Properties of special circulation

Question 1

What are the 3 main characteristics of a special circulation?

Answer 1

Special requirements met by the circulation
Special structural or functional features of the circulation
Specific problems relating to that circulation

Question 2

Where do the left and right coronary arteries originate from within the heart?

Answer 2

From the aorta immediately after the cusps of the aortic valve.

Question 3

What part of the heart does the left coronary artery supply?

Answer 3

Supplies the left ventricle and interventricular septum

Question 4

What part of the heart does the right coronary artery supply?

Answer 4

Supplies mainly the right ventricle

Question 5

What is the function of the coronary veins?

Answer 5

Carry blood from the myocardium to the right atrium. Most of the blood of the coronary veins returns through the coronary sinus.

Question 6

What are the special requirements of the coronary circulation?

Answer 6

Needs to deliver O2 at a high rate to match oxygen demand of myocardium which is 20x the demand of resting skeletal muscle

Needs to Increase O2 supply in proportion to increased demand/cardiac work

Question 7

What is a special structural feature of the coronary circulation?

Answer 7

High myocardial capillary density

Question 8

Explain the effect that the high myocardial capillary density has on 02/nutrient transfer?

Answer 8

Creates a large endothelial surface area for 02 transfer and reduces maximum diffusion distance. These effects increase rate of 02/nutrient transfer.

Question 9

Why is capillary density so much higher in cardiac muscle than in skeletal muscle?

Answer 9

Although both tissues have roughly one capillary per muscle fibre, the myocardial muscle fibres are smaller so more of them can fit in the same space thus increasing density.

Question 10

Describe the special functional features of the coronary circulation during normal activity?

Answer 10

Coronary blood flow per gram of tissue is about 10x the flow per weight of rest of body.

Relatively sparse sympathetic innervation – means arterioles don’t vasoconstrict a lot meaning higher blood flow through them

High amount of endothelial nitric oxide released leading to vasodilation – helps maintain high blood flow through heart

Myocardium must extract 65-75% of the O2 to meet its needs – average in body is 25% at rest.

Question 11

Describe how the special functional features of the coronary circulation change during increased oxygen demand?

Answer 11

Coronary blood flow increases in proportion to demands.

Production of vasodilators (adenosine, K+, acidosis) out-compete relatively low sympathetic vasoconstriction.

Circulating adrenaline dilates coronary vessels due to abundance of β2-adrenoceptors

Question 12

What is the bohr shift?

Answer 12

The shift in the oxygen dissociation curve caused by changes in the concentration of carbon dioxide or the pH of the environment.

Question 13

What causes the oxygen dissociation curve to shift to the right?

Answer 13

Increase in CO2 results in a decrease in blood pH because carbon dioxide reacts with water to form carbonic acid resulting in haemoglobin having a lower affinity for oxygen and so more O2 is given up to the tissues.

Question 14

What causes the oxygen dissociation curve to shift to the left?

Answer 14

Decrease in CO2 results in an increase in blood pH, which results in haemoglobin having a higher affinity for oxygen and so less O2 is given up to the tissues.

Question 15

How can the bohr shift be used to explain why the myocardium is able to extract so much oxygen from the blood?

Answer 15

Coronary sinus blood returning to right atrium from myocardial tissue has a greater carbon dioxide content due to high capillary density, surface area and small diffusion difference.
The high CO2 and low pH mean that haemoglobin has less affinity for oxygen and more O2 is given up to the myocardial tissues.

Question 16

How is the extra 02 needed by the myocardium at high cardiac work rates suplplied?

Answer 16

Supplied mainly by increases in coronary blood flow rather than extraction

Question 17

Why is the extra 02 supplied by an increase in blood flow rather than extraction?

Answer 17

Extraction is already near max during normal activity.

Question 18

What is the relationship between coronary blood flow and cardiac work rate?

Answer 18

Coronary blood flow and cardiac O2 consumption, used to measure cardiac work, increase linearly until very high cardiac work rates where O2 extraction also begins to increase.

Question 19

How is the relationship between coronary blood flow and cardiac work rate achieved by the myocardium?

Answer 19

Linear relationship is achieved by the fact that myocardium metabolism produces vasodilator substances in proportion to its work rate thus increasing blood flow accordingly.

Question 20

Name some of the vasodilator substances released by the myocardium?

Answer 20

Adenosine, pCO2, H+, K+

Question 21

What is this relationship an example of?

Answer 21

Metabolic Hyperaemia

Question 22

What are some of the special problems with the coronary circulation?

Answer 22

Systole obstructs coronary blood flow

Human coronary arteries are functional end arteries

Acute coronary obstruction causes myocardial infarction

Chronic coronary stenosis causes stable angina

Question 23

Explain why Systole obstructs coronary blood flow and why it’s a problem for coronary circulation

Answer 23

2/3 of coronary arterial system is intramural, inside myocardium. Intramural vessels are compressed during systole, especially during isovolumetric contraction, to the point where coronary artery flow stops or is reversed briefly so myocardial perfusion is very much impaired. Full flow is only restored during diastole, around 80% of coronary flow occurs during diastole.

Question 24

Explain the concept of a functional-end artery and how it cause problems for coronary circulation

Answer 24

Functional-end arteries have very little Arterio-arterial anastomoses, cross-branching collateral vessels. Although we have some they are quite narrow and transmit very little flow. All this means that an obstruction reduces downstream tissue perfusion by a very large amount leading to myocardial ischaemia.

Question 25

Explain the problems caused by the obstruction of a coronary artery.

