18. Pulmonary circulation Flashcards
Pulmonary circulation
Blood going to gas exchange surface
not blood supplying pulmonary tissue to keep it alive
How does the pressure in the pulmonary circulation differ from the systemic circulation?
Pressure in the pulmonary circulation is much LOWER than the systemic circulation
State a key difference in the structure of the pulmonary arteries compared to the systemic arteries.
Pulmonary arteries have a greater lumen: wall thickness ratio meaning that they are more compliant
Why is the right ventricular wall thinner?
Pumping to pulmonary circuit
Needs to be low pressure (vs left ventricle)
How does pressure change with distance from the heart?
Pressure gradually decreases with distance from the heart
How does the mean arterial blood pressure vary between the systemic and pulmonary circulation?
MAP in the pulmonary circulation is 15% that of the systemic circulation
How does the pressure gradient differ between the systemic and pulmonary circulation?
10% of systemic
How does the resistance differ between the systemic and pulmonary circulation?
10% of systemic
What volume is carried by the systemic and pulmonary circulations?
Systemic: 4.5L
Pulmonary: 0.5L
List 3 primary functions of the pulmonary circulation
Gas exchange (O2 delivery, CO2 removal)
Metabolism of vasoactive substances
Filtration of blood
Describe the metabolism of vasoactive substances that occurs in the pulmonary circulation
Special endothelial cells expressing ACE for converting Ang I to Ang II
Breaks down Bradykinin (which is a vasodilator)
Specific substrates it binds and converts only expressed in endothelium of lung and kidney
Where is ACE expressed?
In the lung endothelium and in the kidneys
What does ACE do?
Converts Angiotensin I to Angiotensin II
Breaks down bradykinin
Embolus
a ‘mass’ within the circulation capable of causing obstruction
Embolism
an ‘event’ characterised by obstruction of a major artery
Describe the protective role of the pulmonary circulation.
It filters the blood before it reaches the systemic circulation.
Small emboli are eliminated
Large emboli are trapped (causing occlusion to blood flow)
Pulmonary shunts
circumstances associated with bypassing the respiratory exchange surface
State 3 pulmonary shunts.
Bronchial Circulation
Foetal circulation
Congenital defect
Describe the pulmonary shunts in Foetal circulation
Lungs not being ventilated, so beneficial to bypass: Foramen Ovale (links atria) and Ductus Arteriosus (allows bifurcation of pulmonary arteries of pulmonary trunk to connect to aortic arch)
Describe the pulmonary shunt in Bronchial Circulation
Blood goes through left side of heart to bronchial circulation, some drains back into pulmonary vein and returns back to left side of heart.
Describe the pulmonary shunts in a congenital defects
If foramen ovale does not close= Atrial septal defect (ASD)
Ventricular septal defect (VSD) allows mixing of blood across septum, causes right side becomes stronger ventricle.
Many people have an ASD, even a small opening comprises the filtration function of the pulmonary circulation, potential for plaque to bypass filter and enter systemic circulation
How does the pulmonary circulation respond to an increase in cardiac output?
Pulmonary arteries are more compliant
Can distend to prevent significant increase in pressure
Increased recruitment of hypo-perfused beds (apex) also prevents increase in pressure
Allow increase in CO without increasing fluid leakage and compromising pulmonary function
Describe perfusion at rest
Preferential perfusion down path of least resistance at base of lung (not against gravity)
Describe perfusion during increased CO
Increased vascular recruitment
Increase in flow through apex of lung
Huge capacity for increasing throughput without increasing pressure
Which vessels in the lungs are affected during inspiration and expiration?
Inspiration: Compresses alveolar vessels
Expiration: Compresses extra-alveolar vessels
When is resistance lowest?
At functional residual capacity (default mechanical equilibrium)
When is resistance highest?
At residual volume (empty)
At total lung capacity (full)
Describe and explain the effects of increasing ventilation on pulmonary resistance.
Ventilation increases pulmonary resistance at the extremities (round alveoli)
Near residual volume (forced expiration), intrathoracic pressure presses on the extra-alveolar arteries thus increasing resistance.
Near TLC, expansion of the alveolus presses on the alveolar arteries thus increasing resistance.
Systemic and pulmonary responses to hypoxaemia
Systemic: Vasodilation
Pulmonary: Vasoconstriction
Why does the pulmonary vasculature vasoconstrict in response to hypoxaemia?
To achieve better ventilation-perfusion matching
So it is not sending blood to parts of the lung that are inadequately ventilated
Describe the mechanism causing vasoconstriction of pulmonary vessels in response to hypoxia
Closure of O2 sensitive K+ channels Decrease in K+ efflux Increase in membrane potential Membrane depolarisation: Opening of VGCC Vascular smooth muscle contraction
Describe Hypoxic pulmonary vasoconstriction that may occur at High altitude/ COPD
Breathing hypoxic air
Causes global constriction all over lungs
Significant impact on pressure
Right ventricle working harder against resistance
Gets stronger
Concentric hypertrophy, septum begins to change
Effects EDV on left side of heart
Comprises stroke volume
Give an example of a situation in which the response of the pulmonary circulation to hypoxia is beneficial
During foetal development
Blood follows the path of least resistance
High-resistance pulmonary circuit means increased flow through shunts
1st breath increases alveolar PO2 and dilates pulmonary vessels
Give an example of a situation in which the response of the pulmonary circulation to hypoxia is DETRIMENTAL.
COPD
Reduced alveolar ventilation and air trapping
Increased resistance in pulmonary circuit
Leads to pulmonary hypertension and right ventricular hypertrophy
Leads to congestive heart failure.
Where does greater perfusion of the lungs occur?
Base Higher intravascular pressure (due to the effects of gravity) More recruitment Less resistance Higher flow rate
Where does greater ventilation of the lungs occur?
Base
Smaller transmural pressure gradient
Alveoli smaller and more compliant
More ventilation
What 2 key pressures are important in pulmonary fluid balance?
Hydrostatic pressure (in capillary and interstitium) Oncotic pressure (in capillary and interstitium)
Describe the hydrostatic pressure through a capillary (plasma hydrostatic pressure)
Highest at arterial end (as blood flows down pressure gradient), gradually decreases
Describe interstitial hydrostatic pressure
Tiny, barely noticeable
Describe the plasma oncotic force and what this causes
High osmolarity in plasma
Low osmolarity in interstitial fluid
Causes more fluid to be drawn into vessel than out
What is the net net movement of fluid between plasma and interstitial fluid? How is this dealt with?
1 mmHg out into interstitial fluid
“Mopped up” by lymphatic system, steady fluid accumulation is kept under control
What occurs if too much fluid accumulates, exceeding the maximum rate of clearance of the lymphatic system?
Pulmonary Oedema
Mitral stenosis
Hardening of the valve between the LA and LV
How is the pulmonary fluid balance affected in mitral stenosis?
Pressures back up from left heart through pulmonary circulation Increased plasma hydrostatic pressure More fluid forced into interstitium Max. lymph clearance rate exceeded = OEDEMA develops
How is the pulmonary fluid balance affected in liver failure?
Liver synthesises plasma proteins, failure results in Hypoproteinaemia
Plasma oncotic pressure reduced
Less fluid drawn into capillary “reduced sucking force”
Fluid accumulates in interstitium
Lymph clearance exceeded
= OEDEMA develops
How is the pulmonary fluid balance affected in metastatic breast cancer?
Cancerous cells spread to nearby thoracic lymph nodes/ducts
Tumours obstruct lymphatic drainage
Lymph clearance compromised
= OEDEMA develops
