Page 1

• Introduction

• Respiration has two meanings in biology.

  • At the cellular level, it refers to the O₂ requiring chemical reactions that take place in the mitochondria and are the chief source of energy in the eukaryotic cells.

  • At the level of the whole organism, it designates the process of taking in O₂ from the environment and returning CO₂ to it.

• O₂ consumption is directly related to energy expenditure.

  • Energy requirements are usually calculated by measuring O₂ intake or CO₂ release.

  • Energy expenditure at rest is known as basal metabolism.

• Diffusion and Air Pressure

• In every organism from amoeba to elephant, gas exchange--the exchange of O₂ and CO₂ between cells and the surrounding environment--takes place by diffusion.

  • Diffusion--the net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random movement.

• In describing gases, scientists speak of the pressure of a gas rather than its concentration.

  • At sea level, air exerts a pressure of one (1) atm. (15 lb/in²)

    • This pressure is enough to support a column of mercury 760 mm high.

  • Dalton's Law of Partial Pressure--The total pressure of a mixture of gases is sum of the pressures of the separate gases in the mixture.

    • The pressure of each gas is proportional to its concentration.

      • O₂ makes up 21% of the composition of dry air therefore 21% of the total air pressure or 160 mm of Hg results from the pressure of O₂--partial pressure of O₂--designated as pO₂--if H₂O is present then pO₂ =155 mm Hg.

• If a liquid containing no dissolved gases is exposed to air at atmospheric pressure, each of the gases in the air diffuses into the liquid until the partial pressure of each gas in the liquid is equal to the partial pressure of the gas in the air.

  • pO₂ of blood means the pressure of dry gas with which the dissolved O₂ in the blood is in equilibrium.

    • For example, blood with a pO₂ of 40 mm Hg would be in equilibrium with air in which the partial pressure of O₂ was 40 mm Hg.

    • If blood with a pO₂ of 40 mm Hg was exposed to the usual mixture of air, O₂ will move from the air to the blood until the pO₂ = 155 mm Hg.

• Conversely, if a liquid containing a dissolved gas is exposed to air in which the partial pressure of that gas is lower than the liquid, the gas will leave the liquid until the partial pressures of the air and the liquid are equal.

• In summary, gases move from a region of higher partial pressure to a region of lower partial pressure.

 

• Respiration in Humans--Some Principles

• In humans both diffusion and bulk flow move O₂ molecules between the external environment and actively metabolizing tissues.

• This movement occurs in four (4) stages:

  • . Movement by bulk flow of the O₂ containing air to a thin, moist epithelium close to small blood vessels in the lungs.

  • Diffusion of the O₂ across the epithelium into the blood.

  • Movement by bulk flow with the circulating blood to the tissues where it will be used.

  • Diffusion of the O₂ from the blood into the interstitial fluids, from which it diffuses into the individual cells.

• CO₂--produces in the tissue cells, follows the reverse path as it is eliminated from the body.

 

 

ANATOMY AND HISTOLOGY OF THE RESPIRATORY SYSTEM
 

• Introduction

 

 

• The respiratory system consists of the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.

• Upper respiratory tract refers to:

  • Nasal cavity, pharynx, and associated structures.

• Lower respiratory tract refers to:

  • Larynx, trachea, bronchi, and lungs

• Respiratory movements are accomplished by the diaphragm and the muscles of the thoracic wall.

 

 

 

• Nose and Nasal Cavity

 

• In humans, inspiration and expiration usually takes place through the nose.

 

 

• The nasal cavity is located inside the nose and joins the pharynx.

 

• External openings to the nasal cavity are the external nares or nostrils and the posterior openings from the nasal cavity into the pharynx are the internal nares or conchae.

• The anterior portion of the nasal cavity just inside the external nares is the vestibule.

• The nasal septum divides the nasal cavity into two (2) parts:  

  • Posterior half of the septum is bone (vomer and perpendicular plate of the ethmoid) and the anterior half is nasal cartilage.  

