Respiratory Physiology: Gases in the Bloodstream

Alveolar perfusion

Blood flow to alveoli for gas exchange

Alveolar ventilation

movement of air into and out of the alveoli

For gas exchange to be efficient, alveolar perfusion must ________ alveolar ventilation

match

Ventilation/perfusion matching

The coupling of blood flow with the amount of air reaching the alveoli

Alveolar ventilation/alveolar perfusion mismatch

occurs when airflow into the alveoli (ventilation, V) does not match blood flow through the pulmonary capillaries (perfusion, Q)

Alveolar ventilation/alveolar perfusion mismatch abrievation

V•A/Q• mismatch

What casues V•A/Q• mismatch?

factors that alter pressures in alveoli, pulmonary arteries, and pulmonary veins

V•A/Q• mismatch is the most common cause of ___________

hypoxemia

hypoxemia

reduced systemic pressure pressure of arterial oxygen

Two types of V•A/Q• mismatch:

1.) dead space ventilation

2.) right to left shunt

Dead space venilation

alveolar ventilation without perfusion; no movement of fresh gas from alveolus to blood

Dead space ventillation occrus with zone _____ blood flow

zone 1 (capillaries completely closed)

Right to left shunt

perfusion without alveolar ventilation; arterial blood does not come into contact with alveolus containing fresh gas

Right to left shunt will cause a build up of ________ and reduction of _______ in the blood

CO2; O2

Three scenarios that cause a right to left shunt:

1.) shunt from the right side of heart to left side

2.) shunt connecting a pulmonary artery with a pulmonary vein

3.) foreign body that is preventing alveolar ventilation

There are regional differences in V•A/Q• matching in the lungs as a result of ___________

gravity

Perfusion is better in the ________ portions of the lungs

Ventilation is better in the ________ portions of the lungs

lower

upper

There is is high V•A/Q• matching in the ________ portions of the lungs and low V•A/Q• matching in the __________ portions of the lungs

upper; lower

V•A/Q• matching is important to __________ control of gas exchange

homeostatic

In order to maintain homestatic control of gas exchange, the lung has two reflexes:

1.) hypoxic vasoconstriction

2.) hypocapnic bronchoconstriction

hypoxic vasoconstriction

low oxygen levels that trigger narrowing of blood vessels

Why does hypoxic vasoconstriction occur?

localized hypoventiliation (reduced airflow to alveoli) will reduce local partial pressure of alveolar O2; this causes vasoconstriction of the pulmonary arteriole supplying that alveolus

Purpose of hypoxic vasoconstriction

the constriction of the vessels with reduced airflow will prevent blood flow to under-ventiltaed alveoli; this helps redirect blood flow towards better ventilated regions of the lung

hypocapnic bronchoconstriction

low CO2 levels that trigger narrowing of airways due to no perfusion

CO2 is a natural bronchodialator or bronchoconstrictor?

bronchodialator

Why does hypocapnic bronchoconstriction occur?

-When a part of the lung is poorly perfused, less CO₂ is delivered to the alveoli

-as you continue to breathe, pressure of CO2 in the alveolus drops since no new CO2 is coming in

-the low CO₂ (hypocapnia) in that airway causes the smooth muscle to constrict

Purpose of hypocapnic bronchoconstriction

the constriction of the bronchiole with no perfusion will air flow to under-ventiltaed alveoli; this helps redirect air flow towards better ventilated regions of the lung

Oxygen is carried in the blood in two forms:

1.) combined with hemoglobin

2.) dissolved in plasma

Is most of the oxygen in the blood bound to hemoglobin or dissolved in plasma?

bound to hemoglobin

What is oxygen bound to hemoglobin measured as?

% Hb saturation

What is oxygen dissolved in plasma measured as?

PaO2

Each Hb molecule can exist in two forms:

1.) oxyhemoglobin

2.) deoxyhemoglobin

oxyhemoglobin

Hemoglobin combined with oxygen

deoxyhemoglobin

hemoglobin without oxygen

Structure of hemoglobin molecule

contaisn four protein subunits (2A and 2B)

Each subunit of hemoglobin contains a ________ group with one ion of ________

heme; iron

Purpose of iron ion on the heme group of hemoglobin

reversibly binds to one molecule of O2

Therefore, each Hb can carry ________ oxygen molecules

four

Graph that measure Hb/O2 loading is called..

oxy-hemoglobin dissociation curve

oxy-hemoglobin dissociation curve

the relationship between the extent of oxygen binding to hemoglobin and the PaO2

Why is the oxy-hemoglobin dissociation curve not a linear line?

because the relationship between HbO2 and PaO2 is not linear; reaction of the first subunit of Hb with O2 faciliates the reaction of the next with O2 (binding of one O2 on Hb will help the other O2 molecules bind)

The oxy-hemoglobin dissociation curve is STEEP between ____-______ mm HG PaO2

10-60

The oxy-hemoglobin dissociation curve is FLAT between ____-______ mm HG PaO2

70-100

Why does the oxy-hemoglobin dissociation curve flatten out at higher PaO2s?

because most of the O2 is already bound

The x-axis of a oxy-hemoglobin dissociation curve is ___________

The y-axis of a oxy-hemoglobin dissociation curve is __________

x-axis: PaO2

y-axis: % HbO2 saturation

What is the % HbO2 saturation in the tissues? In the lungs?

tissues: 75%

lungs: 100%

At low PO2, O2 and Hb are ___________ combining and dissociating

at higher a PO2 (greater than 60 PO2) Hb are ___________ combining and dissociating

rapidly; slowly

Below what PO2 does hypoxemia become concerning?

