Respiratory System Part II Chapter 22. Gas Exchange (Blood, Lungs, Tissues) As blood enters the lungs via pulmonary circuit, oxygen levels are low and.

Respiratory System

Part II Chapter 22

Gas Exchange (Blood, Lungs, Tissues) • As blood enters the lungs via pulmonary circuit, oxygen

levels are low and carbon dioxide levels are high

• In the alveoli oxygen is taken up and carbon dioxide is unloaded

• Oxygen is delivered to body tissues and cells

• Three factors influence the movement of these gases– (1). Partial pressure and gas solubilities– (2). Alveolar ventilation and pulmonary blood perfusion– (3). Structural aspects of respiratory membrane

Gas Laws and Properties of Gases • Gas exchange in the lungs and tissues takes place via simple diffusion

• We measure gases in partial pressures – Dalton’s Law of Partial Pressure states that the total pressure exerted by a

mixture of gases is the sum of the pressures exerted independently by each gas (it’s partial pressure) in the mixture.

• N and O (21%) account for 99% of the gas in the atmosphere; 0.5% carbon dioxide

• Total partial pressure of gases in atmosphere = 760 mmHg• PO2 = 159 mmHg (decreases at higher altitude = 110 mmHg)

– Henry’s Law states that when a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to it’s partial pressure

• The amount of gas that dissolves in a liquid is dependent upon the solubility of the gas and the temperature of the liquid.

Gas Solubility • Solubility of gases depends on:

– 1. Solubility of a particular gas in a liquid• Carbon dioxide most soluble in air than O2

– 2. Temperature of the liquid/Pressure• As Temperature rises, gas solubility decreases • As Pressure increases, gas solubility increases

• The composition of gases in the alveoli differs from that of the air (table 22.4)– Atm (mostly O2 and N2)– Alveoli – more CO2 and water vapor and less % of oxygen

• Oxygen moves into the blood (lower in alveoli); mixture of new gases in the conducting tubes

• P02 = 104; PCO2 = 40 mm Hg

• The differences in PP of the gases allows for gas exchange via diffusion

• Alveoli and gas pressure

Factors Influencing Movement of Gases• 1. Partial Pressures and Gas Solubility's

– Pressure difference across membranes allow movement

• 2. Ventilation-Perfusion Coupling– Ventilation (gas reaching the alveoli) must be almost at an equal

rate to perfusion (blood flow in the capillaries)

• 3. Membrane Thickness – 0.5 – 1.0 micron is optimal and efficient

Partial Pressure Gradients • Top: gradients promoting gas

exchange in the lungs

• Bottom: gas exchange at the tissue level

• Gases measured in partial pressures– PO2

– Movement is ‘down’ the concentration pressure gradient

• Gas Exchange • Gas Exchange II

Ventilation-Perfusion Coupling in the Lungs• Definitions

– Ventilation = gas that reaches the alveoli– Perfusion = blood flow in the pulmonary capillaries

• Autoregulatory controls (affecting the alveoli)– 1. If ventilation is poor (low PP of oxygen); terminal arteries contract

shunting blood to areas where PP oxygen is high – 2. If ventilation is high (high PP of oxygen); arterioles dilate which

increases blood flow in the pulmonary capillaries • PP of carbon dioxide affect bronchiole diameter

– 1. Areas where PP CO2 is high, bronchioles dilate so that it can be eliminated

• Results – 1. This balances ventilation with perfusion

• Poorly ventilated alveoli (low oxygen and high carbon dioxide), bronchioles dilate, and more air is brought in (more oxygen).

Ventilation and Perfusion Coupling

Oxygen Transport Most oxygen is transported via (1) hemoglobin and by (2) dissolved in plasma (1.5%)- low solubility in water

– Hemoglobin is very important in that it transports around 98% of the oxygen to the tissues

HbO2 (oxyhemoglobin) ------- HHb + O2 (deoxyhemoglobin)

– Oxygen gas is ‘loosely’ attached to hemoglobin

HHb + O2 ↔ HbO2 + H+ (oxyhemoglobin)

– Loading and unloading of hemoglobin– Right: lungs; Left: tissues

Rate at which Hb binds or releases oxygen depends on temperature, blood pH, PCO2, BPG (organic chemical) Influence of PO2 on hemoglobin saturation

– Oxygen-hemoglobin dissociation curve (22.20)

Oxygen-hemoglobin Saturation Curve

Influence of PO2 on hemoglobin loading and unloading

Influence of PO2 on Hemoglobin Saturation• During resting conditions, PO2 = 100 mm Hg (98%

saturation of Hb in arterial blood)– 100 ml of systemic blood contains 20 ml of oxygen (20 vol %)

• Blood flows into the systemic circuit (capillaries) - Hb saturation is 75%– 5ml of oxygen is released / 100 ml of blood – Oxygen content is now 15% volume

• Important points – 1. At PO2 mm Hg, Hb almost fully saturated with oxygen.

