3-2 Pulmonary Circulation

Blood supply to the lungs comes from what 2 sources

bronchial artery system

pulmonary artery system

Unoxygenated blood leaves the… by..

right ventricle by way of the pulmonary artery trunk, which branches into the right and left pulmonary arteries

where does gas exchange occur

alveolar capillary membrane

the pulmonary arteries further divide into smaller arteries and arterioles that feed into the capillary network

pulmonary artery blood vs blood from pulmonary veins

arteries- unoxygenated

veins- oxygenated

pulmonary and bronchial circulation fxns

facilitates gas exchange

delivers nutrients to lung tissues

acts as blood reservoir for the left ventricle

pulm circ has lower pressure (18) than systemic circ (18)

only 1/3 of vessels are filled with blood at any given time

Pulm circulation pressure and volume

it’s a low pressure, high volume system

what 2 systems do pulm circ use to lower pulmonary pressure

in the setting of increased blood flow

1. recruitment - opening of these closed vessels

2. vessel distention, which allows more blood to circulate through. This dynamic system allows the lungs to efficiently match blood flow with ventilation and oxygen needs, ensuring optimal gas exchange under varying conditions.

bronchial circulation

part of the systemic circulation

received 1% of the cardiac output

does not participate in gas exchange

Under normal resting conditions:

During rest:

When the body requires more oxygen (e.g., during exercise):

some pulmonary capillaries are closed and not perfused.

only a fraction of the pulmonary capillaries are needed to oxygenate the blood, so many remain closed.

increased cardiac output raises pulmonary blood pressure, which recruits (opens) these previously closed capillaries to allow for greater gas exchange

Alveolar capillary membrane is

normal pulmonary arterial pressure in healthy adult vs arterial pressure of systemic circulation

very thin (the width of a single cell), crucial for efficient gas exchange

pulmonary: 22/8 systemic: 120/80

Ventilation:

movement of air into and out of the lungs

Distribute air within lungs to alveoli for maintenance of oxygenated and removal of CO2

carried by pulm system

Diffusion:

movement of gases between air spaced in the lungs and the bloodstream

carried by pulm system

Perfusion

movement of blood into and out of the capillary beds of the lungs to body organs and tissues

carried by cardiovascular system

Dead space

3 dimensions:

Some air stays within the airways and does not participate in gas exchange

anatomic dead space: includes gas in the conducting airways

Alveolar dead space: composed of ventilated but unperfused areas of the lung (portion of the air…)

Physiologic dead space: sum of the anatomic and alveolar dead space

Minute Ventilation

the total volume of air that is inhaled or exhaled from the lungs in one minute

product of tidal volume xRR per minute

can help determine whether a pt can or not come off the vent

Alveolar ventilation

the difference between tidal volume and anatomic dead space volume multiplied by RR per minute, useful when looking at integrity of alveolar capillary membrane on PFTs

4 steps of gas transport for oxygen delivery

1. 1. ventilation of lung

2. 2. diffusion of oxygen from alveoli into capillary bed

3. 3. perfusion of systemic capillaries w oxygenated blood

4. 4. diffusion of oxygen from systemic capillaries into the cell

4 steps in reverse order for removal of CO2

1. 1. Diffusion of CO2 from cells into systemic capillaries

2. 2. Perfusion of pulm capillary bed by venous blood

3. 3. Diffusion of CO2 into the alveoli

4. 4. Removal of CO2 from the lungs by ventilation

Diffusion

movement of gases btw air spaced in lungs and bloodstream

Effective gas exchange:

needs ~ even distribution of ventilation(o2) and blood (perfusion) in all portions of lungs

HOWEVER distribution is uneven

why is distribution of ventilation and perfusion uneven

• - Gravity and alveolar pressure affect distribution of blood and diffusion (depend on body position)

• - Ventilation perfusion ratio (perfusion > ventilations in base of lung) (vent > perfusion in apices of lungs)

Relationship btw ventilation and perfusion:

•is expressed as a ratio

•the normal V/Q ratio is 0.8 - which is the amount perfusion exceeds ventilation under normal circumstances (this means that, overall, the lungs receive slightly more blood flow than air, but this is close to optimal for gas exchange)

Zone 1

reflects blood flow in apices of lung

blood flow is minimal bc enlarged alveolar sacs in apices create alveolar pressure higher than capillary pressure ( V/Q ratio >1)

Zone 2

reflects the middle region of lung

pulm arterial pressure varies due to right ventricle systole, so there’s intermittent perfusion

Zone 3

continuously perfused throughout the cardiac cycle

pulm arterial pressure exceeds alveolar pressure (V/Q <1)

Diffusion across alveolocapillary membrane

idea for oxygen diffusion

large total surface area

very thin= one cell thick under normal circumstance

Diffusion of gas from high to low concentration

Partial pressure of oxygen molecules (Po2) is much greater in alveolar gas (PAO2) than it is in capillary blood (PaO2): Promotes rapid diffusion from the alveolus into the capillary.

