CVS 8 Special Circulations Flashcards
What are the main features of the pulmonary circulation?
Pulmonary which accepts entire cardiac output and works with low resistance due to short, wide vessels and lots of capillaries connected in parallel (low resistance) and arterioles with little smooth muscle. Low resistance means runs under low pressure
What two circulations do the lungs have?
pulmonary and bronchial
What does the bronchial circulation do?
meets metabolic requirements of lungs
What is the mean pressure in the pulmonary artery?
12-15mmHg
What is the mean pressure in the pulmonary capillaries?
9-12mmHg
What is the mean pressure in the pulmonary veins?
5mmHg
What is the cardiac output at rest?
5l/min
What us the max cardiac output in a non-athlete?
20-25l/min
What is the pressure in the right atrium?
0-8mmHg
What is the pressure in the left atrium?
1-10mmHg
What are the pressures in the left ventricle?
Systolic- 100-140mmHg
Diastolic-1-10mmHg
What are the pressures in the right ventricle?
Systolic- 15-30mmHg
Diastolic-0-8mmHg
What are the pressuresin the aorta?
100-140mmHg
60-90mmHg
What are the pressures in the pulmonary artery?
15-30mmHg
4-12mmHg
How are the lungs adapted to promote efficient gas exchange?
High density of capillaries in alveolar wall
Short diffusion distance (combined endothelium and epithelium 0.3um)
large surface area
The rate of what two processes must be matched for efficient oxygenation?
ventilation and perfusion
What is the optimal ventilation/perfusion ratio?
0.8
How do the lungs maintain the optimal ventilation-perfusion ratio?
diverting blood from alveoli that arent well ventilated as alveolar hypoxia results in vasoconstriction of pulmonary vessels ensuring perfusion matches ventilation
When can chronic hypoxia occur?
at altitude and as a consequence of lung disease like emphysema
What can chronic hypoxic vasoconstriction cause and how?
Right ventricular failure
chronic increase in vascular resistance giving chronic pulmonary hypertension
This gives high afterload on right ventricle leading to failure
How does being upright (orthostasis) affect hydrostatic pressure in different areas of the lungs?
Greater hydrostatic pressure in the vessels in lower lung, these are distended by the higher hydroststic pressure due to gravity, while vessels in upper part collapse in diastole
How does exercise effect pulmonary blood flow?
Increased cardiac output
Small increase in pulmonary arterial pressure
opens apical capillaries
increased O2 uptake by lungs
As blood flow increases, capillary transit time reduced from 1 to as little as 0.3s without compromising gas exchange
What pushes fluid out of capillaries?
Hydrostatic pressure
What draws fluid back into capillaries?
oncotic pressure
What pressure has the highest effect on capillary hydrostatic pressure? What does this mean for pulmonary system?
Venous.
Low venous pressure so only a little fluid leaves capillaries
Where does pulmonary oedema occur and what does it do?
Impairs gas exchange
Mainly at base of lungs when standing
Throughout when lying down
What can happen to left atrium if there is pulmonary oedema?
If pressure rises to 20-25mmHg can get mitral valve stenosis and left ventricular failure
How would pulmonary oedema be treated?
Use duiretics to relieve symptoms and treat underlying cause
How much(relative) cardiac output does the brain recieve and whats its relative mass?
15% CO
accounts for 2% of body mass
How does the cerebral circulation meet the high demand for O2?
High capillary density (large SA, reduced diffusion dist)
High basal flow rate
High O2 extraction
Why is O2 supply to brain important?
Neurones very sensitive to hypoxia
Loss of consciousness after a few seconds of cerebral ischaemia
Irreversible damage to neurines in ~4mins
stroke causing neuronal death
How is a secure blood supply to brain ensured structurally?
Anastomoses between basilar and internal carotid arteries
How is a secure blood supply to brain ensured functionally?
Brainstem regulates other circulations
Myogenic autoregulation maintains perfusion during hypotension (vasoconstriction and vasodilation)
metabolic factors control blood flow
At what blood pressure can myogenic autoregulation no longer maintain cerebral blood flow?
50mmHg
How does pCO2 affect cerebral arteries?
Very sensitive to it
Hypercapnia- Inc pO2-vasodilatation
Hypocapnia- Dec pO2- vasoconstriction
What happens when someone panic hyperventilates?
They get hypocapnia (decreased pCO2) so cerebral vasoconstriction leading to dizziness or fainting
Which areas of brain will get increased blood flow?
active areas as they make more CO2 and metabolites like adenosine which is a very powerful dilator
What is cushing’s reflex?
(Rigid cranium protects brain but doesn’t allow volume for expansion
Increases in intracranial pressure impair cerebral blood flow and can cause tumour and haemhorrage)
Impaired blood flow to vasomotor control regions of brainstem increase sympathetic vasomotor activity
What can and can’t diffuse freely through the blood brain barrier and why?
Lipid soluble molecules like O2 and CO2 diffuse freely
Lipid Insoluble solutes such as K+ and catecholamines cant diffues freely as endothelial cells in capillaries have tight junctions
Where do left and right coronary arteries arise from?
Aortic sinuses
Describe the relationship between the mechanical work and coronary blood flow of the myocardium
Almost linear until very high demand where there is an increase in oxygen extracted
How many fold must O2 supply to the heart increase by to cope with high demand?
5
How many of the capillaries in skeletal muscle are perfused at any one time?
half
How is cardiac muscle adapted for oxygen delivery?
High capillary density and continuous NO production by coronary endothelium to maintain high basal flow
Describe the consequences of partial or total occlusion of coronary arteries?
Coronary arteries are functional end arteries and prone to atheroma.
Narrowed coronary arteries lead to angina on exercise due to increased demand. Stress and cold can also cause sympathetic coronary vasoconstriction and angina
Sudden occlusion by thrombus causes MI
What does most of the cutaneous circulation flow through rather than capillaries?
Arterio-venous anastomoses AVA
What is blood flow to skin used for?
Most isn’t nutritive, has special role in temperature regulation being body’s main heat dissipating surface
What happens to cutaneous circulation when core temperature decreases?
Sympathetic innervation which causes increased tone in AVAs decreasing blood flow to apical skin
What happens to cutaneous circulation when core temperature increases?
Less vasomotor drive so AVAs dilate and there’s a low resistance shunt of blood to the venous plexus which is close to surface and allows dissipation of heat
What shuts off capillaries in skeletal muscle?
Pre-capillary sphincters
How is blood flow increased in skeletal muscle?
Mainly by opening up more capillaries via vasodilator nervous activity and local metabolites that reduce sympathetic vasoconstrictor tone
By how many fold can flow to skeletal muscle increase?
20