Increase in pressure causes very little increase in saturation. • Higher altitudes there is enough oxygen (sufficient Hb loading) when

PP oxygen is lower

Influence of PO2 on Hemoglobin Saturation

• Only around 25% of oxygen unloading occurs during one systemic circuit – High reserve of oxygen is left in the venous blood (venous

reserve) • Applications for exercise (more oxygen will dissociate from

hemoglobin during vigorous exercise and move into the tissues with out increasing respiratory rate or CO

Dissociation Curve and the Bohr Shift

Other Factors Influencing Hemoglobin SaturationTemperature, blood pH, PCO2 and BPG(2,3-bisphosphate) affect saturation at a given PO2 in blood

Cells metabolize glucose using oxygen and produce CO2 and H+ (lowers pH) levels in the capillary blood

– Occurs during cell respiration. More oxygen is needed by the cell to continue respiration

All factors modify hemoglobin structure-affects it’s affinity for oxygen

– Increase in all of the factors listed above will decrease Hb affinity for oxygen (curve to shift right)

• Declining pH and increasing PCO2 weakens the Hb bond which is called the Bohr Effect.

• Allows for oxygen to be unloaded when really needed

Decrease in factors causes higher affinity and curve shifts left – Increasing pH causes higher Hb affinity for oxygen

Carbon Monoxide and Hb Affinity

• Any inadequate delivery of oxygen to the tissues is called hypoxia.

• Hb saturation falls below 70%– 1. Anemic– 2. Ischemic – congestive heart failure blocks blood flow– 3. Histotoxic – cells cannot use oxygen due to poisons (CN) – 4. Hypoxemic – reduced air flow to lungs; CO poisoning

• Carbon monoxide poisoning – 200 times Hb affinity for CO compared to oxygen– Takes oxygen’s place – Cyanosis, headache and respiratory distress

Making Sense of CO2 Transport • Transported by following means

– (1). Blood plasma (7%)

– (2). Chemically bonded to hemoglobin (carbaminohemoblobin) (23%)

• CO2 + Hb ↔ HbCO2

– (3). HCO3- (70%)

• Bicarbonate ion in plasma • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

-

• Carbonic anhydrase in RBCs – Carbonic acid converted to H+ and binds to Hb

increasing the RELEASE OF OXYGEN!– Hb loading increases O2 unloading!

• HCO- is released into the plasma and onto the lungs – Cl- moves into the cell to account for the loss of the

negative ion (chloride shift)

Gas Transport

Haldane Effect • Lower PO2 and lower Hb saturation - the more CO2 can

be transported

• As CO2 enters systemic bloodstream, causes more oxygen to dissociate from Hb (Bohr effect) 22.22A– Allows for more CO2 to combine with Hb– HbO2 --------- O2 + Hb

• Opposite in pulmonary circulation– Hb saturated with oxygen, the H+ released combines with HCO3

-

----------- unloads carbon dioxide – O2 + HHb ---------- HbO2 + H+– H+ + HCO3

- --------- H2CO3 ----- CO2 + H2O unloaded in lungs 22.22b

Buffering pH Changes • Carbonic Acid – Bicarbonate Buffer System

• CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

• Points – Resists shifts in pH

– If concentration of H+ rises (pH falls) will be tied up by bicarbonate to form carbonic acid (weak acid- changes pH very little)

– Maintains pH of around 7.3

Neural Pathways • Medullary groups (control respiration)– 1. DRG (dorsally near root of CN IX)

• Integrate peripheral sensory input; modify rhythms of VGR

• Takes input from peripheral stretch and chemoreceptors and sends to the VGR

– VRG • Rhythm generators ‘drive’

respiration• VRG neurons fire which activate

respiratory motor neurons which cause inspiratory muscles to contract (inspiration)

• Expiratory neurons fire inhibiting the VRG and expiration occurs

Neural Pathways

• Pons– PGR (pontine respiratory group)– Sends impulses to the VRG of

the medulla– Helps to modify the rhythm of

the VRG (during sleep, voice, exercise)

Neural and Chemical Influences on Medullary Respiratory Centers

• Irritant Receptors– Mucus, dust constrict

bronchioles (-)

• Inflation Reflex (stretch)– Baroreceptors: lungs

over-inflate sends signals to medulla which causes decrease in inspiration (-)

• Chemical Factors – PCO2 ; PO2; pH

Chemical Factors • Chemoreceptors monitor gas levels and H+ levels in the blood

• 1. Effects of PCO2 (central chemoreceptors)– As carbon dioxide rises, pH falls in the cerebrospinal fluid --- increase in

respiration (hypercapnia)

• 2. Effects of PO2 (peripheral chemoreceptors) – Levels must drop below 60 mm Hg before it causes increased ventilation

• Will stimulate ventilation even if carbon dioxide levels are normal

• 3. Effects of pH– Falling pH levels is monitored by the peripheral chemoreceptors

• Causes increased breathing– Central chemoreceptors are not involved due to the lack of H+ diffusing

from the blood to the cerebrospinal fluid

Location of Chemoreceptors

Adjustments • Exercise

– Breathing becomes hyperpnia but not hyperventilation – Gas levels maintained (actually oxygen may increase slightly) – Higher Exercise induced ventilation caused by

• Psychic stimuli, proprioceptors relay stimuli to respiratory centers

• High Altitude – As oxygen levels drop above 8000 ft. carbon dioxide sensors become more sensitive – Low oxygen levels means increased ventilation– After 2 or 3 days you acclimatize or respiratory volume stabilizes at a level 2-3 L/min

higher than at sea level – Hb saturation is only 67% at 19,000 feet; but Hb unloads only 25% of oxygen so tissue

oxygen needs are met– If all out effort takes place then fatigue sets in. Kidneys will produce EPO and more

erythrocytes