Respiratory

Respiratory Physiology Joyce King, CNM, Ph.D.

Primary Function ❚ The exchange of gases between the environmental air and the blood. ❙ Ventilation ❙ Diffusion ❙ Perfusion

Nonrespiratory Functions of the Respiratory System Water loss and heat elimination Warms and humidifies inspired air Enhances venous return Contributes to normal acid-base balance ❚ Enables speech, singing, etc. ❚ ❚ ❚ ❚

Nonrespiratory Functions of the Respiratory System ❚ Defends against inhaled foreign matter. ❚ Removes, modifies, activates various materials passing through the pulmonary circulation. ❙ Inactivates prostaglandins ❙ Activates angiotensin II

❚ Nose - organ of smell

Structure of the Respiratory System ❚ Respiratory airways ❙ ❙ ❙ ❙ ❙

Nasal passages Pharynx Trachea Bronchi Bronchioles

Structure of the Respiratory System ❚ Lungs - left lung - 2 lobes; right lung - 3 lobes ❙ Alveoli ❘ ❘ ❘ ❘

Type I alveolar cells Type II alveolar cells Alveolar macrophages Pores of Kohn

Alveolar fluid lining with pulmonary surfactant Type II alveolar cell

Alveolar macrophage

Type I alveolar cell Interstitial fluid

Pulmonary capillary

Alveolus

Erythrocyte

Structure of the Respiratory System ❚ Lungs, contd. ❙ Pulmonary capillaries ❙ Elastic connective tissue ❙ Pleural sac

❚ Thorax

Respiratory Mechanics Air moves from an area of high pressure to an area of low pressure, following the pressure gradient.

Respiratory Mechanics ❚ Pressure considerations ❙ Atmospheric pressure ❙ Intra-alveolar pressure ❙ Intrapleural pressure

Major Inspiratory Muscles ❚ Quiet breathing ❙ Diaphragm ❙ External intercostal muscles

❚ Deeper inspirations ❙ Accessory inspiratory muscles

Expiration ❚ ❚ ❚ ❚

Inspiratory muscles relax Chest wall and lungs recoil Intra-alveolar pressure increases Air leaves the lungs

Expiration Expiration is normally a passive process. For more complete and rapid emptying: ❙ Abdominal muscles ❙ Internal intercostal muscles

Airway Resistance ❚ Bronchoconstriction parasympathetic stimulation ❚ Bronchodilation - sympathetic stimulation

Airway Resistance

Lung Elasticity ❚ Elastic recoil and compliance ❙ Connective tissue ❙ Alveolar surface tension

Pulmonary Surfactant ❚ Secreted by Type II alveolar cells ❚ Reduces alveolar surface tension ❙ Increased pulmonary compliance ❙ Reduces the lungs tendency to recoil

Energy Expenditure is Increased When: ❚ ❚ ❚ ❚

Pulmonary compliance is decreased Airway resistance is increased Elastic recoil is decreased There is need for increased ventilation

Lung Volumes ❚ Normal range: ❙ Male - 5.7 liters ❙ Female - 4.2 liters ❘ 500 ml of air are inspired and expired ❘ At end of quiet expiration, the lungs still contain 2,200 ml of air

Lung Volumes ❚ Gas exchange continues during expiration ❙ Maintains constant gas content ❙ Decreased energy utilization

Lung Volumes ❚ ❚ ❚ ❚ ❚

Tidal volume Residual volume Vital capacity Total lung capacity Forced expiratory volume in one second

Alveolar Ventilation ❚ ❚ ❚ ❚ ❚

Pulmonary ventilation Respiratory Rate Anatomical dead space Alveolar ventilation Alveolar dead space

Local Regulation Goal - Maximally match blood flow to airflow ↓Airflow in comparison to blood flow → ↑ CO2 → relaxation of airway → ↑ airflow ↑ blood flow in comparison to air flow → ↓ O2 in alveolus and surrounding tissues → vasoconstriction of

Gas Exchange ❚ Simple diffusion down partial pressure gradients

Partial Pressure The individual pressure exerted independently by a particular gas within a mixture of gases.

Partial Pressure The air we breath exerts a total atmospheric pressure of 760 mm Hg (at sea level) and is composed of: ❙ 79% Nitrogen - PN2 = 760 mm Hg x .79 600 mm Hg ❙ 21% Oxygen - PO2 = 760 mm Hg x .21 = 160 mm Hg ❙ The partial pressure of CO2 is negligible at 0.03 mm Hg

Amount of gas that dissolves in blood depends on: ❚ Solubility of the gas in blood ❚ Partial pressure of the gas The difference in partial pressure between pulmonary blood and alveolar air is known as a partial pressure gradient.

Alveolar Air ❚ Inspired air is humidified - water vapor ❚ Fresh inspired air mixes with the large volume of old air and the dead space at the end of each inspiration, less than 15% of the air in the alveoli is fresh air. The average alveolar PO2 is 100 mm Hg.

O2 and CO2 Exchange ❚ Only small fluctuations occur in alveolar PO2 throughout both inspiration and expiration. ❙ Small proportion of total air exchanged ❙ Oxygen rapidly moves down its partial pressure gradient

O2 and CO2 Exchange ❙ Pulmonary blood PO2 equilibrates with alveolar PO2 ❙ PO2 of arterial blood is fairly constant

O2 and CO2 Exchange – Alveoli Level ❚ Alveolar PCO2 also remains fairly constant at 40 mm Hg. ❚ Partial pressures of gases favor the movement of O2 from the alveoli into the blood and CO2 from the blood into the alveoli.

