Pulmonary Blood Supply Lecture 6 Flashcards
What are the characteristics of the pulmonary blood supply?
Functions of pulmonary circulation : O2/CO2 exchange (perfusion), one of the bodys reservoirs for blood (capacitance), Filter, Synthesis and metabolism of vasoactive substances.
Receives all of the right ventricular output.
Thin walled and highly branched. Low intravascular pressure- reflects thin muscle wall of right ventricle
Low pressure due to short and large vessels and due to no high resistance vessels in pulmonary circulation. Low resistance due to short blood vessels, larger radius vessels.
Both arterioles and venules are distensible, thin walled- resemble systemic venules (reservoir function)
Huge capillary surface area- for gas exchange. varied by recruitment, distension and collapse .
All blood is returned to the left ventricle
What are the effects of gravity on pulmonary circulation?
Gravity has a significant effect because the resistance/pressures are so low
At the level of pulmonary valve- arterioles blood pressure 28/8mmHg and the capillary pressure is 10-8mmHg
Near base- arterioles pressure is 40/20mmHg (+12/12mmHg due to gravity)
Near apex- arterioles pressure (-12/12mmHg) not sufficient to hold capillaries open during diastole and so the vessels collapse
Gravity creates a gradient for pulmonary blood flow (perfusion) from apex to base. Pressure is lower at apex than at the base due to gravity causing the vessels to collapse
What are the capacitance functions of the pulmonary circulation?
- Standing up- redistributes blood towards the base of lungs and increases return to the left side of the heart (~400ml resistributed to systemic circulation). When lying down pulmonary perfusion is evenly distributed from apex to base. Blood vessels contain greater blood volume when supine (lying), they distend.
2 +3. Increase cardiac output and PABP and increased blood volume- increases perfusion towards the apex, increases the filling of pulmonary circulation
- Left ventricular failure- left ventricular distension, increases left atrial distension, increases filling of pulmonary circulation, eventually increases pressure throughout pulmonary circulation
List some vasoconstrictor influenced in pulmonary circulation
Sympathetic nerve activity to pulmonary vessels- more activity leads to vasoconstriction and increased pressure throughout pulmonary circulation
Inflammatory mediators, allergens- histamine, bradykinin, 5-HT cause pulmonary vasoconstriction and cna increase the pressure throughout pulmonary circulation
Hypoxia - causes pulmonary vasoconstriction
How does hypoxia affect the pulmonary circulation?
When some alveoli are poorly ventilated and the hypoxia is localised to those alveoli vasoconstriction redistributes blood flow to better ventilated areas. Improving O2 exchange
If there is generalised hypoxia (widespread throughout lungs) which can cause genralised pulmonary vasocontriction and increase pressure on the pulmonary circulation and can lead to pathology- limits O2 exchange
Hypoxia either leads to an increase or decrease in reactive oxygen species (ROS)➡️ then the common pathway leads to increased intracellular calcium which leads to muscle contraction and vasoconstriction
What are the effects of generalised pulmonary vasoconstriction?
Increased resistance- can lead to increased ventricular work and therefore right heart failure
Pulmonary remodelling- hypertrophy of vascular smooth muscle which increases resistance and can then lead to right heart failure
Increased hydrostatic pressure- increased filtration out of the capillaries leading to pulmonary oedema
Capillary hydrostatic pressure can increase from 10/8mmHg to 20/18mmHg leading to greater pressure o more fluid is pushed out of circulation causing interstitial oedema followed by alveolar oedema
Interstitial oedema causes widening of gap between the alveoili and pulmonary capillaries which makes gas exchange harder as diffusion length has increased.
Alveolar oedema is when pressure is higher so more fluid goes into alveoli making it more difficult in ventilation and exchange between capilaries and alveoli.
Pulmonary oedema occurs in
1) Hypoxia (high altitude, respiratory disease)
2) Inflamtion , anaphylactic responses - pulmonary vasocontriction and increased vascular permebaility
3) Extreme excerise in horses (due to increased cardiac output, causes increased filtration out of capillaries into alveoli
Also occurs in left ventricular failure - leads to increase in left atrial pressure, feeds back to pulmonary circulation, leads to increased pulmonary pressure - pulmonary oedema.
Describe the filter function of pulmonary circulation
Microthrombi- small particles of RBCs, platelets and fibrin aggregate.
lodge in pulmonary circulation where they are lysed
Blood clots, air or fat can form a pulmonary embolism- increase dead space. Blocks blood vessels and obstructs blood flow.
Describe the metabolic function of the pulmonary circulation
Pulmonary endothelial cells synthesise:
Nitric oxide (NO) in response to shear stress - tonic pulmonary vasodilation
Prostacyclin (PGI2) for tonic pulmonary dilation. PGI2 infusion or inhalation can be used to treat PPH
Angiotensin converting enzyme (ACE)- converts A1 to A2 (major site of conersion in the body)
Pulmonary endothelium can removes or degrade the followng;
Removes 5-HT released from platelets and noradrenaline from the sympathetic nerves and adrenal medulla
