5.3: Ventilation and Perfusion

What are the three processes of pulmonary gas exchange?

Ventilation, perfusion, and diffusion

What is ventilation?

The flow of gases into and out of the alveoli of the lungs

What is perfusion?

The flow of blood in the pulmonary capillaries

What is diffusion?

The transfer of gases between the alveoli and pulmonary capillaries

What does ventilation depend on?

Airway resistance and lung compliance

What is perfusion responsible for?

Moving blood into tissues for gas exchange

Where does deoxygenated blood flow in the lungs?

Through the pulmonary arteries

Where does oxygenated blood go after the capillaries?

To the pulmonary veins

Where do pulmonary veins carry oxygenated blood?

To the left atrium

What is diffusion in the lungs?

Movement of gases across the alveolar-capillary membrane

What increases gas diffusion?

High concentrations of oxygen

What decreases gas diffusion?

Diseases that destroy lung tissue or thicken the alveolar-capillary membrane

Give examples of conditions that decrease diffusion.

Pulmonary edema and pneumonia

What must match for proper gas exchange?

Ventilation and perfusion

What is anatomic dead space?

Air that does not undergo gas exchange in the conducting airways

What is alveolar dead space?

Alveoli that are ventilated but not perfused

What is physiologic dead space?

The sum of anatomic dead space and alveolar dead space

How does physiologic dead space compare to anatomic dead space normally?

They are approximately the same

What happens to physiologic dead space in lung disease?

It increases

What is a shunt?

Blood moving from pulmonary circulation to systemic circulation without being oxygenated

What happens in an anatomic shunt?

Blood moves from venous to arterial side without passing through the lungs

What is an example cause of an anatomic shunt?

Congenital heart defects

What happens in a physiologic shunt?

Blood moves through unventilated parts of the lung

What does a physiologic shunt cause?

A mismatch of ventilation and perfusion

What conditions commonly cause a physiologic shunt?

Destructive lung disorder or heart failure

What does proper gas exchange depend on?

Equal amounts of air and blood entering the lungs

What causes a mismatch of ventilation and perfusion?

Dead air space and shunt

What characterizes a shunt in terms of ventilation and perfusion?

Perfusion without ventilation

What happens to the ventilation-perfusion ratio in a shunt?

It is low

What conditions are associated with shunt?

Atelectasis and airway obstruction

What characterizes dead air space?

Ventilation without perfusion

What happens to the ventilation-perfusion ratio in dead air space?

It is high

What condition is an example of dead air space?

Pulmonary embolism

What does a pulmonary embolism do in terms of perfusion?

Restricts blood flow to part of the lung

What type of diseases can impair both ventilation and perfusion?

Diseases like chronic obstructive lung disease

In what forms is oxygen transported in the blood?

Dissolved and bound to hemoglobin

In what forms is carbon dioxide transported in the blood?

Dissolved and bound to hemoglobin

What are arterial blood gases (ABGs) used to measure?

Partial pressure of oxygen (PO2) and carbon dioxide (PCO2)

Why are ABGs preferred over venous blood for gas exchange assessment?

Venous blood does not reflect lung gas exchange function

What is the normal pH range in ABGs?

7.35 - 7.45

What is the normal PCO2 range?

35 - 45 mm Hg

What is the normal HCO3 range?

22 - 26 mEq/L

What is the normal PO2 range?

80 - 100 mm Hg

What percentage of oxygen is transported by hemoglobin?

98-99%

What is oxyhemoglobin?

Hemoglobin bound to oxygen

What is the oxygen saturation when blood leaves the left side of the heart?

95-97%

What is the oxygen saturation in venous blood returning to the right heart?

About 75%

What percentage of oxygen is transported in dissolved form?

1-2%

Which form of oxygen can diffuse into cells?

The dissolved form

What is hemoglobin affinity?

The degree to which hemoglobin binds oxygen

How many oxygen molecules can one hemoglobin bind?

Four

What happens to hemoglobin after the first oxygen binds?

It changes shape to make binding easier

How does hemoglobin affinity change with saturation?

It increases as saturation increases

Why must hemoglobin affinity decrease?

To release oxygen to tissues

How does affinity change as oxygen is released?

It decreases with each release

What factors influence hemoglobin affinity for oxygen?

pH, carbon dioxide concentration, and body temperature

When does hemoglobin bind oxygen more readily?

High pH, low CO2, and low temperature

When does hemoglobin release oxygen more readily?

Low pH, high CO2, and high temperature

In what three forms is carbon dioxide transported in the blood?

Dissolved, attached to hemoglobin, and as bicarbonate

What percentage of CO2 is transported dissolved?

10%

What percentage of CO2 is transported bound to hemoglobin?

30%

What percentage of CO2 is transported as bicarbonate?

60%

What influences acid-base balance in relation to CO2?

The amount of dissolved CO2 and bicarbonate levels

How does CO2 solubility compare to oxygen?

CO2 is 20 times more soluble in plasma than oxygen

What happens to CO2 after it diffuses into red blood cells?

It combines with hemoglobin or forms carbonic acid

What reaction forms carbonic acid?

CO2 + H2O = H+ + HCO3-

What enzyme catalyzes the formation of carbonic acid?

Carbonic anhydrase

What does carbonic acid ionize into?

Bicarbonate (HCO3-) and hydrogen ions (H+)

What happens to the hydrogen ions formed?

They combine with hemoglobin

What happens to the bicarbonate ions formed?

They diffuse into plasma in exchange for a chloride ion

What is the chloride shift?

Bicarbonate leaving the red blood cell in exchange for chloride ions

What is formed when CO2 binds to hemoglobin?

Carbaminohemoglobin

What is the function of carbaminohemoglobin?

Transport CO2 from tissues to the lungs

Where does CO2 go after reaching the lungs?

Into the alveoli and then into the environment

How is breathing controlled?

By automatic and voluntary mechanisms

What controls automatic regulation of breathing?

Chemoreceptors and lung receptors

What do chemoreceptors monitor?

Blood levels of oxygen, carbon dioxide, and pH

What do chemoreceptors adjust?

Ventilation rates

What do lung receptors monitor?

Breathing patterns and lung function

What is voluntary regulation of breathing?

Temporary control of breathing during activities like speaking, singing, or holding breath

What type of receptors regulate breathing based on CO2 and oxygen levels?

Chemoreceptors

What are the two types of chemoreceptors?

Central and peripheral

Where are central chemoreceptors located?

In the brain surrounded by cerebrospinal fluid (CSF)

What do central chemoreceptors detect?

Changes in blood PCO2 levels

How does CO2 affect central chemoreceptors?

CO2 diffuses into CSF, releases hydrogen ions, and stimulates the receptors

What is the effect of increased PCO2 on ventilation?

A short-term increase in ventilation

What happens to central chemoreceptor response in chronic high PCO2 (COPD)?

The response is reduced or absent

What stimulates ventilation in COPD patients instead?

A decrease in blood PO2 levels

Where are peripheral chemoreceptors located?

In the carotid and aortic bodies

What do peripheral chemoreceptors monitor?

Arterial blood oxygen levels

When are peripheral chemoreceptors stimulated?

When PO2 falls below 60 mm Hg