9 Special Circulation

Question 1

What is the blood flow like to the heart and brain compared to skin and muscle?

Answer 1

much higher

Question 2

how much of the available oxygen goes to the heart?

Answer 2

1/3-2/3

Question 3

What does a large a-vO2 difference suggest?

Answer 3

the vascular bed has a high oxygen consumption

Question 4

What parts of the heart have a high blood supply?

Answer 4

the ventricles, particularly the left ventricle

Question 5

What do coronary veins drain into?

Answer 5

the coronary sinus, and then into the right atrium

some is drained via thesbian veins

Question 6

When might there be an increased demand for oxygen in the heart?

How is this met?

Answer 6

any increase in heart rate or contracitlity

this is met by an increase in flow

Question 7

What is the distribution of blood vessels like in cardiac muscles compared to skeletal muscle?

Answer 7

much much denser in cardiac muscle, 3000/mm2 compared to 400/mm2 in skeletal muscle

Question 8

How does myoglobin help cardiac muscle?

Answer 8

it has a much higher affinity to oxygen than haemoglobin, even if it can only bind to one molecule

the oxygen is passed along a chain of myoglobin, to maintin the diffusion of oxygen to mitochondria

Question 9

How would you increase coronary flow?

Answer 9

vasodilating coronary vessels

Question 10

What is the coronary flow reserve?

Answer 10

the difference between the resting level of flow, and the maximum flow that can be made by dilating the vessels

Question 11

What does the coronary flow reserve allow?

Answer 11

blood flow to increase by up to 5x than resting autoregulated levels when cardiac output is increased (this ability is reduced in some cardiac conditions)

Question 12

What would stimulate the coronary release of adenosine?

what does adenosine do?

Answer 12

increased metabolic activity
fall in coronary blood flow
fall in myocardial PO2

vasodilation - increase flow

Question 13

What effect does extracellular potassium have on flow?

Answer 13

it contributes to the inital increase in coronary perfusion, although it is unlikely to contribute much to the sustained rises in coronary flow

Question 14

How can acidosis alter vascular resistance?

Answer 14

changes in O2 and CO2 may cause acidosis, which can act to lower vascular resistance and increase oxygen supply

Question 15

Where does stenosis usually occur?

how significant does it need to be to have effect?

Answer 15

large epicardial arteries, needs to exceed 60-70% reduction in lumen diameter to have a significant impact on flow

Question 16

What happens in coronary artery disease?

Answer 16

affects artery lumen and causes endothelial damage and dysfunction

falls in NO and prostacyclin, which act as vasodilators

Question 17

How might you treat unstable angina?

Answer 17

balloon angioplasty
implanting a stent
coronary bypass graft

Question 18

Does grey or white matter receive more blood flow?

Answer 18

grey matter, despite the fact that there is less of it

Question 19

What is the effect of grey matter being sensitive to hypoxia?

Answer 19

fainting occurs within seconds of hypoxia

hypoxia for over 4 minutes leads to neuronal damage

Question 20

where do peel arteries go?

Answer 20

run over the brain surface

Question 21

What is the benefit to the grey matter having a dense network of capillaries?

Answer 21

large surface area

low diffusion distance

Question 22

Where are the fenestrations in cerebral blood vessels?

Answer 22

there are none, they are completely sealed by tight junctions

Question 23

How can substances pass into the brain?

Answer 23

over the 2 cell membranes and cytoplasm of the endothelial cells

Question 24

How do O2, Co2, alcohol, nicotine, and caffeine pass over the blood endothelium into the brain?

Answer 24

they are lipophilic, so passive diffusion

Question 25

How do glucose and amino acids pass into the brain?

Answer 25

transport proteins

Question 26

What happens to excess K+ in the brain? what adaptation facilitates this?

Answer 26

it is pumped out via Na+/K+ ATPase endothelial cells in the brain have 5x more mitochondria than muscle endothelium

Question 27

how is high cerebral basal flow maintained?

Answer 27

peripheral vasoconstriction sacrifices perfusion to the peripheries, but not the heart

Question 28

What is the autoregulatory range in the brain?

Answer 28

60-150mmHg

Question 29

What happens in extreme cerebral hypoxia?

Answer 29

mental confusion | syncopy

Question 30

What does cerebral hypercapnia cause? How?

Answer 30

vasodilation, especially in small peel arteries endothelial NO fall in myocyte pH

Question 31

What does cerebral hypocapnia cause? what effect will an arterial CO2 of 2kPa have on cerebral flow?

