S2: Special Circulations

Question 1

How do circulations adapt to the particular needs of end organs?

Answer 1

e.g. circulations to brain, heart, skin, kidney, lungs, skeletal muscle

All have their unique characteristics:
· Special requirements
· Special features
· Structural
· Functional
. Special problems

Question 2

What is the special requirement of the coronary circulation?

Answer 2

• Heart needs a high basal supply of O2 (x20 more than skeletal muscle)
• There is an increase in O2 supply in proportion to increased demand/cardiac work.

Question 3

What are the special structural features of the coronary circulation?

Answer 3

• Cardiac Muscle has high capillary density
• Large surface area for O2 transfer

Together these reduce diffusion distance to myocytes so O2 transport is fast
t = x2

Question 4

What are the special functional features of the coronary circulation during normal activity and increased demand?

Answer 4

During normal activity:

• High blood flow - 10x the flow per weight of rest of the body
• Relatively sparse sympathetic innervation and high NO released (not much vascular tone)
• Resting vasodilation
• High blood flow and large surface area = high O2 extraction compared to average body

During increased demand:

• Coronary blood flow increases in proportion to demand
• Production of vasodialators (adenosine, K+, acidosis) ‘outcompete’ sympathetic vasoconstriction during excersize
• Circulating adrenaline dilates coronary vessels due to abundance of B2 adrenoreceptors
• Relationship between activity of the heart and vasodilation helping meet demands of the heart providing more oxygen

Question 5

What enables good oxygen extraction into capillaries?

Answer 5

High capillary density, surface area, good blood flow and small diffusion distance

Question 6

Compare coronary sinus blood to rest of body

Answer 6

Coronary sinus blood (venous system of coronary circulation) has the highest ppCO2 as it is able to uptake more CO2 and extract more O2
- It has an extraction of around 75% compared to 25% in mixed venous blood.

Question 7

Explain metabolic hyperaemia in the coronary circulation

Answer 7

• Extraction is near max during normal activity
• Therefore to provide more O2 during demand, we must increase blood flow
• Myocardium metabolism generates metabolites to produce vasodilatation, increase blood flow (metabolic hyperaemia)

e.g. Adenosine, produced by ATP metabolism and is released from cardiac myocytes.

Also increases in pCO2, H+, K+

Question 8

Describe Ischemic heart disease

Answer 8

Decreased blood flow and oxygen to the heart muscle

• Coronary arteries are functional end-arteries (arteries providing blood flow and O2 to particular areas of the heart) and therefore a decrease in perfusion produces major problems
• Heart is very susceptible to sudden and slow obstruction

· Sudden: acute thrombosis, produce myocardial infarction
. Slow: atheroma (sub-endothelium lipid plaques) chronic narrowing of lumen, produces angina

Question 9

Why must a good resting period be maintained in the heart for coronary blood flow?

Answer 9

• Systole obstructs coronary blood flow
• Coronary blood flow only occurs during diastole, so when the heart contracts, it causes mechanical obstruction of blood flow. A good resting period of the heart therefore needs to be maintained - this is difficult when BP drops low, HR is high, ventricles don’t empty completely.

Question 10

Explain the problem of coronary arteries being functional end arteries

Answer 10

• Partial occlusion reduces blood flow to that area
• Low numbers of cross-branching collateral vessels (Arterio-arterial anastomoses) as coronary arteries are functional end arteries
• Blood flow cannot by diverted to ischemic area as other arteries cannot divert blood to area

Question 11

Explain what will happen if there was total occlusion of LAD coronary artery

Answer 11

1. Ischemic tissue - MI
2. Acidosis caused by anaerobic respiration
3. Pain (stimulation of C fibres by H+)
4. Impaired contractility
5. Sympathetic activation
6. Arrhythmias
7. Cell death (necrosis)

Question 12

Compare the blood flow in an normal artery and occluded artery - explain how angina occurs

Answer 12

Normal artery

During normal activity, resistance (R) in a series add together and there is low resistance in a large coronary artery and high resistance in an arteriole.
In excersise, metabolic vasodilation of arterioles reduce total R so there is increased blood flow to meet increased O2 demands

Artery with atheroma

With stenosis in large coronary artery, increased resistance occurs. Metabolic hyperaemia occurs at rest so blood flow meets needs and arterioles reduces in resistance (by dilating).

