18. Pulmonary circulation Flashcards by Georgia Leggett

Pulmonary circulation

Blood going to gas exchange surface

not blood supplying pulmonary tissue to keep it alive

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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

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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

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Why is the right ventricular wall thinner?

Pumping to pulmonary circuit

Needs to be low pressure (vs left ventricle)

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How does pressure change with distance from the heart?

Pressure gradually decreases with distance from the heart

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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

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How does the pressure gradient differ between the systemic and pulmonary circulation?

10% of systemic

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How does the resistance differ between the systemic and pulmonary circulation?

10% of systemic

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What volume is carried by the systemic and pulmonary circulations?

Systemic: 4.5L
Pulmonary: 0.5L

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List 3 primary functions of the pulmonary circulation

Gas exchange (O2 delivery, CO2 removal)
Metabolism of vasoactive substances
Filtration of blood

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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

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Where is ACE expressed?

In the lung endothelium and in the kidneys

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What does ACE do?

Converts Angiotensin I to Angiotensin II

Breaks down bradykinin

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Embolus

a ‘mass’ within the circulation capable of causing obstruction

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Embolism

an ‘event’ characterised by obstruction of a major artery

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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)

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Pulmonary shunts

circumstances associated with bypassing the respiratory exchange surface

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State 3 pulmonary shunts.

Bronchial Circulation
Foetal circulation
Congenital defect

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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