16. Pulmonary Circulation Flashcards
Outline the bronchial circulation
Systemic supply to the lungs; supports tissue viability
Branches from the aorta
Drains into the bronchial veins and small broncho-pulmonary anastomoses
Define anastomosis
A connection made surgically between adjacent blood vessels, parts of the intestine, or other channels of the body
Outline the pulmonary circulation
From the left ventricle, it receives the entire cardiac output each cycle
The primary function is gas exchange
Other functions include the filtering of small emboli, metabolism of vasoactive substances (angiotensin I, bradykinin, serotonin, noradrenaline, prostaglandins, leukotrienes), and others
Structure/function relationship:
- Key point; vast alveolar surface area needs to be supplied (70m^2; grows from child to adult), therefore its structure needs to optimise diffusion
- From the right ventricle; deoxygenated blood flows to the alveoli, then oxygenated blood flows to the left atrium
- Has to accommodate the entire cardiac output each cycle, therefore has to be a high-capacity low-resistance circuit
The cardiac output may also increase (exercise, pyrexia, etc.), therefore it needs to accommodate these changes
Spare capacity is also required for its filtratory function
Define pyrexia
Raised body temperature; fever
Contrast the pulmonary circulation with the systemic circulation
Pulmonary circulation does not require such high pressures due to close proximity with the heart, therefore does not have the same gravity to overcome like with normal venous return
This means that the structure of the arteries, veins and capillaries are different
Arteries/veins:
- Pulmonary:
o Thinner walls
o Less smooth muscle
- Systemic:
o Thicker wall
o Abundant smooth muscle
Capillaries:
- Pulmonary:
o Very thin walls; mesh provides a thin sheet of blood
- Systemic:
o Network structure throughout tissues
Pulse pressures: systemic (120/80mmHg), pulmonary (25/8mmHg); because the wall of the right ventricle is thinner
o Mean systemic = 100mmHg, mean pulmonary = 5mmHg
Pulmonary vascular resistance (= pressure gradient / blood flow = arterial pressure - venous pressure / cardiac output)
o Therefore, PVR = 15 - 5/5 = 2mmHg/l/min
o In contrast, SVR = 18mmHg/l/min; this is due to reduced arterial pressure
Outline the embryological development of the pulmonary circulation
Development completely separate from systemic circulation
The dorsal aorta splits, leading to a left dorsal aorta and a right dorsal aorta
6 branchial arches arise from the dorsal aorta
The 1st, 2nd and 5th branchial arches regress on both sides
The 3rd branchial arch gives rise to the common carotid arteries on both sides
The 4th branchial arch gives rise to the aorta
The 6th branchial arch gives rise to the pulmonary circulation (i.e. pulmonary artery):
o RHS: loses its link to the aorta
o LHS: retains its link to the aorta during embryonic life; ductus arteriosus
Outline the foetal circulation
The ductus arteriosus forms the link between the pulmonary artery and the aorta
Blood flow: RV –> ductus arteriosus –> aorta –> systemic circulation –> RA
Foramen ovale: allows blood to flow from the RA to the LA
The placenta is the site of gas exchange, therefore most blood flow (60%) is via the placenta
The pulmonary circulation forms a high resistance circuit since the lungs are fluid-filled
In the foetus, the RV is hypertrophied, therefore both ventricles have a similar appearance (the RV regresses after birth)
Outline the placenta
The placenta is an organ that develops in your uterus during pregnancy
This structure provides oxygen and nutrients to your growing baby and removes waste products from your baby’s blood
The placenta attaches to the wall of your uterus, and your baby’s umbilical cord arises from it
It allows nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother’s blood supply; it also fights against internal infection; and produces hormones which support pregnancy
Outline the pulmonary circulation at birth
The first breath causes the lungs to expand, increases the alveolar pressure and decreases the pulmonary resistance
This leads to:
- Reflex closure of the foramen ovale and the ductus arteriosus
- This is followed by remodelling and permanent closure of both structures
Outline the regulation of blood flow at birth
The effects of gravity; blood goes down
The close proximity of alveoli and capillaries from gas exchange results in the exposure of vessels to alveolar pressure
The distribution of blood-flow in the lungs is then considered to be 3 zone model:
- Zone 1: Alveolar pressure > Arterial pressure > Venous pressure (tend vessel compression)
- Zone 2: Arterial pressure > Alveolar pressure > Venous pressure (tend vessel compression on the venous side; this is important because the pulmonary circulation needs to restrict recruit vessels, and it is easy to open the tiny closure; progressive restriction
Zone 3: Arterial pressure > Venous pressure > Alveolar pressure
Outline the effects of progressive restriction on the pulmonary vascular bed
Right and left lungs: pulmonary arterial pressure (PAP) is normal
Right lung only: PAP is normal since the right lung has the sufficient spare capacity to accommodate the entire cardiac output without an increase in PAP
Right upper lobe only: PAP increases
Increased capacity of the pulmonary bed (required for increased CO accommodation) can then be achieved by recruitment or distension
Both lead to a fall in pulmonary vascular resistance
Recruitment is the opening of vessels that were previously closed (Zone 2), via a slight increase in venous pressure which is achieved through the integration of pulmonary and cardiovascular system
By decreasing the pulmonary vascular resistance, it enables to the pulmonary arterial pressure to remain normal even if the venous pressure increases
Define distention