Answer 31

vasoconstriction this will halve total cerebral flow

Question 32

How can cerebral flow be measured?

Answer 32

retinal flow, as the retina is an embryological derivative of the nervous system

Question 33

What does severe hypoxia in the cerebrum stimulate?

Answer 33

vasodilation | hyperventilation via stimulation of carotid chemoreceptors

Question 34

What is the effect of hyperventilation in response to systemic hypoxia?

Answer 34

systemic hypoxia evokes hyperventilation so hypoxic dilation is often masked by hypocapnic vasocontriction

Question 35

What is the effect of systemic hypoxia on blood flow? what might have a different effect?

Answer 35

only a minor effect, it has to be really really hypoxic before an increase in cerebral flow is observed it is much more susceptible to changes in CO2 though

Question 36

what can astrocytes do?

Answer 36

they surround the synapses and may also be stimulated, signalling to smooth muscle to control vessel diameter

Question 37

What factors are important in coupling tissue metabolism to local flow?

Answer 37

``` increased interstitial [K+] adenosine neuronal NO metabolites released from astrocytes during increased activity CO2 (not as significant as the others) ```

Question 38

What might ICP be increased by?

Answer 38

Intracranial Pressure intracranial bleeding cerebral edema tumour

Question 39

What might the effects of increased ICP be?

Answer 39

venous collapsing | decreased effective CPP, causing a reduced blood flow

Question 40

How would we calculate CPP?

Answer 40

Central Perfusion Pressure = mean ABP - ICP

Question 41

When might postural hypotention lead to syncopy?

Answer 41

only really if the baroreceptor reflex is impaired

Question 42

What is a TIA?

Answer 42

transcient ischaemic attack temporary reduction in flow (minutes to hours)

Question 43

what is a stroke?

Answer 43

total interruption in blood flow

Question 44

What is the range of blood flow exhibited by the skin?

Answer 44

1-200 ml/min/100g

Question 45

What is the structure of cutaneous blood supply?

Answer 45

small arteries give rise to arterioles penetrating into the dermis these give rise to capillaries looping under the epidermis venules return the blood to the venous plexus the venous plexus could also receive blood directly from subcutaneous arteries via atriovenous anastomoses

Question 46

What do atriovenous anastomoses allow?

Answer 46

the blood to bypass cutaneous circulation

Question 47

How much of cutaneous blood volume is in the venous plexus?

Answer 47

most of it | can be up to 1.5L

Question 48

What affects changes in AVAs?

Answer 48

sympathetic activity, alpha adrenoreceptors | they do not show functional hyperaemia or autoregulation

Question 49

What happens when temperature rises?

Answer 49

sympathetic nervous system stimulates vasodilation and sweat release

Question 50

What stimulates sweat release and what effect does it have?

Answer 50

hypothalamic stimulation of sympathetic cholinergic fibres innervating the sweat glands evaporates to remove heat has an enzyme telling tissue to release bradykinin, a vasodilator acting on smooth muscle

Question 51

How does an increased temperature affect baroreceptor afferent activity to CNS?

Answer 51

vasodilatoin of arterioles and AVAs decreases TPR decreases ABO decreases baroreceptor afferent activity to CNS

Question 52

What effect does decreasing baroreceptor afferent activity to CNS have?

Answer 52

decreased vagal activity to SA node | increased sympathetic activity to SA node

Question 53

What happens to cutaneous circulation when temperature drops?

Answer 53

increase in sympathetic activity and noradrenaline release causes vasoconstriction of cutaneous arterioles increased sympathetic activity to arteriovenous anastomoses causes constriction to prevent flow to veins this increases resistance to blood flow, diverting flow to the interior deeper veins in fat

Question 54

What happens in prolonged exposure to cold?

Answer 54

vasoconstriction in cutaneous circulation changes to a paradoxical cold circulation

Question 55

What causes paradoxical cold circulation?

Answer 55

paralysis of noradrenergic neurotransmission in response to cold, and the release of vasodilators like prostacyclin redness caused by increased affinity of O2 for haemoglobin (left shift of O2 dissociation curve)

Question 56

Why is it useful to have arteries and veins next to each other?

Answer 56

counter-current heat exchange preserves body temperature

Question 57

What happens in Raynaud's disease?

Answer 57

cold / emotional stimuli lead to vasoconstriction white, blue, then red numbness, pain, burning sensation