Pulmonary Circulation

Pulmonary Capillary Pressure normal value

10 mm Hg

Colloidal Osmotic Pressure normal value

25 mm Hg

What keeps alveoli dry

Suction force of 15 mmHg (colloidal osmotic pressure - pulmonary capillary pressure) draws fluid from the pulmonary interstitial fluid into pulmonary capillaries

What happens if pulmonary capillary pressure rises above 25 mm Hg

Fluid escapes into interstitial spaces

Leads to pulmonary oedema

What conditions would raise pulmonary capillary pressure

Exercise at high altitude

Left heart failure

Mitral stenosis

Pulmonary fibrosis

Four factors influencing blood flow in the lung

Arterial to Venous Pressure Gradient

Pulmonary vascular resistance (PVR)

Gravity

Alveolar pressure

Mean Pulmonary Artery Pressure

15mmHg (25/8mmHg)

Left Atrial Pressure

5mmHg

Arterial to Venous Pressure Difference Driving Blood into Pulmonary Circulation

15-5 = 10mmHg

Pulmonary pressure is what compared to systemic circulation

about 1/10 of the pressure in the systemic circulation

Greatest pressure drop in pulmonary sys is where

across capillary bed

What are these signs of

Pulmonary Vascular Disease (PVD)

What can Pulmonary Vascular Disease (PVD) lead to

Right ventricular failure

Loss of left atrial compliance

Mitral regurgitation

Left ventricular dysfunction

Passive backwards transmission of left-sided filling pressures (from left side to pulmonary circulation)

Vascular resistance

The degree to which the blood vessels impede the flow of blood

Vascular resistance formula

Pulmonary vascular resistance (PVR) formula

Pulmonary vascular resistance is high/low

Very low

Systemic vascular resistance (SVR) value

17mmHg/L/min

Hypoxia in the systemic circulation causes vasodilation/vasoconstriction

vasodilation

Hypoxia in the pulmonary circulation causes vasodilation/vasoconstriction

vasoconstriction

(Pulmonary hypoxia causes vasoconstriction to redirect blood flow away from poorly ventilated areas, improving V/Q matching and gas exchange)

Factors that affect PVR

Blood pressure

Hypoxia

Lung volume

Increased blood pressure increases/decreases PVR

decreases

By what mechanisms does increased blood pressure decrease PVR

Recruitment - open previously closed vessels (& vessels that were open but not conducting now conduct blood)

Distension - increase in caliber of vessels (vessels widen with pressure)

How does pulmonary artery pressure (PAP) change during exercise

During exercise, sympathetic stimulation increases cardiac output, leading to higher pulmonary artery pressure (PAP)

During exercise how does the pulmonary capillary transit time remain sufficient for oxygenation

increased blood flow distribution

Effect of recruitment & distention during exercise

Decreases PVR - Prevents pulmonary hypertension during exercise and ensures efficient oxygen delivery to tissues

Pregnancy leads to a ______% increase in blood volume and cardiac output,

30-50%

Effect of pregnancy on PAP

Raises it slightly

What is the physiological effect of a slightly increased PAP in pregnancy & significance of this

The increased PAP opens additional pulmonary capillaries (recruitment) and widens existing vessels (distension), decreasing PVR and improving pulmonary circulation.

Significance:

• Prevents pulmonary hypertension

• Increases pulmonary blood flow to meet the increased oxygen demand of both the mother and foetus.

What is Pulmonary Arterial Hypertension characterised by

narrowing, remodelling, and fibrosis of the pulmonary arteries, leading to increased resistance to blood flow

Effect of pulmonary arterial hypertension on pressure & resistance in the circulatory system

The increased resistance to blood flow forces the right ventricle (RV) to generate higher pressures to push blood into the pulmonary circulation

PVR is increased

To overcome the increased PVR, the right ventricle pumps with greater force, raising pulmonary artery systolic and mean pressures

(In summary PAH leads to elevated RVP, PVR & PAP)

At what mPAP is PAH diagnosed

Pulmonary artery pressure is diagnosed when mean pulmonary artery pressure is >20 mmHg at rest

Results of decreased pulmonary blood flow in PAH

Capillary recruitment and distension are impaired

Hypoxia

Exercise intolerance

The increased afterload on the right ventricle leads to right ventricular hypertrophy and eventually right heart failure due to the inability to sustain high pressures.

Difference between normoxia, acute hypoxia & chronic hypoxia

How does the pulmonary system respond to hypoxia

hypoxia-induced pulmonary vasoconstriction

How does hypoxia-induced pulmonary vasoconstriction work

There are oxygen sensitive K+ channels in the smooth muscle that surrounds blood vessels within alveolar walls

When P_O2 within alveoli falls, K+ channels close.

