10) Properties of special circulation

Question 1

What vessels perfuse the heart and what vessels drain the heart?

Answer 1

• Coronary arteries perfuse the heart. There are two of them which originate from the left side of the heart at the beginning (root) of the aorta as it leaves the left ventricle
• Coronary veins carry deoxygenated blood from the myocardium to the right atrium. Most of the blood of the coronary veins returns through the coronary sinus

Question 2

When does the heart get its blood supply?

Answer 2

• The heart gets its blood supply during diastole (when it is relaxed) rather than systole
• When the myocardium contracts it squeezes blood vessels found in the heart
• However when the heart relaxes, pressure in the aorta forces blood to the capillaries for gas exchange and back

Question 3

What are the special requirements for the coronary circulation?

Answer 3

• Needs a high supply of O2

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

Question 4

What are the special structural features of coronary circulation?

Answer 4

• High capillary density so shorter distances for diffusion
• Large surface area of O2 transfer
• These factors reduce diffusion distance to myocytes. Diffusion time is proportional to distance squared.
• Therefore O2 transport is fast

Question 5

How do cardiac muscles differ from skeletal muscles?

Answer 5

• Cardiac muscles contain higher numbers of fibres and capillaries which give rise to shorter diffusion distances

Question 6

How does the heart change when there is an increased demand of oxygen?

Answer 6

• During increased demand coronary blood flow increases.
• Production of vasodilators increases causing dilation
• Circulating adrenaline dilates coronary vessels due to abundance of B2-adrenoreceptors

Question 7

When do we experience coronary blood flow?

Answer 7

• Blood flow in the coronary vessels only occurs in diastole
• This is because during systole the heart muscles contract which squeeze these vessels shut

Question 8

Explain the Bohr shift in the heart

Answer 8

• Blood returning in the coronary sinus of the right atrium, from the myocardial tissue, has a greater carbon dioxide content due to high capillary density, large surface area and small diffusion difference
• This high CO2 and low pH shifts the Bohr Oxygen Dissociation curve to the right
• This means that haemoglobin has a lower affinity for oxygen and more oxygen is given into myocardial tissues

Question 9

What is metabolic hyperaemia?

Answer 9

• Extraction of oxygen is near max at normal activity
• However as oxygen demand increases we must increase blood flow.
• The myocardium produces metabolites which produce vasodilation which increases blood flow (called metabolic hyperaemia).

Question 10

Why are coronary arteries called functional end-arteries?

Answer 10

• They have no branching between them.

- This is a problem as decreased perfusion can produce major problems

Question 11

What is Ischaemic Heart Disease?

Answer 11

• A disease related to the narrowing of coronary arteries

Question 12

What are the different ways in which the coronary arteries can get obstructued?

Answer 12

• Sudden: Acute thrombosis which produce myocardial infarction
• Slow: Atheroma (sub-endothelium lipid plaques) leading to the chronic narrowing of the lumen resulting in angina (pain in the chest)
• Systole also obstructs coronary blood flow

Question 13

What is thrombosis?

Answer 13

• When a blood clot formed elsewhere in the body gets dislodged in the coronary artery

Question 14

What can thrombosis cause?

Answer 14

• Ischemic tissue
• Acidosis
• Pain (Stimulation of C-fibres)
• Impaired contractility
• Arrhythmias
• Cell death (necrosis)

Question 15

How does the heart reduce resistance to meet O2 demands when exercising?

Answer 15

• In normal hearts the resistance in the large coronary artery is low while the resistance in the arterioles are high
• During exercise the arterioles are vasodilated to reduce the resistance of the arterioles.
• As a result the total resistance is reduced and we have increased blood flow to meet increased oxygen demands

Question 16

Why do atheromas cause problem for people during exercise?

Answer 16

• At rest atheromas in the coronary artery increases resistance which lowers resistance in the arterioles making them dilated slightly
• Metabolic hyperaemia occurs at rest to meet blood flow needs
• During exercise the arterioles further dilate to reduce resistance
• Despite this total resistance is too high and so O2 demand cannot be met
• This causes anginas

Question 17

What are different mechanical factors that can reduce coronary flow?

Answer 17

• Shortening diastole (e.g. increasing heart rate) as this reduces the window in which we pump blood from the ventricles through the aorta
• Increasing ventricular end-diastolic pressure (e.g. heart failure through stiffening of ventricles) which will lower the efficiency of blood flow through the heart
• Reduced diastolic arterial pressure (e.g. hypotension) causing blood flow to be hindered and can even go on to increase heart rate

Question 18

What is cutaneous circulation?