O2 and CO2 Exchange ❚ O2 uptake matches O2 use even when O2 consumption increases due to increased metabolism.

Rate of Diffusion ❚ Surface area ❚ Thickness of the membrane ❚ Diffusion coefficient of the particular gas.

Rate of Diffusion These factors are relatively constant under resting conditions, therefore the partial pressure is the primary factor that determines rate of exchange.

O2 and CO2 Exchange – Tissue Level ❚ Partial pressures of gases favor the movement of O2 from the blood into the adjacent cells and CO2 from the cells into the capillary blood.

Gas Transport ❚ Oxygen ❙ Dissolved oxygen (1.5%) ❙ Chemically bound to hemoglobin (98.5%)

The PO2 of the blood is a measurement only of the dissolved oxygen.

Hemoglobin ❚ Four polypeptide chains ❚ Four iron-containing heme groups that are able to combine with an O2 molecule. If Hb is carrying its maximum O2 load, it is considered to be fully saturated.

Oxyhemoglobin Dissociation Curve ❚ Plateau portion ❙ Range that exists at the pulmonary capillaries.

Note: Minimal reduction of oxygen transported until the PO2 falls below 60 mm Hg.

Oxyhemoglobin Dissociation Curve ❚ Steep portion of the curve ❙ Range that exists at the systemic capillaries

A small drop in systemic capillary PO2 can result in the release of large amounts of oxygen for the metabolically active cells.

Hemoglobin ❚ Storage for oxygen ❚ Maintains a low PO2 ❚ At the tissue level, as the PO2 falls the Hb unloads some of its stored oxygen.

Factors that Affect the Binding of Hemoglobin and Oxygen ❚ Carbon dioxide ❚ Acidity ❚ Temperature Bohr effect – Reduction in the affinity of hemoglobin for oxygen in response to an increase in blood carbon dioxide and a decrease in pH.

Gas Transport ❚ Carbon Dioxide ❙ ❙ ❙ ❙

Dissolved CO2 (10%) Carbamino hemoglobin (30%) Bicarbonate (60%) Chloride shift Haldane effect – the ability of hemoglobin to pick up CO2 and CO2generated H+.

Control of Respiration ❚ Neural control ❚ Chemical stimuli

Neural Control ❚ Respiratory control centers housed in the brain stem ❙ Inspiratory and expiratory neurons in the medullary respiratory center ❘ Dorsal respiratory group ❘ Ventral respiratory group ❘ Rostral ventromedial medulla

Neural Control ❚ Respiratory control centers housed in the brain stem, contd. ❙ Apneustic ❙ Pneumotaxic center

❚ Stretch receptors in the lung (Hering Breuer reflex) ❙ Prevents overinflation of the lung

Neural Control ❚ Ventilation is matched to the body’s needs for oxygen uptake and carbon dioxide removal ❙ Medullary respiratory center receives input ❙ Appropriate signals sent to motor neurons ❙ Rate and depth of ventilation adjusted

Chemical Stimuli ❚ Arterial PO2 ❚ Arterial PCO2 ❚ Arterial H+

Arterial PO2 ❚ Monitored by peripheral chemoreceptors ❚ PO2 must fall below 60 mm Hg to stimulate increased respiration ❚ Peripheral chemoreceptors respond to the PO2 and not the total oxygen content.

Arterial PCO2 ❚ Major regulator of ventilation under resting conditions. ❚ CO2 crosses the blood-brain barrier forming H+

Arterial PCO2 ❚ Central chemoreceptors are sensitive to changes in CO2-induced H+ in the brain ECF ❚ An elevation of H+ stimulates increased ventilation.

Arterial H+ ❚ Monitored by aortic and carotid body peripheral chemoreceptors ❚ Plays a role in adjusting ventilation in response to alterations in arterial H+ concentrations unrelated to fluctuations in PCO2

Every day it’s the same old thing: Breathe, breathe, breathe

Respiratory Questions Contraction of the abdominal muscles is important in: a. normal (quiet) inspiration b. forced (maximum) inspiration c. normal (quiet) expiration d. forced (maximum) expiration

Respiratory Questions Alveolar surfactant acts to increase pulmonary: a. surface tension b. compliance c. airway resistance d. blood flow

Respiratory Questions At the top of a 3000 meter high mountain, which of the following alveolar partial pressures would be expected to be lower than normal? a. PA-O2 b. water vapor c. PA-CO2 d. All of the above d. Only a and b

Respiratory Questions Compared with systemic arterial blood, systemic venous blood has a higher: a. oxygen content b. pH c. bicarbonate ion concentration d. hemoglobin concentration

Respiratory Questions An oxyhemoglobin saturation of systemic venous blood of 25% for a person at rest is: a. above normal b. below normal c. within the normal range

Respiratory Questions As blood passes through systemic capillaries, the enzyme carbonic anhydrase catalyzes: a. conversion of dissolved CO2 to carbonic acid b. conversion of carbonic acid to bicarbonate ion c. binding of carbon dioxide to hemoglobin, thus displacing oxygen d. binding of hydrogen ions to