The Pulmonary System

The Pulmonary System

Respiratory Physiology

I. Lung structure and function

A. Upper respiratory tract

1. Nasopharynx

a. Clean/filter

b. Warm and humidify inhaled air

c. Protective mucous lining

(1) Goblet cells: respond to local irritation

(2) Subepithelial gland tissue: vagal stimulation

2. Glottis

B. Lower respiratory tract

1. Trachea

a. Shape/stability

2. Lungs

a. Left: 2 lobes

b. Right: 3 lobes

3. Generations of bronchial tree

a. Transport/conducting airways

(1) 0 - trachea

(2) 1 - main stem bronchi

(3) 2 - lobar bronchi

(4) 3 - segmental bronchi

(5) 4-15 bronchioles

(a) cartilage support thru 15th generation

(6) 16 terminal bronchiole

(a) Decreased flow pressure (slow air flow allows fine dust particles to settle out here)

(b) Banded by smooth muscle

b. Respiratory zone

(1) 17-19 respiratory bronchioles

** At 17^th generation, first place to have exchange of CO2 and O2

(2) 20-22 alveolar ducts

(3) 23 alveoli

C. Alveoli (acinus: terminal respiratory gas exchange unit of the lung, composed of airways and alveoli distal to a terminal bronchiole)

1. Characteristics

a. Area: 50-100 square meters

b. Volume about 3000 ml.

c. Single layer of epithelium

2. Alveolar stability

a. Surface tension (= attraction between surfaces)

(1) force between liquid molecules that tend to decrease the surface area

(2) Changes with surface area

b. Surfactant - lowers surface tension

(1) Secreted by alveolar type II epithelial cells

(2) Bipolar molecules: hydrophobic at surface

(a) Repel like molecules

(3) Helps keep alveoli dry

(a) Surface tension reduces tissue hydrostatic pressure

(b) Surfactant reduces surface forces and prevents transudation of fluid

c. Interdependence of alveoli

D. Blood flow

1. Bronchial circulation

a. Nourishes bronchi and conducting airways

2. Pulmonary blood vessels

a. Receive total output from right heart

b. Low pressure system - highly distensible

(1) Mean pulmonary pressure = 15 mm Hg

3. Pulmonary capillaries

a. Dense network of short interconnected capillaries

b. Diameter 10 microns - about same as RBC

c. RBC transverses capillary in about 3/4 second

E. Innervation - Autonomic nervous system

1. Sympathetic

a. Bronchodilation

2. Parasympathetic

a. ACh neurotransmitter - bronchoconstriction

II. Ventilation

A. Lung volumes

1. Total lung capacity (TLC): Adult about 6-7 L

2. Vital capacity (VC): maximum expiration after maximum inspiration

3. Tidal volume (TV): Normal breathing, about 500 ml

4. Functional residual capacity (FRC): lung volume after normal exhalation

5. Residual volume (RV): remaining air volume after maximal exhalation

6. Minute volume (MVV): tidal volume X respiratory rate

7. Alveolar ventilation = (tidal volume - dead space volume) X respiratory rate

8. Anatomic dead space - conducting zone

9. Physiologic dead space

a. Non-perfused aerated alveoli

b. For normal young adult - anatomic dead space

B. Compliance of lung tissue - elastic, distensible tissue

1. Compliance = volume change/unit of pressure change

C. Airflow characteristics

1. Turbulent in trachea

2. Transitional: 1st-15th generation

3. Laminar flow in terminal bronchioles

a. short, small diameter, slow flow

4. Diffusion in alveolar ducts and alveoli

D. Regional differences in ventilation

1. Ventilation per unit volume

a. Greatest at dependent portion of lung

2. Changes with posture

III. Diffusion - gas transfer across alveolar walls

A. Fick’s law: factors

1. Diffusion through membrane is directly proportional to:

a. Cross-sectional area

b. Driving pressure (concentration gradient)

c. Gas coefficient (coefficient for characteristics of the medium): CO2 is 20X more soluble than O2

