Special Circulations

Question 1

What are the 2 circulations of the lungs?

What are 2 key characteristics of pulmonary circulation?

Answer 1

1. Pulmonary circulation: blood supply to alveoli for gas exchange. 2 key characteristics: low resistance and low pressure
2. Bronchial circulation: systemic circulation to meet metabolic requirements of the lungs

Question 2

What are 2 factors that promote an efficient gas exchange?

Answer 2

1. Large SA: high density of capillaries

2. Short diffusion distance; a thin layer of tissue separating gas from plasma

Question 3

What is the V/Q ratio? What value indicates an optimal ratio? Why isn’t it 1:1?

Answer 3

The ventilation-perfusion ratio should be 0.8:1

Not 1:1 because 1L of blood can hold 200 mL of O2, and 1L of air contains 210 mL of O2, so there has to be less air.

Question 4

What causes hypoxic pulmonary vasoconstriction and how does it ensure optimal VQ?

Answer 4

Small pulmonary arteries constrict when there is low O2:
This causes a reduction in K+ channels (they’re very sensitive to a decrease in oxygen).

Vasoconstriction ensures that more bloodflow goes to alveoli that are better ventilated

Question 5

What is chronic hypoxic vasoconstriction, what can it lead to and when can it occur?

Answer 5

A chronic increase in vascular resistance leads to chronic pulmonary hypertension, this affects the afterload on the R heart so it has to pump harder = can lead to R heart failure

Occurs due to lung disease or being at altitude

Question 6

Why does gravity have such a big effect on the vessels of the lungs?

Answer 6

Because there are less muscular vessels

Question 7

What are the 3 main layers of vessels in the lungs going from superior-inferior

Answer 7

1. Apex: pressure is not maintained and vessels collapse during diastole
2. Mid heart: In line with mid lungs, vessels continually open
3. Base: gravity pushes more blood, and due to their less muscular nature vessels tend to be distended

Question 8

What happens to the pulmonary vessels during exercise?

Answer 8

CO must match pulmonary circulation, so increased CO (during exercise) increases the pulmonary arteriole pressure; this increase in pressure opens up capillaries at the lung’s apex

Question 9

How low can capillary transit time drop to without compromising gas exchange?

Answer 9

0.3 seconds

Question 10

What is the normal pressure in pulmonary circulation, and what are the 2 main competing forces?

Answer 10

9-12 mmHg
Plasma oncotic pressure and interstitial oncotic pressure, as there isn’t enough lung lymph to exert enough hydrostatic pressure

Interstitial oncotic pressure: proteins/large molecules in the lung interstitium draw fluid out of the vessels

Plasma oncotic pressure: proteins/large molecules in the vessels keeping fluid within the vessels

Question 11

When might the L atrial pressure be 20-25 mmHg?

Answer 11

Mitral valve stenosis, L ventricular failure

Question 12

Explain why pulmonary edema occurs when there is increased pressure on the L side of the heart

Answer 12

Increased pressure increases hydrostatic pressure:
Now, the hydrostatic + interstitial oncotic pressure&raquo_space; plasma oncotic pressure, fluid leaks out of the pulmonary vessels and causes a pulmonary edema

Question 13

What drug and non-drug help could you recommend to treat symptoms in a patient with pulmonary edema?

Answer 13

Non-drug: Sleep upright, as gravity will distribute less of the excess fluid across the apex, mid-heart and base vessels and they’ll be able to breathe better

Drug: diuretics

Question 14

How much of the CO and how much oxygen does the brain require at rest? How does cerebral circulation meet the high O2 demand

Answer 14

15% of CO, 20% of oxygen used by grey matter

1. High capillary density: large SA and reduced diffusion distance.
2. High basal flow rate (X10 average than whole body)
3. High O2 extraction: 35% above the body’s average

Question 15

If the brain loses perfusion, how long will it take to become unconscious and have irreversible neurone damage?

Answer 15

Unconscious: 10 seconds

Irreversible damage: 4 minutes

Question 16

How is the brain ensure its blood supply structurally and functionally?