Distention: The state of being distended, enlarged, swollen from internal pressure
For example, on inhalation there is distention of the lungs due to the increased air pressure within the lungs
The word ‘distention’ comes from a Latin root ‘tendere’, which means ‘to extend’
Draw a diagram of the 3-zone model to describe blood flow in the lungs
[See http://www.icsmsu.com/exec/wp-content/uploads/2011/12/ABS-Respiratory_System.pdf Page 46]
Outline pneumonia and HPV (hypoxic pulmonary constriction)
If alveolar oxygen tension falls, active vasoconstriction of pulmonary arteries may occur (to a diameter of <1000μm)
This diverts blood to the aerated regions of the lung to find more oxygen to meet metabolic demands
Regulated by HIF (hypoxia-inducible factor); regulates systemic changes in haematopoetic, respiratory and cardiovascular physiology that combine to restore adequate oxygenation:
o Acts as an alveolar oxygen sensor in pulmonary arterial myocytes which cause temporary vasoconstriction
HPV in the foetus: this helps to maintain low blood flow to the lungs of the foetus, therefore blood is directed into the systemic circulation
HPV in chronic lung disease:
o Advantage: reduces V/Q mismatch by reducing blood flow to poorly ventilated areas
o Disadvantage: chronic HPV leads to vascular remodelling
o This results in a permanent increase in PVR, leading to pulmonary hypertension, leading to right-sided heart failure
Outline V/Q mismatch
V/Q mismatch is a defect that occurs in the lungs whereby ventilation (the exchange of air between the lungs and the environment) and perfusion (the passage of blood through the lungs) are not evenly matched, a symptom typical of chronic obstructive pulmonary disease (COPD)
Define pulmonary oedema
Fluid infiltration from the pulmonary circulation into the pulmonary interstitium due to a change in the rate of normal dynamic fluid shifts
Outline pulmonary oedema
Consequences include interstitial oedema and alveolar flooding
Fluid flux across the pulmonary capillaries is caused by the difference between hydrostatic pressure and colloid osmotic pressure:
o Normally, if the hydrostatic pressure is greater than the colloid osmotic pressure, there is a small fluid flux into the interstitum, but any excess fluid drains into the lymphatics (act as rate limiting step)
o In pulmonary oedema, the lymphatics become overwhelmed by the higher fluid flux rates, leading to interstitial oedema and alveolar flooding
Effects of pulmonary oedema include impaired gas exchange, reduced lung compliance, increased pulmonary venous pressure
Clinical presentation (often seen following a myocardial infarction):
- Terrified patient (drowning feeling), Severe breathlessness (work of breathing is very high), Pink frothy sputum (due to high intravascular pressures causing rupture of alveolar vessels), Crackles on auscultation:
o Morphine relieves these clinical symptoms
Causes:
o Increased hydrostatic pressure due to high pulmonary venous pressure which is caused by left heart failure or mitral stenosis
o Reduced plasma colloid pressure due to low plasma protein concentration which is caused by starvation or abnormal leakage from the kidney or the gut
o Increased capillary permeability due to endothelial cell damage; this occurs in ARDS (adult respiratory distress syndrome)
Define pulmonary embolus
A clot formation in the pulmonary circulation
Outline pulmonary embolus
Mechanism:
o Small clots form in the deep veins of lower limbs
o These clots enter the pulmonary circulation; they are filtered by spare capacity vessels and broken down by the fibrinolytic system in the lungs
o If a clot is too big, it obstructs blood flow, leading to a rapid increase in pulmonary blood pressure, thus leading to RV failure
Consequences: a spectrum of consequences exists:
o Major clots: RV failure and circulatory collapse
o Intermediate clots: pressures on the right side of the heart increase; gas exchange decreases leading to breathlessness; lung infarction is rare due to the dual blood supply to the lungs (pulmonary circulation and the bronchial circulation)
o Small clots: filtered out of the circulation
Define pulmonary hypertension
Elevated pulmonary artery pressure (PAP), such that it gets closer to systemic pressure
Outline pulmonary hypertension
Mechanism:
- The pulmonary arteries become thicker at higher pressure and PAP increases, therefore the right ventricle must pump at a higher pressure, leading to hypertrophy (compensation)
- Right ventricular failure occurs when the right ventricle can no longer compensate
Causes:
- Pulmonary arterial hypertension (primary defect in the pulmonary arteries), which may be primary or related to collagen/vascular diseases, congenital heart disease, drugs, toxins, HIV etc.:
o Leads to elevated PVR
- Pulmonary venous hypertension (primary defect usually cardiac), which may be left sided atrial, ventricular or valvular disease:
o Leads to elevated pulmonary capillary wedge pressure
- Pulmonary hypertension associated with disorders of the respiratory system and/or hypoxia:
o When right ventricular failure develops in the setting of chronic lung disease and hypoxia, it is termed ‘cor pulmonale’
- Pulmonary hypertension due to chronic thrombotic and/or embolic disease
- Pulmonary hypertension associated with miscellaneous disorders
Outline cor pulmonale
The abnormal enlargement of the right side of the heart as a result of disease of the lungs or the pulmonary blood vessels
Outline pulmonary shunts
Occurs when the alveoli of the lung are perfused with blood as normal, but ventilation fails to supply the perfused region, i.e. V/Q=0
Due to the alveoli filling with fluid, causing parts of the lung to be unventilated though still perfused
Intrapulmonary shunting is the main cause of hypoxaemia (inadequate blood oxygen)
The lack of ventilation may lead to hypoxic pulmonary vasoconstriction to divert blood flow to other regions of the lung
