bio005 chapter 16 respiratory system

Function of the Respiratory System

• Encompasses 3 related functions: ventilation, gas exchange, and O2 utilization (cellular respiration)

  • Ventilation moves air in and out of lungs for gas exchange with blood(external/pulmonary respiration)

  • Gas exchange between blood and tissues, and O2 use by tissues is internal/tissue respiration

• Gas exchange is passive via diffusion

Structure of the Respiratory System pt 1

• Air passes from mouth to trachea to right and left bronchi to bronchioles to terminal bronchioles to respiratory bronchioles to alveoli

Structure of the Respiratory System pt 2

• Gas exchange occurs only in respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli (= respiratory zone)

Structure of the Respiratory System pt 3

• All other structures are the conducting zone

• Alveoli are polyhedral in shape and clustered at the ends of respiratory bronchioles, like units of honeycombs.

Alveoli

• Gas exchange occurs across the 300 million alveoli (60-80 m2 total surface area)

  • Only 2 thin cells are between lung air and blood: 1 alveolar and 1 endothelial cells.

  • This is called the respiratory membrane.

Physical Aspects of Ventilation

• Ventilation results from pressure differences induced by changes in lung volumes

  • Air moves from higher to lower pressure

  • Compliance, elasticity, and surface tension of lungs influence ease of ventilation

Pneumothorax

• Pneumothorax is a collapsed lung (air in the intrapleural space)

• Cause: When atmospheric air invades the pleural cavity

• Treatment: Needle aspiration, or chest tube insertion to remove air in the pleural space.

Physical Aspects of Ventilation

• Compliance: distensibility/stretchiness

• Elasticity: tendency to recoil after distension

• Surface tension: the force directed inward which resists distension. Surfactant reduces surface tension which prevents alveoli from collapsing.

  • Ex: Neonates born before 33 weeks

Surfactant pt 1

• Premature neonates are often born with insufficient surfactant (= Respiratory Distress Syndrome or RDS)

  • Have trouble inflating lungs

• In adults, septic shock may cause Acute Respiratory Distress Syndrome (ARDS) which decreases compliance and surfactant secretion

Surfactant pt 2

• Surfactants are phospholipids and lecithin secreted by Type II alveolar cells found in the respiratory membrane

• Lowers surface tension by getting between H2O molecules, reducing their ability to attract each other via hydrogen bonding

Mechanics of Breathing

• Pulmonary ventilation consists of inspiration (= inhalation) and expiration (= exhalation)

  • Accomplished by alternately increasing and decreasing volumes of thorax and lungs

Quiet Breathing

• Inspiration occurs mainly because diaphragm contracts, increasing thoracic volume vertically (75%)

• External intercostal: contraction contributes a little by raising ribs, increasing thoracic volume laterally (25%)

• Expiration is due to passive recoil

Deep Breathing

• Inspiration involves contraction of extra muscles to elevate ribs: scalenes, pectoralis major/minor, and sternocleidomastoid muscles

• Expiration involves contraction of internal intercostals and abdominal muscles

Boyle’s Law

• Boyle’s law describes how pressure changes drive inhalation & exhalation

• The volume of a gas varies inversely with its pressure

Boyle’s Law

• Think of cylinder = lungs, think of piston = muscles

• This causes pressure changes and affects airflow

Pressure changes during respiration

• During inspiration, intrapulmonary pressure is -3 mm Hg pressure; during expiration is about +3 mm Hg

• Positive transmural pressure (intrapulmonary minus

  intrapleural pressure) keeps lungs inflated

Pneumothorax

• Pneumothorax is a collapsed lung.

• Cause: When atmospheric air invades the pleural cavity

• Treatment: Needle aspiration, or chest tube insertion to remove air in the pleural space.

Partial Pressure of Gases

• Partial pressure is pressure that a particular gas in a mixture exerts independently.

• Dalton’s Law states that total pressure of a gas mixture is the sum of partial pressures of each gas in the mixture.

• Atmospheric pressure at sea level is 760 mm Hg

  • PATM = PN2 + PO2 + PCO2 + PH2O = 760 mm Hg

Gas Exchange in Lungs

Is driven by differences in partial pressures of gases between alveoli and capillaries

Gas Exchange in Lungs

Is facilitated by large surface area of alveoli, short diffusion distance between alveolar air and capillaries, and tremendous density of capillaries

Partial Pressures of Gases in Blood

• When blood and alveolar air are at equilibrium the amount of O2 in blood reaches a maximum value

• Henry’s Law states that this value depends on solubility of O2 in blood (a constant), temperature of blood (a constant), and partial pressure of O2

• So the amount of O2 dissolved in blood depends directly on its partial pressure (PO2), which varies with altitude

Blood PO2 and PCO2 Measurements

• Provides good index of lung

  function

• At normal arterial blood has

  about PO2 = 100mmHg and

  PCO2 = 40 mmHg

• PO2 = 40 mmHg in systemic veins

• PCO2 = 46 mmHg in systemic veins

Brain Stem Respiratory Centers

• Automatic breathing is generated by a rhythmicity center in medulla oblongata

  • Consists of inspiratory neurons that drive inspiration and expiratory neurons that inhibit inspiratory neurons

    • Their activity varies in a reciprocal way and may be due to pacemaker neurons

Chemoreceptors

• Automatic breathing is influenced by activity of chemoreceptors that monitor blood PCO2, PO2, and pH

  • Central chemoreceptors are in medulla

• Peripheral chemoreceptors are in large arteries near heart (aortic bodies) and in carotids (carotid bodies)