Answer 18

• Blood supply to the skin

Question 19

What problem can inefficient cutaneous circulation cause?

Answer 19

• Skin ulcers due to impaired microvascular flow

Question 20

What are the different roles of cutaneous circulation?

Answer 20

• Defence against the environment as a physical barrier
• Lewis triple response to trauma which is where we increase blood flow to bring more immune cells to areas of damage in the skin
• Temperature regulation where blood delivers heat to core, radiation, sweating, conduction to skin and convection from skin

Question 21

What does poikilothermic mean?

Answer 21

• Skin is poikilothermic which means its temperature can vary over a large range without damage

Question 22

What does skin temperature depend on?

Answer 22

• Skin blood flow

- Ambient temperature

Question 23

What are arterio-venous anastomoses (AVAs)?

Answer 23

• They are a direct connection from the arterioles to the venules.
• They allow blood to completely bypass the capillaries located in the dermis

Question 24

How are vessels in the skin regulated by temperature?

Answer 24

• Vessels in the skin are regulated by temperature regulation nerves in the hypothalamus
• When it is too cold sympathetic vasoconstrictor fibres release noradrenaline which act on a1 receptors which cause them to constrict arterioles.
• This prevents them from entering the capillary loop in the dermis and instead they pass into the venules through the anastomoses
• This decreases the rate at which heat is lost from the body
• When it is too hot sudomotor vasodilator fibres release acetylcholine which act on the endothelium to produce nitric oxide
• This causes the arterioles to dilate allowing more blood to flow into the capillary loops in the dermis.
• This increases the rate at which heat is lost from the body

Question 25

What is paradoxical cold vasodilatation?

Answer 25

• This occurs in severe cold.
• Initially in cold the vasoconstriction of arterioles occurs which causes the blood to flow through the anastomoses
• This prevents blood and oxygen reaching the capillary beds in the dermis (cold-induced vasoconstriction)
• After a while the vessels become hypoxic and are unable to be be maintain their tone (be constricted)
• They relax and blood rushes into the capillary loops in the dermis to supply the skin with oxygen
• Once sufficiently supplied with oxygen the arterioles are able to vasoconstrict again to prevent blood from reaching the capillary bed and from loosing too much heat (Paradoxical cold vasodilation)

Question 26

Why is paradoxical cold vasodilation important for the body?

Answer 26

• It protects the skin from damage

- This is because if blood supply to the skin is inhibited for too long it can lead to skin damage and even necrosis

Question 27

What happens to cutaneous perfusion as you increase core temperature?

Answer 27

• As you increase core temperature cutaneous perfusion increases

Question 28

What are the effects of an increase in core temperature on your body?

Answer 28

• Sweating: increased sympathetic activity to sweat glands which is mediated by acetylcholine
• Vasodilation: Increased sympathetic sudomotor activity causes acetylcholine to act on endothelium to produce nitric oxide which dilates arterioles allowing blood flow to capillary loops

Question 29

What other factors can affect blood flow in vessels found in the skin?

Answer 29

• Baroreflex/RAAS/ADH-stimulated vasoconstriction of skin blood vessels: Blood is directed to more important organs/tissues during loss of blood pressure following haemorrhage, sepsis, acute cardiac failure.
• Emotional communication such as blushing ( caused by sympathetic sudomotor nerves)
• Response to skin injury for example The Lewis triple response

Question 30

What is The Lewis triple response of skin to trauma?

Answer 30

• When we sustain trauma to skin, the C-fibres detect this trauma and send an impulse to the brain
• There are some collateral axons that branch off the main C-fibre and go to the skin nearby which releases substance P.
• Substance P triggers histamine release which causes vasodilation
• First we experience redness due to capillary vasodilation caused by histamine
• Then we experience flare (a redness in the surrounding area) due to arteriolar dilation mediated by an axon reflex
• Finally we experience wheal (secretion of extracellular fluid from capillaries and venules).
• Overall there is an increased delivery of immune cells and antibodies to the site of damage to deal with invading pathogens

Question 31

What are some special problems with the skin?

Answer 31

• Prolonged obstruction of flow by compression. When we lay on part of a skin for a long time, parts of the skin are not perfused. This can lead to severe skin necrosis. This is countered by changing positions which can trigger reactive hyperaemia.
• Postural hypotension. Caused by standing for long periods of time in hot weather which decreases venous pressure (as blood is pulled towards the feet by gravity) and causes hypotension and increases capillary permeability (leading to oedema). Furthermore cardiac output is also reduced as the heart is not being stretched as much so the force of contraction is less (Starling’s law)