 

 

 

• Floor of the nasal cavity is the hard palate and the lateral wall is modified by the presence of three (3) bony ridges called conchae 

  • Deep to each concha is a passage way called a meatus.

    • Within the superior and middle meatus are openings from the various paranasal sinuses and the opening of the nasolacrimal duct is within the inferior meatus.

• Vestibule is lined with stratified squamous epithelial cells that are continuous with the stratified epithelia of the skin.

• Mucous membrane that lines the nasal cavity consists of pseudostratified ciliated columnar epithelium with goblet cells that secrete a thick layer of mucus.

• In the most superior part of the nasal cavity is the olfactory epithelium, which functions in the sense of smell.

• Air enters the nasal cavity through the external nares, and the vestibule is lined with hairs that trap some of the large particles of dust in the air.

  • Mucus also traps debris and the cilia on the surface of the mucus membrane sweep the mucus posteriorly to the pharynx where it is swallowed and eliminated by the digestive system.

• Air is also humidified by the addition of moisture from the mucous membrane and is warmed within the nasal cavity before it passes into the pharynx, preventing damage to the more delicate linings in the rest of the respiratory passages.

Page 2

• Pharynx

 

 

• Is the common opening of the digestive and respiratory systems.

  • Receives air from the nasal cavity and air, food and water from the mouth.

• Inferiorly, the pharynx leads to separate openings of the respiratory system (larynx) and digestive system (esophagus).

• Pharynx can be divided into three (3) regions:

  • Nasopharynx

  • Oropharynx

  • Laryngopharynx

• Nasopharynx

  • Is the superior region of the pharynx and extends from the external nares to the level of the uvula--a soft process that extends from the posterior edge of the soft palate.

  • Is lined with a mucous membrane.

  • Auditory tubes open here.

  • Posterior surface contains the pharyngeal tonsil that protects the body from infection.

• Oropharynx

  • Extends from the uvula to the epiglottis.

  • Oral cavity opens into the oropharynx through the fauces.

    • Food, drink, and air pass through the oropharynx.

  • Is lined with stratified squamous epithelium that provides protection against abrasion.

  • Two (2) sets of tonsils (palatine and lingual) are located near the fauces.

• Laryngopharynx

  • Extends from the tip of the epiglottis to the openings of the larynx and esophagus.

  • Is lined with squamous epithelium.

 

• Larynx 

 

 

• Consists of an outer casing of nine (9) cartilages connected to each other by muscles and ligaments.

  • Six (6) of the cartilages are paired and three (3) are unpaired.

 

• Largest and most superior of the cartilages is the thyroid cartilage or Adam's Apple.

• Most inferior cartilage is the unpaired cricoid cartilage which forms the base of the larynx on which the others rest.

• Third unpaired cartilage is the epiglottis.

  • Consists of elastic cartilage rather than hyaline.

  • During swallowing the epiglottis covers the opening of the larynx and prevents material from entering the larynx.

• Six (6) paired cartilages are stacked in two pillars between the cricoid and thyroid cartilages.

 

• Arytenoid cartilages--largest--most inferior.

• Corniculate cartilages--middle pair.

• Cuneiform cartilages--Most superior and smallest.

  • Two (2) pairs of ligaments extend from the anterior surface of the arytenoid cartilages to the posterior surface of the thyroid cartilage.

 

 

• Superior pair forms the vestibular folds or false vocal cords.

  • When the vestibular folds come together, they prevent air from coming from the lungs and prevent food and liquids from entering the larynx.

• Inferior pair of ligaments form the vocal cords or true vocal cords.

 

• The true vocal cords and the opening between them is called the glottis.

• The vestibular folds and the vocal cords are lined with stratified squamous epithelium.

• Remainder of the larynx is lined with pseudostratified ciliated columnar epithelium.

• Inflammation of the mucosal epithelium of the vocal cords is called laryngitis.

  • When speech is produced, air moving past the vocal cords causes them to vibrate producing sound.

• The greater the amplitude of the vibration, the louder the sound will be.

• Pitch is controlled by the frequency of the vibrations.

  • The cricoid cartilage and the arytenoid cartilages can be moved by various muscles to change the length of the vocal cords.