2. Diffusion through membrane is inversely proportional to:

a. thickness of wall/membrane

B. Concept of partial pressure

1. Atmospheric pressure = 760 mm Hg at sea level

2. Combination of gases in ambient air (inspired)

a. Nitrogen, 79%, Oxygen, 21%

3. Each gas behaves independently

4. Total pressure is sum of pressure of individual gases

5. Inhaled

a. Dry air humidified in nasopharynx

(1) water vapor partial pressure = 47 mm Hg

(2) 760-47=713 mm Hg dry air

b. Partial pressure of oxygen available

(1) 713 X .21 = 149 mm Hg

IV. Perfusion of the lungs

A. Characteristics of pulmonary vessels

1. Vessel walls

a. thin, highly distensible, little smooth muscle

2. Pathway

a. arteries follow bronchi to terminal bronchioles

b. form capillary bed

B. Distribution of pulmonary blood flow

1. Low pressure system (B/P 25/8 mm Hg)

2. Low pulmonary vascular resistance

a. Maintain against increased blood flow and/or pressure

b. Mechanism for maintaining low pressure

(1) Recruitment

(2) Distention

3. Affected by posture - hydrostatic pressure

a. Upright human lung: flow decreases linearly base to apex

(1) 25/8 mm Hg minimum to adequately perfuse apices

(2) horizontal human lung

(a) apical and basal flow similar

(b) increased flow to dependent region

4. Affected by exercise

a. Regional differences in flow decrease with exercise

V. Ventilation/Perfusion relationships

A. Normal balance under ideal conditions

1. Atmosphere pressure = 760 mm Hg

2. Saturated inspired air water vapor pressure = 47 mm Hg

3. Oxygen content 21% = 149 mm Hg

B. Alteration in ventilation

1. Hypoventilation

a. low alveolar ventilation = low alveolar oxygen = increased P_CO2

b. Etiology

(1) Drugs that depress CNS

(2) Damage to chest wall

(3) Paralysis of respiratory muscles

(4) Very high resistance to breathing

c. can abolish effect by administering oxygen

2. Hyperventilation

a. “Blow off” CO2

VI. Oxygen Transport

A. Dissolved

1. Amount dissolved is proportional to partial pressure of that gas (Henry’s Law)

2. For each mm Hg PO2 = 0.003 ml O2/100 ml. blood

a. Normal arterial blood PO2 contains 0.3 ml dissolved oxygen/100 ml blood

B. Bound to hemoglobin (heme = iron-porphyrin compound)

1. Oxygen forms easily reversible bond

a. Reduced Hgb is purple

b. Oxygen bound - red

2. O2 capacity

a. 1 Gm pure Hgb can combine with 1.34 ml oxygen

b. If Hgb = 15 Gm, Oxygen capacity = 20.1 ml/100 ml blood

3. O2 saturation Hgb = O2 combined with Hgb/O2 capacity X 100 (to get %)

a. O2 sat. arterial blood = 97.5% (SaO2)

b. O2 sat. venous blood = 75% (SvO2)

4. Oxygen content of blood

a. (1.34 X Hgb X Sat/100) + 0.003 PO2

b. Normal

(1.34 ml/g X 15 gm/dl Hgb X 97.5/100) + 0.003 ml/dl (100 mm Hg) = 19.9 ml oxygen in 100 ml of blood

c. Anemia with Hgb = 10 Gm

(1.34 X 10 gm Hgb X .975) + .3 = 13.36 ml. oxygen in 100 ml of blood

C. Oxygen dissociation curve

1. S shaped curve

a. amount oxygen carried by hemoglobin increases rapidly to PO2 50 mm Hg, then flattens

2. Physiologic advantage of curve

a. Flat upper portion: insures high concentration gradient for oxygen diffusion into pulmonary capillaries

b. Steep lower portion of curve: peripheral tissue can withdraw large amounts of oxygen for small drop in capillary PO2

3. Shifts in the oxygen dissociation curve

a. Right shift

(1) Increased unloading of oxygen at given PO2 - increases tissue O2 supply

(2) Etiology

(a) Increase in H+ concentration

(b) Increased PCO2

(d) Increased concentration of 2,3 diphosphoglycerate in RBCs

(3) Exercising muscle is acid, hypercarbic, hot and benefits from increased unloading of oxygen

