• Lungs OH-Lungs OH-ADAM

• 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). 23374, 23380

• 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. OH-Bronchial Tree in Relation to the Lungs 23345, 23348

• 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. OH-36 Microcilia in Human Lungs 23352, 23353

• 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 OH-Histology of the Lungs

• The actual exchange of gases takes place in the alveoli which are clustered like grapes around the ends of the smallest bronchioles. 23360

• Each alveolus is about 0.1 or 0.2 mm in diameter and each is surrounded by capillaries. 23360, 23364

• 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 OH-Pulmonary Ventilation: Pressure Changes

• 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 OH-Hemoglobin

• 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: OH-T110- 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 OH-T-112 Chloride Shift

• 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 neuronswhen 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 OH-156 Respirometer Tracings of Lung Volumes and Capacities

• 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

• Nonrespiratory Air Movements

• Air movements that occur in addition to breathing are called nonrespiratory 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.

Read pages 778-780 Homeostatic Imbalances of the Respiratory System

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