This triggers increased intracellular calcium → increased smooth muscle cell depolarisation → contraction → vasoconstriction of pulmonary vessels

Blood flow to poorly ventilated lung regions is reduced - blood is diverted to alveoli that are better ventilated

How does hypoxia-induced pulmonary vasoconstriction help in high altitude

High altitude hypoxia Increases pressure in pulmonary artery - better perfusion of lung apex

How does hypoxia-induced pulmonary vasoconstriction help in utero

It limits blood flow through the (not-yet-functioning) lungs of the foetus

Possible risk of chronic hypoxia

How does hypoxia-induced pulmonary vasoconstriction affect people with COPD

In COPD persistent vasoconstriction leads to vascular remodeling and fibrosis, raising mean pulmonary artery pressure (mPAP) and causing pulmonary hypertension. The right ventricle (RV) faces increased afterload, resulting in right ventricular hypertrophy and eventually cor pulmonale (right heart failure).

Symptoms of right heart failure

dyspnea, peripheral edema, jugular venous distension (JVD), and hepatomegaly

Effect of oxygen therapy on COPD patients

Oxygen therapy helps reduce HPV (🫁 Hypoxic Pulmonary Vasoconstriction), lowering PVR and alleviating right ventricular strain

COPD patients have a large liver why?

Increased heart size irritates liver, creating an inflammatory response

When is pulmonary vascular resistance (PVR) at its lowest

FRC

PVR Increases at Both High and Low Lung Volumes

FRC

Amount of air left in lungs after a normal expiration

Pulmonary circulatory vessels have thick/thin walls

Thin

(pulmonary arteries have less smooth muscle in their walls than systemic arteries)

(they are distensible & compressible)

Pressures surrounding pulmonary vessels can distend/compress them. How does this work

What keeps large pulmonary vessels open

Large blood vessels near the hilum are exposed to negative intrapleural pressure which keeps them open

What is the effect of high lung volume on PVR

What is the effect of low lung volume on PVR

Best perfused area of lungs

Base (bottom) of the lung

Why is the top of the lung not as well perfused as the bottom

Due to gravity, pulmonary arterial pressure is greatest at the base of the lungs

There is just about enough pressure to bring blood to the top of the lung due to gravity

Pressure Differs from Top to Bottom of 30cm high Lung

about 23mmHg

Mean pressure in lung

15 mmHg

Pressure at base of lung

25 mmHg

Pressure at apex of lung

2 mmHg

How is perfusion throughout the lung measured

A 64-year-old male with severe emphysema presents with progressive dyspnea and reduced exercise tolerance. Pulmonary function tests show increased total lung capacity (TLC) and residual volume (RV), consistent with lung hyperinflation. On examination, the patient has barrel chest, distant heart sounds, and an accentuated P2 heart sound.

What is the problem?

What treatment can be given?

What could happen without treatment?

The excessive lung volume compresses pulmonary capillaries, increasing pulmonary vascular resistance (PVR) and impairing pulmonary blood flow. Over time, the right ventricle (RV) must work harder to overcome the elevated PVR, leading to right ventricular hypertrophy and pulmonary hypertension.

Long-term oxygen therapy and bronchodilators help reduce hyperinflation, lower PVR, and improve pulmonary circulation.

Without treatment, persistent high lung volumes may lead to cor pulmonale (right heart failure) and worsening dyspnea.

This case shows the effects of high lung volume on PVR

A 45-year-old patient with acute respiratory distress syndrome (ARDS) is placed on mechanical ventilation with high tidal volumes due to severe hypoxemia. This leads to acute right ventricular strain and then failure. Why?

The patient develops systemic hypotension and poor oxygenation. Why?

What should be done instead?

What could happen if left untreated?

The high lung volumes lead to alveolar overdistension, compressing pulmonary capillaries and increasing pulmonary vascular resistance (PVR). As a result, pulmonary artery pressure (PAP) rises, forcing the right ventricle (RV) to work harder, which can lead to acute right ventricular strain or failure.

This suggests ventilator-induced pulmonary hypertension

To reduce PVR, the ventilator settings are adjusted to lower tidal volumes (lung-protective ventilation) and optimize positive end-expiratory pressure (PEEP).

If untreated, persistent high lung volumes can lead to cor pulmonale and worsening oxygenation.

