properties of special circulations

Question 1

Give example of some special circulations

Answer 1

Cerebral
Pulmonary
Skeletal muscle
Renal GI

Question 2

Describe the structure of the coronary circulation

Answer 2

• Two coronary arteries branch from the aorta to the right and left side of the heart
  The left branches into left anterior descending, left marginal and circumflex arteries
  The right branches into posterior descending and right marginal artery
• Cardiac veins carry poorly oxygenated blood from the myocardium to the right atrium via the coronary sinus (large vein formed of a collection of coronary veins)

Question 3

Describe the special requirements for the coronary circulation

Answer 3

→ Needs a high basal supply of oxygen

→ Oxygen supply needs to increase and adjust according to increased demand

Question 4

Describe the special features of the coronary circulation

Answer 4

High capillary density:
→ Large surface area for oxygen transfer
→ Reduce the diffusion distance between blood and myocytes = oxygen transport is faster
Cardiac muscles contain high numbers of fibres and capillaries resulting in shorter diffusion distances and faster oxygen transport

Question 5

Describe the special functional features of the coronary circulation during normal activity compared to during increased demand

Answer 5

Normal activity:
	· High blood flow 
	· Not much sympathetic innervation 
	· High NO released = vasodilation 
	· High oxygen extraction

Increased demand: summary is basically more vasodilation occurs to meet demands
· Coronary blood flow increases with demand
· Production of vasodilators (adenosine, K+, acidosis) out compete the low sympathetic vasoconstriction resulting in > vasodilation > blood flow
Circulating adrenaline also dilates the coronary vessels due to the abundance of B2-adrenoreceptors

Question 6

Describe how the body unloads 75% of its oxygen to myocardium during normal activity

Answer 6

Coronary sinus blood returning to the right atrium has a high concentration of carbon dioxide

High CO2 (and therefore low pH) shifts the curve to the right

Right bohr shift means that Hb has less affinity for oxygen meaning oxygen is unloaded to the myocardial tissues

Question 7

What causes the increased blood flow in myocardial tissues during increased demand?

Answer 7

To provide more oxygen during high activity, the blood flow needs to increase
Myocardium metabolism generates metabolites to produce vasodilation which in turn increases blood flow (metabolic hyperaemia)

Question 8

Give some examples of metabolites produced by myocardium metabolism during increased demand

Answer 8

· Adenosine (produced by ATP metabolism released from cardiac myocytes)
· K+
· H+ (acidosis)

Question 9

Describe ischaemic heart disease

Answer 9

Coronary arteries are functional end arteries and therefore decreased perfusion = major problems (increased susceptibility to sudden or slow problems)
Sudden: acute thrombosis produces myocardial infarction(heart attack from oxygen starvation)
Slow: atheroma chronic narrowing of lumen produces angina
Systole obstructs coronary blood flow

Question 10

What is meant by a functional end artery?

Answer 10

The sole artery that supplies oxygenated blood to the heart (for example: coronary arteries are the functional end arteries for the heart)

Question 11

What are some effects of thrombosis in coronary vessels on the heart?

Answer 11

Ischaemic tissues, acidosis, pain 
Impaired contractility 
Sympathetic activation (causing vasoconstriction)
Arrhythmia 
Cell death (necrosis)

Question 12

What is thrombosis?

Answer 12

Formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system

Question 13

Describe how a myocardial infarction can occur

Answer 13

Blockage leading to obstruction of blood flow to anterior left ventricle

Question 14

What is angina?

