Resp 5 - Ventilation-Perfusion Relationship, Hb, Dissociation curve

Speed of gas diffusion in blood

Fast

Where does P02 of capillaries increase the most?

The first thrid of the capillary length

Use of PO2 increasing increasing in first third of capillary

In disease process, diffuison can continue for an extra length of time

What determines arterial levels of gases

Partial alveoli pressure of gas

Why is the pulmonary circulation low pressure

Respiratory membrane is extremely fragile

Effect of hypertension on respiratory membrane

Damages membrane, causing edema and influx of plasma/RBCs that

Why is the pulmonary circulatory system a low resistance systen

Radius of vessels is shorter and wider

R = 8nl/pir^4

Pulmonary circulation has high compliance vessels: what is the use?

- high number of arterioles with low resting tone

- Thin walls and paucity of smooth muscle can accept large amounts of blood

- Can dilate in response to modest increases in arterial pressure

What happens if alveoli capillaries are collapsible

If capillary pressure falls below alveolar pressure, the capillaries close off, diverting blood to other pulmonary capillary beds with higher pressures

Capillary system in lungs

Crates strong network within alveolar structures and that has the advantage of having a large respiratory membrane (large SA for exchange)

Ventilation perfusion

Inspired air has to be delivered to regions of the lung where the blood is going and vice versa

Ventilation/perfusion (V/q) ratio

Balance between the ventilation (brining O2 in/removing Co2 from alveoli) and perfusion (removing O2 from alveoli and adding CO2)

Role of the ventilation and perfusion ratio

Major factors that effect alveolar (and therefore arterial) levels of O2 and CO2

The greater the ventilation...

There is a large amount of air per unit time that enters the alveoli, which makes alveolar PO2 and PCO2 approach their respective atmospheric pressure

The greater perfusion...

the more closely the composition of local alveolar air will approach that of mixed-venous blood

Ventilation

Bringing O2 in, removing Co2 from alveoli

Perfusion

Removing O2 from alveoli and adding CO2

Optimal ventilation/perfusion ratios

PO2 - 100-105 mmHg

PCO2 - 40 mmhG

How to increase ventilation/perfusion ratio

Must be increase in ventilation, or decrease in diffusion

Where is a high ventilation/perfusion ratio seen

Collapsing of the lung capillaries, pleurisy, or other diseases

Also seen with alveolar or physiologic dead spaces

What causes an increased V/P ratio in alveoli

Alveoli are normally ventilated but there is very little gas exchange that occurs because no gas exchange occurs. Usually a pathological condition. Region is overventilated or under perfused

How does a alveolar dead space cause a increased V/P ratio

PO2 is increased because no O2 is passed to the lungs

PCO2 is decreased because no CO2 is delivered and diffuses from the capillary system to the alveoli

Anatomical dead volume

Conductive zone

What causes a low V/P ratio

Air way obstruction (collapsed bronchi or bronchioles)

How does an airway obstruction cause a low V/P ratio

no gas exchange between alveolar air and atmosphere. Causes decrease in PO2 and increase in PCO2

Airway obstruction shunt

Amount of blood that passes through airway obstruction - venous blood is not oxygenated and not available for gas exchange because of alveolar occlusion

Regional differences in lung perfusion and ventilation

There are regional differences in perfusion and lung ventilation - local ratio determines local alveolar PO2 and PC02

What causes regional diffeeences in lung perfusion and ventilarion

- Weight of lungs: increases pressure in bottom regions (makes interpleural pressure less negative, causing deflated alveoli)

Where does perfusion occur in an upright person

At the bottom of the lungs - weight increases pressure = deflated alveoli = can expand more

How to test changes in lung perfusion

§ injecting a patient with radioactive xenon

o Patient holds breath as soon as the radioactive material is injected to measure radioactivity in the different regions of the lung

Changing position effects lung ventilation AND perfusion

Is the ventilation-perfusion relationship in healthy lungs uniform?

No. Blood flow and alveolar ventilation are reduced in the apical lung compared to the basal

What causes the non-uniform relationship of ventilation-perfusion

Basal Ventilation-Perfusion is 0.6X the ideal

- Decreased PO2 level and increased PCO2 level compared to ideal

Apical Ventilation-Perfusion is 3X the ideal

- Increased PO2 level and decreased PCO2 level compared to ideal

Ventilation perfusion matching

Respiratory system can maintain appropriate ventilation and perfusion in different lung regions - allows for elimination of mismatch of ventilation and perfusion

What mechanisms limit the ventilation perfusion mismatch

Hemostatic mechanisms - mostly pulmonary capillaries response to low O2

Pulmonary hypoxic vasoconstriction

-blood is shunted to areas with better ventilation

-minimization of V/Q mismatch

How is ventilation and perfusion matched in a bronchoconsreiction

Bronchoconstriction makes diameter of airway smaller, leading to less ventilation (increasing alveolar PCO2, and lowering O2)

- Reduction in PO2 causes vasoconstriction of arterioles - reduces ventilation in response of reduced perfusion

