• RESPIRATORY SYSTEM

• 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.

• Introduction OH-Organs of the Respiratory System 23286

• 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 OH-Respiratory Organs of the Head and Neck

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

• The nasal cavity is located inside the nose and joins the pharynx. 23312

• 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: OH-ADAM
  • Posterior half of the septum is bone (vomer and perpendicular plate of the ethmoid) and the anterior half is nasal cartilage. 23311

• 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 (See Figure 1).
  • Deep to each concha is a passage way called a meatus (See Figure 1).
    • 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.
• 10.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.
• 11.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.

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