    • Higher pitched tones are produced when only the anterior portions of the cords vibrates.

• Progressively lower tones result when longer sections of the cords vibrate.

  • Males normally have lower voices than females because males usually have longer vocal cord.

  • The sound produced by the vibrating vocal cords is modified by the tongue, lips, and teeth to form words.

  • People with the larynx removed can produce sound by swallowing air and causing the esophagus to vibrate

 

• Trachea

 

 

• Is a membraneous tube that consists of dense regular connective and smooth muscle reinforced with 15-20 "C"-shaped pieces of cartilage.

  • Cartilages form the anterior and lateral sides.

    • Protect the trachea and maintain an open passageway for air.

• Posterior wall contains no cartilage and consists of a ligamentous membrane and smooth muscle which can alter the diameter of the trachea.

  • Esophagus lies immediately posterior to the cartilage-free posterior wall of the trachea.

• Trachea is lined with pseudostratified ciliated columnar epithelium that contains numerous goblet cells.

 

 

 

• Cilia propel mucus and foreign particles toward the larynx where they can enter the esophagus and be swallowed. 

 

 

 

 

• Bronchi

 

• Trachea divides into the right and left primary bronchi.

 

• Right bronchus is shorter and wider and is more vertical than the left bronchus.

• Primary bronchi extend from the mediastinum to the lungs.

• The lining of the bronchi is the same as the trachea and the bronchi are supported by "C"-shaped cartilage rings.

Page 3

• Lungs

 

 

 

 

• Are the principal organs of respiration and on a volume basis, they are one of the largest organs of the body.

• Each lung us conical in shape with its base resting on the diaphragm and its apex extending superiorly to a point approximately 2.5 cm superior to each clavicle.

• Right lung is larger than the left lung.

• Right lung has three (3) lobes and left lung two (2).

 

 

 

 

• Lobes are separated by deep prominent fissures on the surface of the lung.

  • Each lobe is divided into lobules that are separated from each other by connective tissue but the separations are not visible as surface fissures.

  • Because major blood vessels and bronchi do not cross the connective tissues, individual diseased lobules can be surgically removed.

  • Nine (9) lobules in the left lung and ten (10) lobules in the right lung.

• Primary bronchi divide into secondary bronchi as they enter their respective lungs. 

 

 

 

• Point of entry of the bronchi, vessels, and nerves is called the hilus of the lung.

  • The secondary bronchi, two (2) to the left lung and three (3) to the right lung, conduct air to each lung.

• Secondary bronchi give rise to tertiary bronchi which extend to the lobules.

• The bronchial tree continues to branch several times, finally giving rise to bronchioles.

  • Bronchioles also divide numerous times to become terminal bronchioles which then divide into respiratory bronchioles.

    • Each respiratory bronchiole divides to form alveolar ducts that end as clusters of air sacs called alveoli.

    • An alveolar sac is composed of two (2) or more alveoli that share a common opening.

  • The bronchi are lined with pseudostratified ciliated columnar epithelium.

 

 

 

 

• The bronchi, other than the primary bronchi are supported by small cartilage plates embedded in their walls rather than "C"-shaped rings.

• Farther into the respiratory tree, the cartilage becomes more and more sparse and smaller and smooth muscle becomes more abundant.

• The bronchioles, devoid of cartilage in their walls, are very small tubes one (1) mm or less in diameter.

  • Because of much smooth muscle and no cartilage in their walls, they can constrict if the smooth muscle contracts forcefully, which occurs during an asthma attack.

 

• Exchange of Gases

 

• The actual exchange of gases takes place in the alveoli which are clustered like grapes around the ends of the smallest bronchioles.

 

• Each alveolus is about 0.1 or 0.2 mm in diameter and each is surrounded by capillaries.

 

 

• The walls of the capillaries and of the alveoli each consist of a single layer of flattened squamous epithelial cells separated from one another by a thin basement membrane.

  • As a result, the barrier between the air in an alveolus and the blood in its capillaries is only about 0.5m.