2. Left shift

(a) Hangs on to O2; won’t give up to tissues

VII. Transport of carbon dioxide

A. Dissolved: 20X more soluble than O2

1. About 10% of CO2 from lung carried dissolved

B. Bicarbonate - bulk of transport

CO2 + H2O <------> H2CO3 <-----> H+ + HCO3-

CA = enzyme carbonic anhydrase

VIII. Acid-base balance

A. pH: What is it?

The negative logarithm of H+ concentration in moles/L

B. Normal values

1. pH: 7.35-7.45

2. PCO2: 35-45 mm Hg

3. HCO3: 22-26 mM/L

4. Base excess: -2 to +2 mmol/L

C. Compensated imbalance

1. Completely (fully) compensated: pH returned to normal level

a. will not overcorrect

2. Partially compensated: pH toward normal level

IX. Regulation of acid-base balance

A. Buffer system – Henderson-Hasselbalch equation (1 part acid to 20 parts base)

B. Respiratory contribution to acid-base balance

1. Rapid response to change of blood pH

a. with increased pH (alkaline) ---> lowered respiratory rate

b. with lower pH (acidic) ---> increased respiratory rate

C. Renal contribution to acid-base balance

1. Alter acid excretion/resorption/excretion

2. Alter bicarb (HCO3) resorption/excretion

3. Slow response to change of blood pH

a. with increased pH (alkaline) ---> decrease bicarb reabsorption

b. with lower pH (acidic) ---> increase bicarb reabsorption

D. Acidosis: pH < 7.35

1. Respiratory acidosis

a. Etiology: accumulation of volatile acids

b. Compensation

(1) Renal - rarely complete compensation

(a) excretion of H+

(b) reabsorption of more HCO3

2. Metabolic acidosis

a. Etiology: accumulation of nonvolatile acids

b. Compensation: respiratory

(1) increase respiratory rate, decreasing acids

E. Alkalosis: pH > 7.45

1. Respiratory alkalosis

a. Etiology: hyperventilation, iatrogenic

b. Compensation: Renal - delayed

(1) decrease bicarb resorption

(2) decrease H+ excretion

(3) nearly complete compensation

2. Metabolic alkalosis

a. Etiology: accumulation of excess base

(1) large amount of bicarb for indigestion

(2) loss of acids

b. Compensation:

(1) Respiratory - fast response

(a) increase H+ by decreasing resp. rate

(b) only partial at best

(2) Renal - delayed

(a) decreased excretion of H+ ions

F. Differentiating the cause of pH imbalance

1. Acute imbalance

a. Check pH

b. Determine acid/alkalosis for each component

c. Match up

d. Acidosis

(1) Respiratory origin: increased CO2, normal base excess

(2) Metabolic origin: decreased base excess, “normal” CO2

e. Alkalosis

(1) Respiratory origin: decreased CO2, normal base excess

(2) Metabolic origin: increased base excess, “normal” CO2

2. Compensated

a. Look at pH, CO2, and bicarb, then base excess

MENNONITE COLLEGE OF NURSING

Pathophysiological Bases of Health Deviation

Arterial Blood Gas Interpretation

For each of the following blood gas results, indicate whether the problem is metabolic acidosis, metabolic alkalosis, respiratory acidosis, or respiratory. Also note whether there is no compensation, partial compensation, or full compensation.

