Unit II: Transport Breathing Mechanism Chapter 20 pp 736-759

Pulmonary Ventilation Breathing – repeating cycle of inspiration and expiration quiet respiration – at rest forced respiration – during exercise Flow of air in and out of lungs requires a pressure difference between air pressure within lungs and outside body

Pulmonary Ventilation Accessory Inspiratory Muscles Primary Inspiratory Muscle External intercostal muscles Sternocleido- mastoid muscle Expiratory Scalene muscles Pectoralis minor muscle Serratus anterior muscle Primary Muscle Diaphragm Internal intercostal Transversus thoracis muscle External oblique Rectus abdominis

Pressure and Flow Atmospheric pressure drives respiration 1 atmosphere (atm) = 760 mmHg Boyle’s Law: pressure is inversely proportional to volume for a given amount of gas, as volume , pressure  and as volume , pressure  Charles’s Law: Volume is directly proportional to its absolute temperature for a given amount of gas, if absolute temperature is doubled, so is the volume

Inspiration Phrenic nerves stimulate diaphragm to flatten and drop & intercostal muscles cause ribs to move slightly up and out  volume of thoracic cavity  intrapleural pressure visceral pleura clings to parietal pleura  lungs expand with visceral pleura -  intrapulmonary pressure net result: 757 mmHg Inflation aided by warming of inhaled air 500 ml of air flows with a quiet breath

Passive Expiration Expiration achieved by elasticity of thoracic cage After inspiration, phrenic nerves continue to stimulate diaphragm to produce a braking action to elastic recoil As volume of thoracic cavity , intrapulmonary pressure  and air is expelled During quiet breathing: +3mmHg During forced breathing: +30mmHg

Respiratory Cycle

Resistance to Airflow Diameter of bronchioles Surface Tension Thin film of water needed for gas exchange creates surface tension that would collapse alveoli Pulmonary surfactant (great alveolar cells) an agent that disrupts the H+ bonds of water decreases surface tension

Alveolar Ventilation Anatomical dead space conducting division of airway Physiological dead space sum of anatomical dead space + any pathological alveolar dead space Alveolar ventilation rate directly relevant to ability to exchange gases

Lung Volumes and Capacities

Neural Control of Breathing Breathing depends on repetitive stimuli from brain Controlled at 2 levels: Voluntary control provided by motor cortex Neurons in medulla oblongata and pons control unconscious breathing

Respiratory Control Centers Medulla oblongata: Ventral respiratory group (VRG) primary generator of respiratory rhythm Dorsal respiratory group (DRG) modifies respiratory rhythm Integrating center

Respiratory Control Centers Pons: Pontine respiratory group (PRG) or Pneumotaxic center regulates shift from inspiration to exhalation as frequency rises, breathe shorter and shallower

Respiratory Control Centers Quiet Breathing INHALATION (2 seconds) Diaphragm and external intercostal muscles contract and inhalation occurs. Activity in the VRG stimulates the inspiratory muscles. Neurons in the VRG become inactive. They remain quiet for the next 3 seconds and allow the inspiratory muscles to relax. intercostal muscles relax and passive exhalation EXHALATION (3 seconds) Forced Breathing INHALATION EXHALATION Inspiratory muscles contract, and expiratory muscles relax. Inhalation occurs. Increased activity in the DRG stimulates neurons of the VRG that in turn activate the accessory muscles involved in inhalation. The expiratory center of the VRG is inhibited. DRG and inspiratory center of VRG are inhibited. Expiratory is active. After each inhalation, active exhalation occurs as the neurons of the VRG stimulate the appropriate accessory muscles. Inspiratory muscles relax.

Air-Water Interface Gases diffuse down their pressure gradients Henry’s law amount of gas that dissolves in water is determined by its solubility in water and its partial pressure in air Temperature of liquid Example: Soda is put into the can under pressure, and the gas (carbon dioxide) is in solution at equilibrium. Open the can: internal pressure falls gas molecules come out of solution Volume difference is so great that within a half hour you are left with “flat” soda.

Systemic circuit Pulmonary capillary Interstitial fluid PO2 = 40 PCO2 = 45 PO2 = 95 PCO2 = 40 Internal Respiration Respiratory membrane Alveolus PO2 = 100 mm Hg PCO2 = 40 mm Hg PO2 = 100 External Respiration Alveolar Gas Exchange Exchange across the water film covering alveolar epithelium and respiratory membrane Factors affecting gas exchange: Pressure gradients of gases

Other Factors Affecting Gas Exchange Gas solubility CO2 20 times as soluble as O2 Membrane thickness - only 0.5 m thick Membrane surface area - 100 ml blood in alveolar capillaries, spread over 70 m2 Ventilation-perfusion coupling - areas of good ventilation need good perfusion (vasodilation)

Oxygen Transport Concentration in arterial blood 98.5% bound to hemoglobin 1.5% dissolved Binding to hemoglobin each heme group of 4 globin chains may bind O2 oxyhemoglobin (HbO2 ) deoxyhemoglobin (HHb) Protein subunits Iron ion Heme unit

Oxyhemoglobin Dissociation Curve Steep slope = a large change in amount of oxygen associated with Hb. Blood entering the systemic circuit has a PO2 of 95 mm Hg. Blood leaving peripheral tissues has an average PO2 of 40 mm Hg. The PO2 in active muscle tissue may drop to 15–20 mm Hg. Oxyhemoglobin (% saturation) PO2 (mm Hg)

Oxyhemoglobin Dissociation Curve Bohr Effect PO2 (mm Hg) Oxyhemoglobin (% saturation) pH increase = curve shifts to the left Hemoglobin releases less oxygen. pH decrease = curve shifts to the right Hemoglobin releases more oxygen. 7.4 7.6 7.2

Carbon Dioxide Transport H2CO3 H+ + HCO3– CO2 diffuses into the bloodstream. 93% diffuses into RBCs. ~7% remains in solution. ~23% CO2 is carried as Carbaminohemoglobin (HbCO2) ~70% is converted to carbonic acid by carbonic anhydrase Hydrogen ions bind to hemoglobin acting as pH buffers. Bicarbonate ions move into the surrounding plasma in exchange for extracellular chloride ions (Cl–). Chloride shift

Summary of Transport Alveolar air space Pulmonary capillary Systemic Plasma Red blood cell Chloride shift Cells in peripheral tissues CO2 delivery O2 delivery O2 pickup CO2 pickup