• Gases are exchanged between the air and the blood by diffusion.

• A pair of human lungs has about 300 million alveoli, providing a respiratory surface of about 70 m².

• 12.The lungs are surrounded by a thin membrane known as the pleura which lines the thoracic cavity.

  • The pleura secretes a small amount of fluid that lubricates the surfaces so that they slide past one another as the lungs expand and contract.

  • Pleurisy is an inflammation of these membranes that causes them to secrete fluid that collects in the thoracic cavity.

 

• Mechanics of Respiration  

 

• Air flows into and out of the lungs when air pressure within the alveoli differs from the pressure of external air.

• When alveolar pressure is less than atmospheric pressure, air flows into the lungs, and inspiration occurs.

• The pressure in the lungs is varied by changes in the volume of the thoracic cavity.

  • These changes are brought about by the contraction and relaxation of the muscular diaphragm and the intercostal muscles. OH-Pulmonary Ventilation: Muscles of Inspiration and Expiration

    • Inhalation is accomplished by contracting the diaphragm, which flattens it and lengthens the thoracic cavity and by contracting the intercostal muscles that pull the rib cages up and out.

      • These movements enlarge the thoracic cavity, the pressure within it falls, and air moves into the lungs.

    • Air is forced out of the lungs as the muscles relax, reducing the volume of the chest cavity and increasing the pressure.

 

• Transport and Exchange of Gases--Hemoglobin and Its Function

 

• Oxygen is relatively insoluble in blood plasma; only about 0.3 mL of O₂ will dissolve in 100 mL of plasma. at normal atmospheric pressure.

• Hemoglobin is the respiratory pigment of humans.

• Hemoglobin is made up of four (4) subunits each of which comprises a heme unit and a polypeptide chain.

• The heme unit consists of a porphyrin ring with one atom of iron (Fe) at its center.

  • The Fe in each heme unit can unite with one molecule of O₂, thus each hemoglobin molecule can carry four molecules of O₂.

  • The O₂ molecules are added one at a time:

    • Hb₄ + O₂ Hb₄O₂

    • Hb₄O₂ + O₂ Hb₄O₄

    • Hb₄O₄ + O₂ Hb₄O₆

    • Hb₄O₆ + O₂ Hb₄O₈

  • The combination of the first subunit of Hb with O₂ increases the affinity of the second and oxygenation of the second increases the affinity of the third, etc.

• Whether O₂ combines with hemoglobin or is released from it depends on the pO₂ in the surrounding blood plasma.

• O₂ diffuses from the air into the alveolar capillaries.

  • In the capillaries where the pO₂ is high, most of the hemoglobin is combined with O₂.

  • In the tissues where the pO₂ is lower, O₂ is released from the hemoglobin molecules and diffuses into the tissues.

  • The system compensates automatically for the O₂ requirements of the tissues.

  • In adult humans: O₂ and CO₂ Diffusion Gradients

    • The pO₂ as the blood leaves the lungs is about 95 mm Hg.

      • At this pressure, the hemoglobin is saturated with O₂.

    • As the hemoglobin molecules move through the tissue capillaries, the pO₂ drops, and as it drops the oxygen bound to the hemoglobin molecules is given up.

 

• Carbon Dioxide Diffusion Gradients

 

• CO₂ is continually produced as a byproduct of cellular respiration and a diffusion gradient is established from tissue cells to the blood within the tissue capillaries.

  • Intracellular pCO₂ is about 46 mm Hg whereas that in the interstitial fluid is about 45 mm Hg.

  • At the arteriolar end of the tissue capillaries the pCO₂ is close to 40 mm Hg.

• After blood leaves the venous end of the capillaries, it is transported to the lungs.

 

• Transport of CO₂  

 

• CO₂ is transports in the blood in three (3) major ways.

  • Approximately 8% is transports as CO₂ dissolved in plasma.

  • Approximately 20% is transported in combination with blood proteins (including hemoglobin).

  • 72% is transports as HCO₃.

• Blood proteins that bind to CO₂ are called carbamino compounds.