Answer 16

Structurally: anastomoses throughout the brain and different links to arteries

Functionally:
1. Brain stem ensures its circulation by stimulating a sympathetic response (increases resistance to other pathways to make sure brain gets perfusion)

2. Myogenic autoregulation: when BP drops there’s an inbuilt mechanism for smooth muscle to adjust the muscle tone and maintain perfusion (high BP = vasoconstriction as you want to reduce blood flow, low BP = vasodilation)

Question 17

When does the myogenic autoregulation fail?

Answer 17

<50 mmHG and upwards of 180 mmHg

Question 18

What is the vascular response to hypercapnia and hypocapnia?

Answer 18

Hypercapnia: high pCO2 means vessels will vasodilate to remove it more efficiently

Hypocapnia: Low pCO2 means vessels will vasoconstrict

Question 19

How would you manage panic hyperventilation?

Answer 19

Can cause hypocapnia and cerebral vasoconstriction: leading to dizziness and fainting, need to increase the patient’s pCO2 by having them breathe into a paper bag

Question 20

What is Cushing’s reflex?

What is the result?

Answer 20

Any brains space-occupying lesion (tumour or hemorrhage) increases the intracranial pressure since the rigid cranium protects the brain and doesn’t allow for volume expansion

Impaired blood flow to the vasomotor control regions of the brain stem stimulate the sympathetic response; triggering an increase in HR and BP BUT increased pressure then stimulates the baroreceptors to stimulate the parasympathetic response which slows down the HR

Cushing’s Triad: Bradycardia, increased BP and irregular ventilation
(Initially, hypoxia stimulates the need for an increase in O2, but when the baroreceptors kick in and slow HR - ventilation also slows, so breathing is irregular)

Question 21

Why does most bloodflow to the coronary arteries occur during diastole?

Answer 21

When the heart contracts the resistance in the coronary arteries is much greater, and when the heart relaxes the vessels are more patent

Question 22

Name 2 molecules that can and can’t diffuse across the BBB

Answer 22

Can: lipid-soluble; O2, CO2

Can’t: lipid insoluble; K+ and catecholamines

Question 23

How do metabolically active areas of the brain ensure their own perfusion?

Answer 23

They may release metabolic vasodilators such as H+, K+ adenosine and CO2. Therefore a high pCO2 and low pO2 signals for vasodilation

Question 24

Name 3 factors that assist coronary arteries in maintaining perfusion to the myocardium

Why still then are the coronary arteries prone to atheromas?

Answer 24

1. High capillary density
2. Small diffusion distance
3. Coronary endothelium secretes NO to maintain high basal flow

Still prone to atheromas as they don’t have a lot of collateral circulation (arterial-arterial anastomoses), so a narrowed coronary artery (atherosclerosis, vasoconstriction due to stress or cold, obstruction) can quickly lead to angina and an MI

Question 25

Explain the relationship between myocardial O2 demand and coronary blood flow?

Answer 25

Almost linear relationship until a very high O2 demand

Question 26

Generally, what determines how much perfusion skeletal muscle receives?

Answer 26

1. Capillary density depends on the muscle type

2. Skeletal muscle has a high vascular tone; meaning it can vasodilate well

Question 27

How does skeletal muscle circulation change from being at rest to exercising?

Answer 27

At rest only 1/2 of capillaries may be open and can increase up to 20 fold during exercise: opening more capillaries increases the bloodflow and decreases the diffusion distance

Question 28

What happens to bloodflow during metabolic hyperemia?

Answer 28

Increase in vasodilator metabolites:

H+, K+, adenosine, inorganic phosphates and osmolarity

Question 29

What is the main role of cutaneous circulation?

Answer 29

Temperature regulation, as skin is the main dissipating surface

Question 30

What specialized feature does the apical (acral) skin have in regulating heat loss? How are they regulated?

Answer 30

Has the arteriovenous anastomoses (AVAs)
This provides a high SA: Volume ratio.

They are under sympathetic control and not regulated by local metabolites:
If the core temp decreases the sympathetic tone to AVAs increases; telling them to vasoconstrict to prevent heat loss

If the core temp increases AVAs decreased sympathetic tone tells them to vasodilate allowing for more heat loss