1. pH = 7.30 8. pH = 7.55

pCO₂ = 50 pCO₂ = 30

HCO₃ = 24 HCO₃ = 25

pO₂= 95 pO₂= 55

2. pH = 7.50 9. pH = 7.42

pCO₂ = 50 pCO₂ = 42

HCO₃ = 40 HCO₃ = 23

pO₂= 80 pO₂= 98

3. pH = 7.25 10. pH = 7.52

pCO₂ = 25 pCO₂ = 45

HCO₃ = 16 HCO₃ = 36

pO₂= 95 pO₂= 88

4. pH = 7.35 11. pH = 7.15

pCO₂ = 60 pCO₂ = 24

HCO₃ = 34 HCO₃ = 8

pO₂= 85 pO₂= 88

5. pH = 7.40 12. pH = 7.56

pCO₂ = 80 pCO₂ = 24

HCO₃ = 48 HCO₃ = 22

pO₂= 90 pO₂= 88

6. pH = 7.10 13. pH = 7.17

pCO₂ = 60 pCO₂ = 60

HCO₃ = 18 HCO₃ = 39

pO₂= 80 pO₂= 60

7. pH = 7.55 14. pH = 7.17

pCO₂ = 25 pCO₂ = 98

HCO₃ = 16 HCO₃ = 38

pO₂= 95 pO₂= 37

X. Muscles of respiration

A. Diaphragm - normally about 70% of work of breathing

1. Muscle: thin, dome-shaped skeletal muscle

a. insertion into lower ribs

2. Innervation: bilateral phrenic nerve from C3 to C5

3. Contraction

a. Force abdominal contents lower and forward

b. Lowers pressure in thorax to sub-atmospheric

4. Paradoxical movement

a. Etiology: paralyzed muscle, loss of innervation

b. Diaphragm moves up instead of down with inspiration

B. Intercostal muscles

1. External - muscles of inspiration

a. pivot ribs up and out

b. increase AP and lateral diameter of thorax

2. Internal - muscle of active exhalation

a. pull ribs down and in

C. Accessory muscles

1. Inspiratory - insert on pectoral girdle

a. Scalene - elevate 1st and 2nd ribs

b. Sternocleidomastoid - lift sternum

c. Pectoralis major and minor and small muscles in neck and head - lift upper thorax

2. Expiratory

a. Abdominal muscles for active exhalation

D. Normal exhalation is passive

1. Return of diaphragm to dome position in thorax

2. Elasticity of parenchyma

XI. Control of Ventilation

A. Basic elements of respiratory control system

1. Sensors - input

a. Central chemoreceptors

(1) respond to change in CSF pH

(a) H+ and HCO3 do not cross blood-brain barrier

(b) CO2 diffuses freely

(d) Primary control of breathing (PCO2 of arterial blood)

b. Peripheral chemoreceptors

(1) Carotid and aortic bodies

(2) respond to decrease in pH and O2 < 100 mm Hg

(3) respond to increase CO2

(4) rapid response to immediate change

c. Pulmonary stretch receptors

(1) may be in airway smooth muscle

(2) stimulation slows respirations

(a) increases expiration time

d. Irritant receptors

(1) Airway epithelium and upper airway

(2) Respond to chemical and mechanical stimulation

2. Central controller

a. Brainstem respiratory centers

(1) Poorly defined collection of neurons

(a) control periodic inspiration/expiration

(2) Medullary respiratory center

(a) Dorsal respiratory group

i) primarily inspiratory

(b) Ventral respiratory group

i) primarily expiratory

i) voluntary control of breathing within limits

3. Effectors

a. Respiratory muscles which cause ventilation

XII. Metabolic functions of the lung

A. Synthesis

1. phospholipids

a. dipalmitoyl phosphatidyl choline - surfactant

2. Protein

a. Elastin and collagen - structural framework

b. Immunoglobulins, especially IgA in mucus

3. Carbohydrate

a. mucopolysaccharides of mucus

B. Metabolism of vasoactive substances

1. Rationale - total volume of blood circulates thru lung

2. Activates

a. Angiotensin I to Angiotensin II

(1) Catalyzed by ACE (angiotensin converting enzyme)

3. Inactivates

a. Bradykinin - 80%

b. Serotonin: storage and uptake

c. Prostaglandins E1, E2, F2

(1) PGE2 important; helps constrict ductus arteriosus

d. Norepinephrine - 30%

C. Metabolism of bronchoactive substances

1. Production of leukotrienes

a. Cause bronchoconstriction

b. May be important in asthma

(1) Prostaglandins possibly involved also

Prototypical Health Problems

I. Respiratory tract infections

A. Upper respiratory infections

1. Common cold

B. Lower respiratory infections

1. Pneumonia

2. Bronchitis

3. Tuberculosis

II. Obstructive pulmonary disease

A. Asthma

B. Emphysema

III. Lung cancer

IV. Pediatrics

A. Croup

B. Cystic fibrosis