  • The most abundant protein to bind to CO₂ is hemoglobin and when CO₂ is bound to hemoglobin, the combination is called carbaminohemoglobin.

    • The CO₂ binds to globin and each globin molecule can combine with a single CO₂ molecule.

• Hemoglobin that has released its O₂ binds more readily to CO₂ than hemoglobin that has O₂ bound to it--Haldane Effect.

  • In tissues, after hemoglobin has released O₂, the hemoglobin has an increased ability to pick up CO₂.

  • In the lungs, as hemoglobin binds to O₂. the hemoglobin more readily releases CO₂.

• CO₂ diffuses into red blood cells where some of the CO₂ binds to hemoglobin, but most of the CO₂ reacts with H₂O to form H₂CO₃, a reaction that is catalyzed by carbonic anhydrase inside the red blood cell.

  • The H₂CO₃ ionizes to form H and HCO₃ ions.

• As a result of the above reactions, a higher concentration of HCO₃ is inside the cell than outside, and the HCO₃ readily diffuses out of the red blood cells into the plasma.

  • In response to this movement of negatively charged ions out of the red blood cells, negatively charged Cl ions move into the red blood cells from the plasma maintaining the electrical balance inside and outside the red blood cells.

• The exchange of Cl ions for HCO₃ ions across the cells' membranes is called the chloride shift.

• The H formed by the ionization of H₂CO₃ bind to the hemoglobin of the red blood cells.

  • This prevents the H ions from leaving the cells and increasing the [H] in the plasma.

 

• Control of Respiration

 

• The rate and depth of respiration are controlled by respiratory neurons in the brainstem.

  • These neurons are responsible for normal breathing, which is rhythmic and automatic like the beating of the heart.

  • Unlike the beating of the heart, breathing may be brought under voluntary control within limits.

• The respiratory neurons in the brainstem activate motor neurons in the spinal cord causing the diaphragm and intercostal muscles to contract.

• In addition to their spontaneous activity, the respiratory neurons receive signals from receptors sensitive to CO₂, O₂, and H as well as receptors sensitive to the degree of stretch of the lungs and chest.

• Chemoreceptor cells located in the carotid arteries, which supply O₂ to the brain, signal the respiratory neurons when the concentration of O₂ decreases.

• The concentration of CO₂ and H is simultaneously monitored by centers in the brain and also by chemoreceptors in the carotid arteries.

 

• Respiratory Air Volumes  

 

• Tidal Volume--Volume of air moved in or out of the lungs during quiet breathing--about 500 mL.

• Inspiratory Reserve Volume--Volume that can be inhaled during forced breathing in addition to tidal volume--3000mL.

• Expiratory Reserve Volume--Volume that can be exhaled during forced breathing in addition to tidal volume--1100 mL.

• Vital Capacity--Maximum volume that can be exhaled after taking the deepest breath. VC = TV + IRV + ERV

• Residual Volume--Volume that remains in the lungs at all times--1200 mL.

• Total Lung Capacity--Total volume of air that the lungs can hold. TLC = VC + RV

 

• Non-respiratory Air Movements

 

• Air movements that occur in addition to breathing are called non-respiratory air movements.

• Cough

  • Involves taking a deep breath, closing the glottis, and forcing air upward from the lungs against the closure.

  • Then the glottis is suddenly opened and a blast of air is forced upward from the lower respiratory tract.

• Sneezing

  • Is much like a cough, but it clears the upper respiratory tract rather than the lower.

  • Is a reflex act that is usually initiated by a mild irritation in the lining of the nasal cavity and, in response, a blast of air is forced up through the glottis.

• Laughing

  • Involves taking a breath and releasing it in a series of short expirations.

• Crying

  • Consists of movements similar to laughing.

• Hiccup

  • Caused by a sudden inspiration due to a spasmodic contraction of the diaphragm while the glottis is closed.

  • Sound of a hiccup is created by air striking the vocal cords.

• Yawning

  • Thought to aid respiration by providing an occasional deep breath.

  • Believed to be caused by lower than usual oxygenated blood.