This case shows the effects of high lung volume on PVR

A 58-year-old female with idiopathic pulmonary fibrosis (IPF) presents with progressive shortness of breath and dry cough over the past year. Pulmonary function tests reveal a low total lung capacity (TLC) and reduced forced vital capacity (FVC), consistent with restrictive lung disease.

Would you expect her PVR to be high/low?

How would you confirm this?

Over time, what would the persistent vascular remodelling and fibrosis lead to?

As the disease progresses, the right ventricle struggles to overcome the elevated afterload, leading to what?

The low lung volume leads to compression of pulmonary capillaries, increasing pulmonary vascular resistance (PVR) and reducing pulmonary blood flow.

Right heart catheterization confirms increased PVR

Elevated mean pulmonary artery pressure (mPAP), leading to pulmonary hypertension.

Peripheral edema, jugular venous distension (JVD), and exertional dyspnea.

This case shows the effects of low lung volume on PVR

what 2 things does pressure in pulmonary arterioles depend on

both mean pulmonary artery pressure and the vertical position of the vessel in the chest, relative to the heart.

In a healthy in individual, capillary/alveolar pressure is always greater than the pressure in capillaries/alveoli

capillary

alveoli

(otherwise the capillaries would be squashed!)

What west zones are generally found in the lungs

Zone 2 and 3 are found in healthy individuals

Zone 1 is only found in unhealthy individuals (e.g. during a haemorrhage)

How does flow in each of the west zones differ

What pressure is responsible for opening alveoli

Transpulmonary pressure

How is transpulmonary pressure calculated

Alveolar pressure - (inter)pleural pressure

How does ventilation & perfusion compare at the top & bottom of the lung

Why are neonatal heart beats so high (140bpm)

Their stroke volume is very low

What is the ventilation-perfusion ratio formula

Normal ventilation-perfusion value

around 0.8 - 1

Why does the V and the Q have to be measured in the same alveolar unit

How many alveoli do we have

300 million

What does each line represent in this graph representing flow of blood & air

1 line is perfusion

1 line is ventilation

KNOW THIS DIAGRAM

Know where there’s a high/low ventilation perfusion match

The ventilation perfusion ratio is highest at the top/bottom of the lung

Top

What is happening in alveolus 1

Shunting

There’s a blockage

Perfusion of blood without ventilation

Unoxygenated blood enters systemic circulation

What is happening in alveolus 3

Dead space

Ventilation of lungs without perfusion

Gas enters and leaves lungs without contacting blood - wasted ventilation

Is this low, normal or high V_A/Q

low ventilation/perfusion (V/Q) ratio

Is this low, normal or high V_A/Q

This represents a case of very high V/Q ratio — in fact, it's essentially infinite V/Q, like in dead space ventilation.

Is this low, normal or high V_A/Q

Normal

Normal alveolar ventilation Approx ___L/min

4

Normal pulmonary capillary perfusion Approx ___L/min

5

Why is normal ventilation perfusion ratio around 0.8

V/Q ratio changes throughout alveoli in lung

Alveoli at apex are underperfused (overventilated)

Alveoli at the base are underventilated (overperfused)

Put in higher & lower (& last 3 columns values)

Which is more dangerous - a high or low V/Q ratio

Low

High V/Q ratio adds relatively little more O₂ to the blood

Low V/Q ratio can be fatal - causes hypoxemia - low blood oxygen levels

Patient Profile: Age/Sex: 45-year-old male

Presentation: Sudden onset of shortness of breath, pleuritic chest pain (sharp and worsened by deep breaths), and a rapid heart rate.

Risk Factors: Recent prolonged travel and a history of deep vein thrombosis (DVT).

Clinical Findings:

•Vital Signs:

• Tachypnea (rapid breathing rate)

• Tachycardia (rapid heart rate)

• Slight hypotension

Examination:

• Normal lung auscultation (no wheezes or crackles)

• Signs of right heart strain on an ECG may be present

What is diagnosed: Pulmonary embolism

What is the resulting V/Q ratio

V/Q > 1

Areas that are ventilated but not perfused cannot effectively exchange oxygen into the blood, leading to an overall decrease in arterial oxygen levels (PaO₂). The body responds by hyperventilating, which often leads to a low arterial carbon dioxide level (PaCO₂).

In the alveolar oxygen tension (PAO₂) formula what does each letter stand for

We use P_atm instead of P_B

Calculate if the A-a gradient is normal

Alkalosis can indicate that the person is breathing quickly/slowly

Quickly - Hyperventilating

How does the body deal with decreased air flow to a region of a lung.

Do pulmonary blood oxygen levels go up or down

How does the body deal with decreased blood flow to a region of a lung.

Do alveolar carbon dioxide levels go up or down