Viva F5(i)

Outline the blood supply of the lung

• The lung is supplied with pulmonary venous blood and bronchial arterial blood
  • The pulmonary venous blood originates from the pulmonary trunk and the blood flow through this is the same as the cardiac output, i.e. about 5L/min
  • The bronchial arterial blood supply comes from the systemic arterial circulation and is about 20-70ml/min
    • The right bronchial artery arises from an intercostal artery
    • On the left, there are usually two bronchial arteries with separate origins arising from the aorta. 
• The cells in the walls of pulmonary vessels do not derive their nutrition from the pulmonary venous blood, but rather from the systemic circulation.
• The walls of these vessels contain vasa vasorum which originate from bronchial arteries. 
• Pulmonary vessels and bronchi co-occupy the space of a fibrous sheath together with a few other ancillary structures.
• These are described as "bronchovascular bundles".

What are the components of a bronchovascular bundle?

• A bronchus or bronchiole

• A pulmonary artery
• A bronchial artery
• A pulmonary vein
• Vasa vasorum
• Nerve fibres
• Lymphatic vessels
• Loose connective tissue
• A fibrous sheath to bind them
  • These sheaths of connective tissue are viewed as extensions of the pleura.
• At the level of the terminal air spaces, the smallest of the arteries and veins (under 50 μm) are surrounded by and are in direct contact with the walls of alveoli.

What are the dimensions and relations of the pulmonary trunk and left and right pulmonary arteries?

• The pulmonary trunk is short and wide – approximately 5 cm in length and max 3 cm in diameter
• The right pulmonary artery is the longest.
  • It is mostly within the pericardium (for about 75% of its length)
• The left pulmonary artery is shorter then the right. 
  • It passes inferiorly and posteriorly and exits the pericardium below the aortic arch at the ligamentum arteriosum.
  • It has a long extrapericardial length before giving off its first branch. 
• The pulmonary arteries taper from about 25mm down to about 17mm
• These dimensions are important radiologically, as enlargement of these vessels raises the suspicion of pulmonary hypertension.

What is the main difference, anatomically, between pulmonary arteries and veins?

• Thinner walls, which are less muscular, less elastic, and more collagenous. 
• The largest pulmonary veins flow seamlessly into the left atrium.
  • At their widest, these are about half the width of the pulmonary arteries (i.e. about 15mm in diameter).
  • Unlike the large arteries, these vessels have less elastic fibres in them and are mainly muscular.
  • There is no anatomical fold or border to act as a convenient distinction between atrium and vein
  • Endothelium of the veins is indistinguishable from the endocardium of the atria (Klimek-Piotrowska, 2016).  

What are the origins of the blood flowing in the pulmonary arteries? 

• The pulmonary artery receives:
  • Deoxygenated venous blood returning from the systemic circulation
  • Deoxygenated venous blood returning from the bronchial circulation (1/3rd of the total bronchial venous drainage)
  • Oxygenated arterial blood via Sperr arteries (from the bronchial circulation)

What are the origins of the blood flowing in the pulmonary veins? 

• Oxygenated venous blood from the pulmonary capillaries
• Deoxygenated venous blood from intrapulmonary shunts
• Deoxygenated venous blood returning from the bronchial circulation (2/3rds of the total bronchial venous drainage)
• Deoxygenated venous blood from the Thebesian circulation of the heart.

What are the thebesian vessels?

• These veins are which drain the myocardium
• Otherwise known as venae cordis minimae.
• Tiny valveless veins in the walls of the four cardiac chambers.
• Only smol:  0.5mm in diameter
• Total flow is very low: 0.12% to 0.43% of the total aortic flow.
• They drain randomly and directly into the chambers of the ventricles and atria themselves, with openings apparent even on papillary muscles.
  • Some of these drain into the right ventricle and atrium, where they would not contribute to shunt.
  • Others drain into the left side of the circulation, and contribute a certain (tiny) fixed physiological  shunt fraction. 

What is the total pulmonary blood volume?

• About 10% of the circulating blood volume,  500ml or so
• Only really relevant when discussing transpulmonary thermodilution measurement of cardiac output
• It varies over the course of the respiratory and cardiac cycle
• It varies in response to gravity (it is lower in the erect position, by one-third according to Nunn's)
• It increases in states of sympathetic stimulation (as systemic vasoconstriction "flushes" blood into the pulmonary circulation)
• At rest, from beat to beat, the pulmonary blood volume varies by around 50ml over the course of a single cardiac cycle

What is normal pulmonary arterial pressure?

• Pulmonary arterial pressure is low
• Normal systolic maximum of about 18-25 mmHg,
• Normal mean pulmonary arterial pressure =  9-16 mmHg.
• Any increase in the mean pressure beyond 20 mmHg is described as pulmonary arterial hypertension

What is normal pulmonary capillary pressure?

• Pulmonary capillary pressure is usually about 8-10mmHg
• It is usually a little higher in the arterial (pre-alveolar) capillaries
• It can be estimated from the Gaar equation: 

PPC = LAP + 0.4 × (mPAP - LAP)

Where

• LAP = left atrial pressure
• mPAP = mean pulmonary arterial pressure

• bonus marks: it differs regionally, according to gravity
  • Pulmonary arterial and capillary pressures are affected by the hydrostatic pressure gradient in the erect lung
  • A 25cm lung has a pressure gradient of 18mmHg along its height
  • The pressure at the hilum may be 16 mmHg, but this will translate into 7mmHg at the apex, and 24 mmHg at the base
  • This is the fundamental premise underlying West's Zones

What is the normal pulmonary venous pressure?

• The mean pressure in the pulmonary venous circulation is usually 6-12 mm Hg
• The pulmonary venous pressure waveform resembles the CVP waveform 
• This waveform can be seen during pulmonary artery wedge occlusion

What is the normal pulmonary vascular resistance?

• The normal value for PVR is 100-200 dynes/sec/cm^-5, or  255 - 285 dynes-sec/cm^–5/m² for PRVI (indexed to body surface area)
• These values are approximately 1/10th of what one might expect from the systemic circulation
• The resistance in the pulmonary circulation can be calculated as follows:

PVR = 80 × (mPAP- PAOP)/CO

Where

• mPAP is the mean pulmonary arterial pressure, and
• PAOP is the pulmonary artery occlusion pressure which we assume is a reasonable surrogate for mean left atrial pressure
• 80 is a fudge factor which converts the equation's output (which ends up in mmHg⋅min⋅mL^-1) into dynes-sec/cm^–5.

Which pulmonary arterial structures are most important for pulmonary vascular resistance?

• Large pulmonary arteries capillaries and veins all contribute a roughly similar amount of resistance to the total
• The biggest pressure drop in the pulmonary circulation occurs in the capillary bed, which accounts for about 40% of the total resistance.
• In contrast in the systemic circulation the resistance in the arterioles is by far the dominant influence.