L15: Special circulation

Question 1

What are the two circulations to the lungs?

Answer 1

Pulmonary circulation–> blood goes to lungs to get oxygenated
–> blood to alveoli via pulmonary trunk (artery)
Bronchial circulation–> blood supply to the lung tissue
–> supplies the tissue to far away to get oxygen via diffusion
–> systemic circulation

Question 2

How do the pulmonary and systemic circulation work?

Answer 2

Work in series

Pulmonary circulation receives the entire cardiac output

Question 3

How does the diastolic pressure in the artia compare to the diastolic pressure in the ventricles?

Answer 3

Pressure in the atria greater than the ventricle
Need a pressure gradient from high (atria) to low (ventricles)
Can be the same

Question 4

What is the diastolic pressure in each chamber of the heart and the great vessels?

Answer 4

RA–> 0-8 mmHg
RV–> 0-8 mmHg
LA–> 1-10 mmHg
LV–> 1-10 mmHg
Pulmonary trunk/artery–> 4-12 mmHg small amount of elastic recoil
Aorta –> 60-90 mmHg –> higher diastolic pressure because of elastic recoil of aorta maintains the pressure

Question 5

What is the systolic pressure each of the chambers and the great vessels?

Answer 5

RA and LA --> no systolic pressure because they don't really contract
RV--> 15-30 mmHg
LV--> 110-140mmHg
Pulmonary trunk--> 15-30mmHg
Aorta--> 100-140 mmHg

Question 6

What are the main features of the pulmonary system?

Answer 6

Low pressure–> MAP (12-15mmHg), Mean capillary pressure (9-12mmHg), mean venous pressure (5mmHg)

Low resistance–> need blood to flow through easily

• Short and wide vessels
• Lots of capillaries (many parallel elements)
• Arterioles relatively little smooth muscle–> keep lumen open allowing good blood flow

Question 7

What is included in the pulmonary system?

Answer 7

RA, RV, pulmonary trunk, lungs, pulmonary veins

Question 8

How has the pulmonary circulation adapted for efficient gaseous exchange?

Answer 8

Very high density of capillaries –> large capillary SA
Short diffusion distance –> thin layer of tissue separating gas phase from plasma (endo and epithelium 0.3 micrometres)
Good O2 and CO2 exchange/ transport capacity

Question 9

What is the ventilation: perfusion ratio?

Answer 9

V/Q ratio- optimal 0.8
Efficient oxygenation
Ventilation needs to match perfusion

Question 10

What is hypoxic pulmonary vasoconstriction?

Answer 10

Important regulator of vascular tone
Ensure optimal V/Q ratio
Vasoconstriction–> divert blood away from poorly ventilated alveoli (hypoxic alveoli)
Optimise gaseous exchange

Question 11

What is one of the downsides to hypoxic pulmonary vasoconstriction?

Answer 11

Chronic hypoxia –> high altitude or consequence of lung disease (emphysema)
–> ↑ vasoconstriction–> increased resistance–> chronic pulmonary hypertension–> high afterload on right ventricle–> right sided heart failure–> pumping against higher pressure

Question 12

How does gravity affect the pulmonary circulation?

Answer 12

Vessels–> thin compliant vessels, easily distend and easily collapse
Upright (orthostasis)
Vessels in lung above the heart–> collapse during diastole (decreased hydrostatic pressure), open in systole due to increased pressure
Vessels at level of heart–> patent all the time
Vessels below the level of the heart–> vessels distend (increased hydrostatic pressure)

Question 13

How does exercise impact pulmonary blood flow?

Answer 13

Increase cardiac output
Pulmonary circulation takes all the blood
Small ↑ pulmonary arterial pressure
Opens apical capillaries (normally collapsed)
Increase O2 uptake in the lungs
Blood flow increases transit time is reduced
–> RBC loaded with oxygen quicker
(normally 1 second, reduced to 0.3 seconds)

Question 14

What are the two forces acting on the blood in the capillaries?

Answer 14

Hydrostatic pressure–> Blood in capillary, pushes fluid out the capillary
Oncotic pressure–> pressure exerted by large molecules (plasma proteins), draws fluid into the capillary
Normally balanced

Question 15

What influence the hydrostatic pressure more?

Answer 15

The venous pressure in the systemic circulation

Venous pressure increases the hydrostatic pressure increases

Question 16

How is the formation of lymph in the lung kept to a minimum?

Answer 16

Hydrostatic pressure is much lower compared to peripheral capillaries
As well as oncotic plasma pressure also an oncotic interstitial pressure
Filtration = reabsorption
Minimum lung fluid

Question 17

What happens if the hydrostatic pressure in the lung increases?

Answer 17

More fluid is filtered out leading to oedema
Filtration > reabsorption
Increase in pressure in left side of the heart
ALSO increase in arterial pressure (before the lungs in pulmonary artery) affects pulmonary hydrostatic pressure because resistance is lower

Question 18

What could cause pulmonary oedema?

Answer 18

Capillary pressure increase due to:
Mitral stenosis–> difficult for blood to get out so ↑ pressure in LA which increase hydrostatic pressure in the lungs
Left ventricular failure–> more blood in LV in systole so difficult to get blood in during diastole –> back pressure

Question 19

What pressure does pulmonary oedema occur at?

Answer 19

Normally kept low at 9-12mmHg

Pressure increase to 20-25mmHg get oedema

Question 20

What does pulmonary oedema result in? How is it treated?