60 PO2

True or false: any oxygen bound to Hb does contribute to PaO2

false!! oxygen bound to Hb DOES NOT contribute to PaO2

What form of oxygen does contribute to PaO2?

oxygen dissolved in plasma

However, Hb does determine the amount of O2 that will __________

diffuse

Why does Hb determine the amount of O2 that will diffuse?

because it binds oxygen as soon as it enters the blood, keeping the blood PO₂ low and maintaining a strong diffusion gradient from alveoli → blood

Diffusion from the alveolus to the blood is governed only by HbO2 or dissolved O2?

dissolved O2

How much arterial oxygen is typically extracted by tissues?

25%

Hb sink

refers to hemoglobin acting like a sink that rapidly soaks up (binds) incoming oxygen, keeping blood PO₂ low and allowing more oxygen to diffuse from the alveoli

Pulmonary arterial blood has a PaO2 of _______ mmHg and ______% Hb saturation

40 mmHg

75% Hb saturation

What promotes the gradient for diffusion of oxygen from avleolus to the blood?

-Pulmonary arterial blood has PaO2 of 40 mmHg, while the alveolus has PAO2 of 105 mmHg

-this causes air to diffuse from the alveolus to the blood

As oxygen diffuses from the alveolus to the blood, it enters the blood bound to Hb or dissolved in plasma?

dissolved in plasma

Oxygen being bound to plasma in the blood after diffusion from the alveolus will then promote diffusion into an _______________

erythrocyte

Increased erythrocyte PO2 then promotes...

O2 binding to Hb

Because Hb will bind to oxygen as soon as it is dissolved in plasma...

this creates a diffusion gradient between the alveolus and blood vessels

In the alveolus, the PO2 is _________ than in the pulmonary arteries

In the tissues, the PO2 is __________ than in the systemic arteries

higher

lower

What promotes the gradient for diffusion of oxygen from tissues to the blood?

-systemic arterial blood has PaO2 of 95 mmHg, while the tissues have PO2 of 75 mmHg

-this causes oxygen to diffuse from blood to the tissues

Going from the blood to to the tissues, oxygen will diffuse from the _______ of the blood to the ________ ________ of the tissues

plasma; intersitial fluid

Diffusion of oxygen from the plasma to intersitial fluid of tissues will then promote diffusion into ________

cells

Diffusion into cells then reduces...

Partial pressure of intersitial fluid of tissues

The reduction of pressure of intersitial fluid of tissues after oxygen diffuses into cells promotes diffusion of...

more O2 from the blood into the tissues

Diffusion of O2 from plasma into the tissues will lower plasma PO2. This will promote __________ of HbO2

dissoication

Therefore, the overall consequence of O2 exchange in tissues is....

trasfer of O2 from Hb to plasma to interstitial fluid

At any given PO2, Hb saturation is modified by four things:

1.) blood PCO2

2.) blood [H+]

3.) blood temperature

4.) 2,3 diphosphoglycerate (DPG)

Increase in any of the factors that modify Hb saturation willl cause the dissociation curve to shift _________

right

If the dissociation curve is shifted to the right, this means that the affinity of Hb for oxygen has ___________

decreased

A decreased affinity of Hb for oxygen means that Hb is...

releasing oxygen

Why does Hb release oxygen with increased blood PCO2, [H+], temperature, and 2,3 diphosphoglycerate (DPG)?

these are all products of increased metabolism, and many metabolic pathways requires oxygen function

The total amount of oxygen in the blood depends on four things:

1.) amount of hemoglobin

2.) carrying capacity of hemoglobin

3.) PO2 in the blood

4.) shape of the O2 dissociation curve

The total carrying capacity of oxygen is equal to...

dissolved O2 + Hb bound O2

At PaO2 of 100 mmHg, CaO2 = ____ml O2 /_______mL blood

20 mL O2/ 100 mL blood

Therefore, since CaO2 = 20 mL O2/ 100 mL blood, there will be ___ mL of dissolved O2 and ____mL of oxygen bound Hb

0.30 mL of dissolved O2

19.70 mL of oxygen bound by Hb

Equation for total carrying capacity of oxygen (CaO2)

= 1.34 (Hb) (SO2) + 0.003 (PaO2)

Delivery of O2 to tissues must be sufficient enough to meet __________ needs of tissues

metabolic

Delivery of O2 to tissues is dependent on...

both carrying capacity of oxygen AND cardiac output

In the absence of Hb, cardiac output would need to be...

much larger in order to sustain life! This is why Hb is essential for deliver of Hb and prolonged existence in general

Which of these is the most common cause of hypoxemia?

A. hypoventilation

B. anatomical shunt

C. diffusion impairment

D. V/Q mismatch

E. decreased inspired oxygen

D. V/Q mismatch

Which of the following is the primary trigger for hypoxic vasoconstriction?

A. PaO2

B. PAO2

C. PvO2

D. PACO2

E. PaCO2

B. PAO2

True or False: With regard to overall O2 content of the blood, PaO2 is a much bigger contributor than SO2

false

Below what PO2 does hypoxemia become very concerning?

A. 90

B. 80

C. 70

D. 60

E. 50

D. 60

How much arterial oxygen is typically extracted by tissues?

A. 5%

B. 15%

C. 25%

D. 35%

E. 45%

C. 25%

Which of the following will cause a right shift of the OHDC?

A. alkalosis

B. decreased PCO2

C. hypothermia

D. increased 2,3 DPG

E. none of these cause a right shift

D. increased 2,3 DPG