Answer 25

Sudden obstruction of coronary artery by thrombus produces a myocardial infarct downstream from the obstruction. Accumulation of interstitial H+ and K+ stimulates nociceptors causing chest pain. Contractility is impaired causing acute cardiac failure. Sympathetic activity increases due to shock/pain and arrhythmias can also occur because conduction of depolarisation across necrotic tissue doesn’t occur

Question 26

Explain how coronary stenosis leads to stable angina

Answer 26

Stenosis of artery due to atheroma formation may not have effect on flow during rest. This is because rise in resistance in coronary artery caused by atheroma is offset by fall in resistance further away due to distal arteriogenesis and dilatation. However, during exercise the stenosed artery can’t reduce the distal resistance enough to combat the large increase in resistance due to the atheroma. This prevents flow from increasing sufficiently enough to meet O2 demands of myocardium. This leads to angina during exercise.

Question 27

What are some of the factors that can reduce the window of coronary flow during diastole?

Answer 27

Shortening diastole, eg. high heart rate

Increased ventricular end-diastolic pressure, eg. Heart failure (aortic stenosis, stiffening of ventricle).

Reduced diastolic arterial pressure, eg. hypotension, aortic regurgitation

Question 28

What are the special requirements of the cutaneous circulation?

Answer 28

Must regulate core temperature - Skin is main heat-dissipating organ in the human body

Must participate in the defence against the environment: Cutaneous circulation helps with defence through the Lewis triple response to trauma (increased blood flow).

Question 29

What are the 3 ways in which the skin dissipates heat and explain how each of them does this?

Answer 29

Radiation: Rate of heat loss due to radiation proportional to difference between ambient temperature and skin temperature.

Conduction/Convection: Warm skin heats up adjacent air via conduction and warm air removed via convection.

Evaporation: Sweat glands produce and deliver sweat to skin where it evaporates thus consuming 2.4kJ of heat per gram of sweat (latent heat of evaporation).

Question 30

Name a special structural feature of the cutaneous circulation

Answer 30

Contains arterio-venous anastomoses (AVAs)

Question 31

What are some of the characteristics of arterio-venous anastomoses (AVAs)?

Answer 31

Direct connections of arterioles and venules. Occur in exposed regions with high surface area/volume ratio e.g. fingers and toes.
AVAs have little basal tone and are controlled mostly by sympathetic vasoconstrictor fibres.

Question 32

Explain the effect the sympathetic vasoconstrictor fibres have on the AVAs

Answer 32

These sympathetic vasoconstrictor fibres release noradrenaline which binds to and activates α1 adrenoceptors leading to vasoconstriction.
This vasoconstriction reduces flow through the AVAs meaning less blood flows to the venous plexus resulting in reduced cutaneous blood flow meaning reduced heat loss via radiation.

Question 33

What other type of fibre can control AVAs?

Answer 33

Sudomotor vasodilator fibres

Question 34

Explain the effect the sudomotor vasodilator fibres have on the AVAs

Answer 34

Release Acetylcholine which acts on endothelium to produce nitric oxide resulting in vasodilation.
This results in greater cutaneous blood flow resulting in greater heat loss via radiation.

Question 35

What 2 types of temperature affect cutaneous circulation?

Answer 35

Ambient and Core temperature.

Question 36

How do cutaneous vessels respond to heat and to cold?

Answer 36

Cutaneous vessels constrict in response to cold and dilate in response to heat.

Question 37

What effect does ambient temperature have on cutaneous blood flow?

Answer 37

If ambient temp too low cold-induced vasoconstriction occurs. This conserves heat by reducing blood flow to skin. After a while however, to protect skin from damage due to too little blood flow paradoxical cold vasodilatation occurs which increases blood flow.

Question 38

How does cold induced vasoconstriction conserve heat?

Answer 38

Sympathetic nerves react to local cold by releasing noradrenaline which binds to a2 receptors on vascular smooth muscle in skin causing vasoconstriction. Less blood flows through venous plexus so less heat loss via radiation.

Question 39

What is paradoxical cold vasodilation and how does it protect against skin damage?

Answer 39

Caused by paralysis of sympathetic transmission to cutaneous vessels. This causes vessels to relax thus reducing resistance and allowing greater blood flow. This allows for skin to get more oxygen thus protecting skin from necrosis.

Question 40

What affect does core temperature have on cutaneous blood flow?

Answer 40

Increased core temperature, e.g. due to exercise, leads to the stimulation of thermoreceptors in anterior hypothalamus.
This leads to Sweating and vasodilation.

Question 41

How does heat induced vasodilation increase heat loss?

Answer 41

Sympathetic sudomotor activity such that acetylcholine act on endothelium to produce Nitric Oxide which dilates arterioles in extremities and increasing blood flow to those areas. This increases heat loss from the extremities which works to reduce the core body temperature.

Question 42

What are some other functional specialisations of cutaneous circulation?

Answer 42

Baroreflex/RAAS/ADH-stimulated vasoconstriction of skin blood vessels

Emotional communication: E.g. blushing (sympathetic sudomotor nerves)

Response to skin injury: The Lewis triple response to trauma

Question 43

Explain what baroreflex/RAAS/ADH-stimulated vasoconstriction of skin blood vessels does to blood flow and when it would occur?

Answer 43

Blood directed to more important organs/tissues during loss of Blood pressure following haemorrhage, sepsis, acute cardiac failure.

Question 44

What is it mediated by?

Answer 44

sympathetic vasoconstrictor fibres , adrenaline , vasopressin and angiotensin II.

Question 45

What are some special problems that can happen to the cutaneous circulation?

Answer 45

Prolonged obstruction of flow by compression: Leads to Severe tissue necrosis as compression of an area more than a few mm thick means diffusion alone can’t provide the oxygen that area needs.

Postural hypotension / oedema due to gravity