Answer 20

Impairs gas exchange –> affected more by posture, mainly at base when upright, lying down throughout lung
Diuretics–> reduced BP by removing water
Treat underlying cause

Question 21

How much of the CO does the brain (cerebral circulation) receive?

Answer 21

15% of CO–> neurones are active
even though brain only accounts for 2% of body mass
Grey matter consumes 20% of total body comsumption at rest

Question 22

How does the cerebral circulation meet the high demand?

Answer 22

High capillary density 
--> Large SA for diffusion 
--> Reduced diffusion distance (<10 micrometres)
High basal flow rate
--> x10 average of whole body 
--> anastomosis
High O2 extraction 
--> 35% above average for body

Question 23

Why is it crucial to have a good blood supply to the brain? How is a secure blood supply to the brain achieved?

Answer 23

Neurones sensitive to hypoxia
Irreverisble damage after 4 minutes
Stucturally
–> Anastomosis between the basilar (from vertebral artery) and carotid artery –> Circle of Willis
Functionally
–> Myogenic autoregulation maintains perfusion during hypotension
–> Metabolic factors control blood flow
–> Brainstem regulates other circulations

Question 24

How does myogenic autoregulation work?

Answer 24

↑ in blood pressure in the whole body–> vasoconstriciton –> don’t want too much blood going to the brain–> ↑ intracranial pressure skull stops expansion
↓ in blood pressure in whole body–> vasodilation –> ensure enough blood reaches the brain

Question 25

At what pressure does autoregulation fail?

Answer 25

MAP below 50mmHg

Question 26

How is the cerebral blood flow metabolically regulated?

Answer 26

Sensitive to changes in pCO2
↑ pCO2 (hypercapnia)–> vasodilation–> neurones are active require more O2
↓ pCO2 (hypocapnia)–> vasoconstriction–> neurones less active require less O2

Question 27

What does an fMRI scan show?

Answer 27

Areas with increase neuronal activity –> ↑ blood flow
↑ pCO2, [K+], and adenosine –> vasodilator
↓pO2

Question 28

What is Cushing’s reflex?

Answer 28

Cranium–> protection but not expansion
↑ intracranial pressure–> impair cerebral blood flow
Impaired blood flow–> vasomotor control regions of brain stem–> ↑ sympathetic activity–> ↑ arterial BP to maintain blood flow
Can get high BP with tachycardia

Question 29

What does the coronary circulation need to ensure?

Answer 29

Must deliver O2 at high basal rate

Meet increased demand (can increase 5 fold)

Question 30

When does blood flow into the coronary arteries usually occur?

Answer 30

Coronary arteries penetrate the myocardium
Therefore blood flow has to occur in diastole
Systole occlude blood flow to the myocardium
↑ HR–> shorten diastole–> reduce time for blood flow
Not a problem with normal pathology but it is with reduced blood flow

Question 31

How is the coronary circulation adapted to deal with it?

Answer 31

High capillary density–> efficient O2 delivery
Diffusion distance <9 micrometers
Continuous production of NO by coronary endothelium maintains high basal flow

Question 32

How does the coronary blood flow increase with myocardial O2 demand?

Answer 32

Linear relationship until very high O2 demand
Vasodilation due to metabolic hyperaemia
Vasodilators–> adenosine, ↑[K+] and ↓pH

Question 33

What type of arteries are the coronary arteries?

Answer 33

Functional end arteries

Few anastomoses

Question 34

What is the problem with functional end arteries in the coronary system?

Answer 34

Prone to atheromas
Narrowed coronary arteries leads to angina on exercise
↑ O2 demand
–> blood flows most during diastole, diastole reduces as HR increase
Stress and cold can also cause sympathetic coronary vasoconstriciton= angina
Sudden obstruction–> MI

Question 35

What does the skeletal coronary circulation need to ensure? How does it do this?

Answer 35

Increase O2 and nutrient delivery and removal of metabolites during exercise
Help regulate arterial blood pressure
SM–> 40% of body mass
Sympathetic vasoconstrictor regulation–> NA released–> α1 receptor as part of baroreceptor reflex–> maintains BP

Question 36

How does the skeletal muscle circulation ensure that these demands are met?

Answer 36

Capillary density–> depends on muscle type
(postural muscles ↑ density)
High vascular tone–> vasodilation–> 20x increase in flow during exercise
At rest- 1/2 capillaries perfused (allows for increased recruitment)
Opening of precapillary sphincters–> more capillary perfusion –> increased blood flow, reduced diffusion distance

Question 37

How does metabolic hyperaemia increase blood flow?

Answer 37

Vasodilator agents
- ↓pH
- ↑ K+
- ↑ osmolarity
- inorganic phosphates
- Adenosine
- ↑ H+
Adrenaline acts as vasodilator in arterioles- β2 receptor Vasoconstriction--> NA on α1 receptors

Question 38

What is the role of the cutaneous circulation?

Answer 38

Temperature regulation via skin
Core temperature maintained around 37 degrees
–> Balance heat production and loss
Maintain BP

Question 39

What are arterovenous anastomoses? How do they help with cutaneous circulation?

Answer 39

Increase SA: volume ratio
Under neural control (sympathetic vasomotor fibres) (–> not metabolites)
Decrease temp–> ↑ sympathetic tone in AVAs–> decrease blood flow to apical skin
Increase temp–> open AVAs
Reduce vasomotor drive to AVAs allows them to dilate–> divert blood closer to the skin