During excersize, arterioles further dilate to reduce resistance but total resistance is still too high due to dominance of stenosis. O2 demand cannot be met and angina develops

Question 13

What are 3 mechanical factors reducing coronary flow?

Answer 13

1. Shortening diastole e.g. high heart rate
2. Increased ventricular end diastolic pressure e.g. volume overload heart failure. More blood left over in ventircul chamber increases pressure during diastole and window for coronary pressure decreases
3. Reduced diastolic arterial pressure e.g. hypotension, aortic regurgitation

So if pressure in ventricles is = or > than aorta no coronary perfusion occurs

Question 14

What are the special requirements that the cutaneous circulation requires?

Answer 14

• Defence against the enviroment
• Temperature regulation - blood flow delivers heat from body core
• Lewis triple response to trauma (increases blood flow). This is an inflammatory response.

Question 15

Explain regulation of temperature by skin

Answer 15

• Radiation (skin vs. ambient temperature)
• Conduction to skin to air – Convection removal
• Sweating (latent heat of evaporation)

Skin is a poikilo-thermic (not homeo-) organ and its temperature can range from 0oC to 40oC (briefly) without damage for short periods of time

Skin temperature depends on:

• Skin blood flow
• Ambient temperature

Question 16

What are the special structural features of cutaneous circulation?

Answer 16

• Arterio-Venous Anastomoses (AVAs)

There are direct connections of arterioles and venules expose blood to regions of high surface area.

When it is hot, temperature regulation nerves (sympathetic vasoconstrictor and sudomotor vasodilator fibres in the hypothalamus are activated. Arterioles vasodilate and blood goes into veins which act as a reservoir holding blood at surface to cool down. The cool blood is then returned to the heart.

Question 17

Name two temperature regulation nerves in the hypothalamus

Answer 17

Sympathetic vasoconstrictor

Sudomotor vasodilator fibres

Question 18

What are the special functional features of cutaneous circulation?

Answer 18

• Responsible to ambient and core temperatures - Increase in ambient temperature causes vaso and venodilation to help with heat loss
• Severe cold causes ‘paradoxical cold vasodilation’ - acute increase in peripheral blood flow observed during cold exposures.
• Core temperature receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow

Question 19

Describe cold induced vasoconstriction and paradoxial cold vasodilation

Answer 19

Cold get sensed by hypothalamus which sends information to sympathetic system which produces NA

• Cold induced vasodilation conserves heat. It is caused by abundance of a2 receptors (Gai -decrease AC/cAMP/PKA) on VSMCs in skin. They bind NA at lower temperatures than a1 receptors.
• Paradoxical cold vasodilation - to protect skin damage. It is so cold that the sympathetic transmission is paralysed. Long term exposure leads to oscillations of contraction/relaxation which increases blood flow. This is a last resort to prevent the skin from dying in cold conditions.

Question 20

What happens when there is increased core temperature by increased cutaneous perfusion?

Answer 20

Increased core temperature stimulate warmth receptors in the anterior hypothalamus which causes:

• Sweating: increases sympathetic activity (Ach) to sweat gland
• Vasodilation: Increases sympathetic sudomotor activity (Ach act on endothelium to produce NO) to arterioles in extremities

Question 21

What is the Lewis Triple Response of Skin to Trauma?

Answer 21

Local redness: Site of trauma
Local swelling: Inflammatory oedema (wheal)
Spreading flare: Vasodilatation spreading out from site of trauma.

This is important to increase delivery of immune cells, antibodies to site of damage to deal with invading pathogens

Question 22

What special problems does the skin encounter?

Answer 22

• Prolonged obstruction of flow by compression

Severe tissue necrosis can cause ‘bed sores’, heals, buttocks, weight bearing areas

Avoided by:

• Shifting position/ turning causing
• Reactive hyperaemia (on removing of compression)
• High skin tolerance to ischaemia
• Postural hypotension/oedema due to gravity

Often standing for long period will decrease CVP (hypotension - reduce starlings law) as more blood is diverted into arteries to vasodilate in the skin to cool down and increase capillary permeability (oedema).