L.13 Perfusion of the Lung Flashcards
Describe the pressure of the pulmonary circulation
- But CO for both sides of the heart same: 5L/min
Low pressure system with 22/10, Mean 14mmHg.
As you branch from pul art. to pul. capillaries pulsatile flow decreases due to reduced resistance.
Still pressure drop from pul. cap. to LA which drives blood flow to heart.
What is sheet blood flow around alveoli
Capillary walls are eroded away between arteriole and venule with only posts of interstitium holding up space.
This enables more contact of blood with the respiratory surface
What are the physical factors that affect blood flow
Normally at low pul art pressures some cap are collapsed.
As pul. art. pressure increases, pul resistance decreases by distension of open cap. Then more recruitment of collapsed cap.
This helps to prevent pulmonary oedema.
As lung volume increases (during inflation) what is the effect on resistance in the extra alveolar and alveolar bv
The extra alveolar vessels are pulled open, therefore reduction in vascular resistance, but the alveolar vessel resistance increases.
These sum to give a u shaped curve of resistance
What are the chemical factors that affect blood flow
Hypoxia: reduction in PO2 in alveoli OR increased CO2 (hypercapnia) causes a Local vasoconstriction of SM in bv. This diverts the blood away from poorly oxygenated alveoli.
What are chemicals that cause vasoconstriction and vasodilation (pul circuit)
Vasoconstrictors: Hypercapnia Decreased pH in the blood, Angiotensin 2, Adenosine: from breakdown of ATP
Vasodilators: trigger endothelium production of NO. released during inflammatory responses: Bradykinin, histamine, vasopressin (only in pul circulation)
What are the autonomic factors that affect blood flow
Symp: Na–>a1 adrenoreceptor. Cause vasoconstriction. Reduce pul. blood flow
Parasymp: ACh to M3 receptor on vascular SM. Vasodilation
Other dilators: Vasoactive intestinal peptide and CGRP
What is the average ventilation perfusion ratio and why : Va : Q
Due to gravity, blood flow and ventilation is greater in the base than in the apex.
However the blood flow declines more steeply than ventilation, because mass of blood>air.
This is why Ventilation perfusion ratio average at rest = 0.8 but ranges from 0.5 to 3 at different parts of the lung
What is the differences in pressure of the Alveoli, pulmonary artery and vein in the top to bottom 3rd of the lung
Top 3rd: PAl > Par > Pv: blood flow is limited until CO inc recruits them
Middle 3rd: Par >PAl >Pv
Bottom 3rd: Par> Pv > PAl
What is the clinical importance of Va: Q
Can differentiate poor Va or Q.
From blocked airway Va:Q of 0 = no ventilation but lots of perfusion. This is a shunt due to bronchitis.
From Blocked bv: Va:Q of greater than 3: good ventilation but poor blood flow. This is embolism: increases dead space.
How does Pulmonary hypertension come about and what does it lead to
Endothelial damage which reduces NO output = vasoconstriction.
This increases pressure inside pul circuit. Increased resistance: increased stress and it will try to increase CO but Right heart failure. Then hypoxia in bloodflow = vasoconstriction: more stress
How does Pulmonary oedema come about and what does it lead to
Left Heart failure causes congestion causes increased pulmonary vein pressure. Pul hypertension causes increased capillary hydrostatic pressure so fluid –> alveoli. This reduces O2 uptake in the blood, causing systemic hypoxia: breathlessness.