Answer 14

Chest pain due to not enough blood flow to myocardium (heart muscle)

Question 15

Describe how atheroma can be a problem during increased activity

Answer 15

Normal heart (no atheroma):
During exercise metabolic vasodilation reduce total resistance therefore increasing blood to meet the demands

Atheroma heart
· Stenosis (narrowing) in large coronary artery = increased resistance
· At rest, metabolic hyperaemia occurs, so blood flow meets needs
· During exercise, arterioles further dilate, BUT resistance is still TOO HIGH due to stenosis
· This means that the oxygen demand cannot be met, resulting in angina

Question 16

List some mechanical factors that reduce coronary flow

Answer 16

Coronary blood flow is restricted during diastole

• Shortened diastole (due to high heart rate)
• Increased ventricular end diastole pressure (due to heart failure causing aortic stenosis and stiffening of ventricle)
• Reduced diastolic arterial pressure (during hypotension and aortic regurgitation)

Question 17

List some special properties of cutaneous circulation

Answer 17

• Defence against environment
• Lewis triple response to trauma (increased blood flow)
• Temperature regulation

Question 18

What does the skin temperature depend on?

Answer 18

Skin blood flow

Ambient temperature

Question 19

Describe special structural features of cutaneous circulation

Answer 19

Arterio-venous anastomoses (AVA)
· They are direct connections of arterioles and venules which exposes the blood to regions of high surface area. This serves as a way to reduce body heat via convection, conduction, radiation and evaporation
Sympathetic vasoconstrictor fibres
· They release NA, acting on alpha-1 adrenoreceptors
Sudomotor vasodilator fibres
There is Ach acting on the endothelium to produce NO. This is driven by temperature regulation nerves in the hypothalamus

Question 20

Describe how the cutaneous circulation can aid heat loss

Answer 20

Increases ambient temperature causes vasodilatation and vasodilatation

Question 21

Describe how the cutaneous circulation can aid heat conservation

Answer 21

Decreases ambient temperature causes vasoconstriction and venoconstriction

Question 22

Describe the effect of cold temperatures on the skin blood flow

Answer 22

1. Cold induced vasoconstriction
   • This allows the body to conserve heat
   • Sympathetic nerves react to local cold by releasing NA, which binds to alpha-2 adrenoreceptors on vascular smooth muscle in skin
   
   2. Paradoxical cold vasodilation
      • Caused by paralysis of sympathetic transmission
      • Long term exposure can lead to oscillations of contract/relax

Question 23

How does increased core temperature affect cutaneous circulation

Answer 23

Increased core temp = increased cutaneous perfusion
The increased core temperature stimulates warmth receptors in the anterior hypothalamus causing:
1. Sweating
This is caused by increased sympathetic activity to sweat glands, mediated by acetylcholine
2. Vasodilation
Caused by increased sympathetic sudomotor activity, such that ACh acts on the endothelium to produce NO = dilates the arterioles in extremities

Question 24

List some other functional specialisations of the cutaneous circulation

Answer 24

1. Baroreflex/RAAS/ADH stimulated vasoconstriction of skin blood vessels
   The blood is directed to more important organs/tissues during the loss of blood pressure following haemorrhage, sepsis, acute heart failure, etc.
   This response is mediated by sympathetic vasoconstrictor fibres, adrenaline, vasopressin and angiotensin II. It is responsible for pale, cold skin of a patient in shock
   
   2. Emotional communication
      For example, blushing (involves sympathetic sudomotor nerves
   3. Response to skin injury
      The lewis triple response

Question 25

Describe the sequence of events that leads to the expression of the Lewis triple response

Answer 25

There is trauma on the skin. This is recognised by C fibres (nociceptive afferent fibres). There is an increased delivery of immune cells and antibodies to the site of damage to deal with the invading pathogens, so the axon reflex causes mast cell degranulation = releases histamine at trauma site

Question 26

What is the triple Lewis response?

Answer 26

1. Local redness at the site
   
   2. Local swelling
      A spreading flare

Question 27

Describe some problems that can arise due to the complexity of the cutaneous circulation

Answer 27

• Prolonged obstruction of flow by compression
This is severe tissue necrosis. It is found mostly at the heels, buttocks and other weight bearing areas
It can be avoided by shifting positions
• Postural hypotension/oedema due to gravity
Often standing for long periods of time in hot weather will decrease the central venous pressure (hypotension) and increase the capillary permeability (oedema)
This causes you to feel faint