Determinants of arterial oxygenation

Hemoglobin: oxygen-carrying oxygen

Oxygen content: total amount of O2 carried in blood

Cardiac fxn: ability to deliver oxygenated blood to tissues

as oxygen diffuses across alveolar capillary membrane, what does it do FIRST

what other steps does it take

dissolved in plasma, where it exerts pressure - PaO2

PaO2 increases= oxygen moves O2 from plasma into RBC and bind with hemoglobin molecules

•Oxygen continues to bind with hemoglobin molecules until hemoglobin binding site are filled (or saturated)

•Oxygen continues to diffuse across the alveolar-capillary membrane until PAO2 and PaO2 equilibrate, eliminating the pressure gradient

Components of Hemoglobin

what does it contain?

Each can bind up to

heme- oxygen binding pigment containing iron

globin- protein! envelop that provides a pocket for a heme molecule

4 heme groups, 4 globin chains (beta&alpha)

bind up to 4 molecules of oxygen

Cooperative

phenomenon where binding of oxygen to one subunit of a hemoglobin molecule increases the affinity of oxygen at the other binding sites

Oxyhemoglobin:

•Hemoglobin molecules bind with oxygen: Oxyhemoglobin

•the majority of oxygen in the blood (97%) is bound to hemoglobin

•the oxygen saturation (SaO2) is the percentage of hemoglobin that is bound to oxygen. This can be measured with an oximeter (pulse oximeter-SpO2)

what happens at the tissues where partial pressure of Oxygen is low

o2 is released from the hemoglobin molecule

•When hemoglobin saturation and desaturation are plotted on a graph, the result is

a distinctive S-shaped curve known as the oxyhemoglobin dissociation curve.

Oxyhemoglobin association and dissociation

•Hemoglobin molecules bind with oxygen: Oxyhemoglobin

•the majority of oxygen in the blood (97%) is bound to hemoglobin

•the oxygen saturation (SaO2) is the percentage of hemoglobin that is bound to oxygen. This can be measured with an oximeter (pulse oximeter)

•The PaO2 measures only thr small amount of oxygen dissolved in plasma and is measured by obtaining an arterial blood gas (ABG)

•A po2 of 60 mm Hg corresponds to an spo2 of about 90%

•At the tissues where partial pressure of Oxygen is low, O2 is released from the hemoglobin molecule

•When hemoglobin saturation and desaturation are plotted on a graph, the result is a distinctive S-shaped curve known as the oxyhemoglobin dissociation curve.

For PaO2 lvls <60, oxygen is

For PaO2 lvls >60:

readily unloaded to peripheral tissues and hemoglobin’s affinity for oxygen diminishes

curve flattens reflecting maximum saturation of hemoglobin w oxygen

Oxygen-Hemoglobin Dissociation Curve

Shift to the left

Shift to the right

left- depicts the hemoglobin’s increased affinity for oxygen, which promotes association in the lungs and inhibits dissociation in the tissues.

•Alkalosis (high pH) and hypocapnia and hypothermia

right- depicts the hemoglobin’s decreased affinity for oxygen or an increase in the ease with which oxyhemoglobin dissociates and oxygen moves into the cells.

•Acidosis (low pH) and hypercapnia and hyperthermia

large changes in PaO2 causes

only small changes in hemoglobin saturation (SaO2)

CO2 Transport

what 3 ways is it transported in blood:

~ 200 ml of CO2 produced by tissues per min= byproduct of cellular metabolism

1. 1. Dissolved in plasma

2. 2. As Bicarbonate (60-70%)

3. 3. Attached to hemoglobin

Solubility of CO2 vs O2

Reduced hemoglobin is able to..

CO2 is 20x more soluble than O2= it wants to get out quickly! (from tissue cell into blood)

carry move CO2: •therefore, the drop in SaO2 at the tissue level increases the ability of hemoglobin to carry CO2 back to the lungs

Presence of RBS enzyme carbonic anhydrase, what happens

CO2 combines w water to form carbonic acid —>

Carbonic acid almost instantaneously breaks down into H+ and Bicarbonate ions —>

•Hemoglobin attaches to H+ (RBC buffer) and HCO3- diffuses into the plasma

•in exchange for chloride - a process known as “chloride shift”

In CO2 transport, when venous blood reaches the lungs,

the chemical process is reversed!

oxygen diffuses into blood and attached to HG → HG releases H to bind with O2 → HCO3- reenters RBC in xchange for Chloride → HCO3 combines w H ( bc of enzyme carbonic anhydrase) to form carbonic acid →

CO2 transport to the lungs

diffusion gradient for CO2 from blood to the lung is only 6mm Hg (46-40)

BUT, because CO2 is so soluble in the alveolar capillary membrane, it quickly diffuses into alveoli (Co2 wants to get out!)

•Diffusion of CO2 in the lung is so efficient that diffusion defects that cause hypoxemia do not cause hypercapnia