- Blood is diverted to regions where ventilation is effective

How does the circulatory system react to decreased airflow to lung

1. Decreased airflow to a region of the lung

2. decreased alveolar pressure and decreased PO2

3. decreased partial pressure for oxygen at the level of the alveoli will influence the pulmonary blood PO2

4. decrease PO2 at the level of the arterial blood

5. vasoconstriction of the pulmonary vessels

6. vasoconstriction of the pulmonary vessels will decrease blood flow (local perfusion decreased to match a local decrease in ventilation)

7. blood will be diverted to regions where ventilation is improved or is still effective

local adaptive effect to this decreased airflow to a region of the lungs is to decrease blood flow

How does the respiratory system respond to decreased airflow to the lung

1. Decreased blood flow to region of the lung →

2. increase alveolar PO2 and decrease alveolar PCO2

3. bronchoconstriction

4. less air moves into the region that has decreased blood flow (local ventilation decreased to match local decrease in perfusion)

5. diversion of blood flow from a region that has low ventilation to a region that has high ventilation to match a local decrease in perfusion

2 ways oxygen is carried in the lungs

1. Dissolved (2%)

2. Combined with hemoglobin (98%)

Why is oxygen present in such low amounts in plasma compared to hemoglobin

Low solubility. The oxygen ends up diffusing into RBCs in the pulmoary capillaries

Dissolved O2 follows Henry's law - what does this mean?

O2 content is proportional to PO2 solubility

Composition of hemoglobin

Protein composed of 4 globin subunits (2 alpha, 2 beta) and 4 hemegroups

Structure of Heme group

Porphyrin ring structure containing ferrous iron for O2 to bind

How many oxygen molecules can bind to one hemoglobin

4. As there is 4 heme units

Deoxyhemoglobin

Hemoglobin without oxygen

What causes interaction between Oxygen and hemoglobin (O2 binding to Hb)

PO2

Oxygen dissociatation curve

Interaction between hemoglobin and arterial partial pressure of oxygen. Y axis is Hb saturaiton, X is PO2

Significance of 100 mmHg PO2 on dissociation curve

Value of PO2 in the blood when it exits the lung capillaries after gas exchange occurs. Hb saturation is 100% (systemic arterial)

Significance of 40 mmHg PO2 on dissociation curve

PO2 level in peripheral tissue where Hb saturation is 75%. (systemic venous) 40 mmHg and 100mmHg is the pressures where the amount of O2 unloaded in tissue capillaries

Physiological pressures of O2 at rest

between 40-100

When blood moves back into the venous system, what percentage of blood is oxygenated

70%

Oxygen capacity

The max amount of oxygen that can combine with hemoglobin

What factors effect oxygen capacity

- How much hemoglobin is in the blood

Hemoglobin saturation

Percentage of available hemoglobin binding sites that have O2 attached

= O2 combined with Hb/O2 capacity x 100

Does hemoglobin saturation tell us about oxygen capacity

No

At the level of the arterial blood where PO2 is 100 mmHg, hemoglobin saturation is about...

97.5%

At the level of the mixed venous blood were PO2 is about 40 mmHg, hemoglobin saturation is about

75%

What determines Hb saturation

PO2 (most important)

PCO2

Temperature

Why is the Hb dissociation curve sigmoidal

cooperative binding of hemoglobin

What is cooperative binding

When O2 binds to Heme, deforms the shape of associated globin chains from tense (T) to relaxed state. When one globin chain deforms, the other do and can bind with O2

Important features of the sigmoidal dissociaiton curve

Plateau between 60-100 mmHg

Steep portion between 10-60 mmHg

What causes Dissociation curve plateau

Caused by reduced alveolar PO2 (therefore arteriolar PO2)

Use dissociation curve plateau

Provides safety factor so that even a significant limitation of lung function still allows almost normal O2 saturation of Hb

- o Makes sure that the majority of the hemoglobin is bound to oxygen despite the fact that there are small changes in arterial PO2

2 regions of steep portion of dissociation curve

40-60 mmHg and 10-40 mmHg

Steep portion of dissociaiton curve (40-60 mmHg)

Unload large amounts of O2 with small decrease in PO2

Even at 40 mmHg pO2, Hemoglobin saturation is still 75%. Why

PO2 must be high in peripheral tissue capillary as this pressure drives diffusion of O2 from RBC to blood to cells and mitochondria

Since PO2 in peripehral tissue is on the steep portion of the curve, what is the effect of pH and temp?

pH and temperature enhance O2 unloading

Where is the steepest portion of the dissociaiton curve?

Between 1040 mmHg

In what situations would P02 go below 40 mmHg?

- High metabolic rate when peripheral tissues use O2

Effect of metabolic rate on tissue PO2

oncreases in metabolic rate cause further decrease in tissue PO2, which facilitates diffusion from plasma which leads to drop in plasma PO2, diffusion of O2 from RBC, drop in PO2 in RBC, additional dissociation of O2 from Hb

Thus excreting muscle can extract much more O2 from blood compared to resting conditions

Use of PO2 increasing increasing in first third of capillary