acid, a weak acid which partially dissociates into

                                     hydrogen and bicarbonate ions.

                       

                        CO2  +  H2O ------> H2CO3 -------->  H+  +  HCO3

           

                   CO2 entering the red blood cell also reacts with an enzyme, carbonic

                              anhydrase, which accelerates the above reaction.  The newly formed

                              bicarbonate then leaves the red blood cell and enters the plasma.

http://www.bmb.psu.edu/courses/bisci004a/respir/b4respir.htm

 

III. REGULATION

            A- Neural regulation

We do not consciously breathe. Breathing is due to the spontaneous firing of a breathing center,   located in the medulla oblongata and called the medullary rhythmicity area.  This center induces spontaneous inspiration and expiration on a regular rhythm.  However, the need for oxygen varies with activity and the state of the body, so the breathing rate must adapt to these needs.  Sensing these, are 2 respiratory centers located in the pons, the apneustic area and the pneumotaxic area.  These 2 centers receive informations from the body, integrate them and in turn, influence the rate and depth of breathing of the medullary rhythmicity.  The nerve impulses from this center are sent to the diaphragm and intercostal nerves through the phrenic nerve.  This nerve exits the spinal cord between C2 and C4.  Therefore, a broken neck above C4 will result in an inability to breathe, leading to death or to a life on a respirator.

 

            B- Factors affecting breathing

    1- cortical influx - Though, emotions can influence the depth and rate of breath

    2- inflation reflex - Receptors located in the chest are stretched during inspiration and send

            message to the brain, leading to a cessation of inspiration.

    3- chemical regulation- An increased pCO2, a more acidotic pH or a decreased in pO2

           level lead to an increase in the breathing rate.  The chemoreceptors sensing these

            parameters are the same ones, already seen in heart rate regulation.

            pH and CO2 are sensed in the medulla oblongata, O2 is sensed in the aortic and carotid

            bodies

            hypercapnia - high level of CO2

            hypocapnia - low level of CO2

     4- proprioreceptor - stretch receptor - they sensed that the body is moving, thus there will be

             need of O2 soon after.  This response anticipates the future need of O2.

     5- stretch anal sphincter - stretched anus triggers an increase BR

     6- pain - increases BR

     7- temperature - increases BR