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Cardiovascular System > 7. Microcirculation > Flashcards

7. Microcirculation Flashcards

1
Q

Microcirculation

A

Every organ in body has its own microcirculation

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2
Q

Why do we regulate blood flow to each microcirculation?

A

Different amounts of blood go to different tissues dependent on need

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3
Q

What equation connects blood flow, pressure gradient and resistance?

A

Flow = pressure gradient/ resistance

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4
Q

What factors affect vascular resistance?

A

Vessel radius
Vessel length
Blood viscosity

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5
Q

What is the relationship between resistance and radius?

A

Poiseuille’s law: resistance is inversely proportional to r^4
Halving the radius decreases the flow 16x

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6
Q

What are the major resistance vessels?

A

Arterioles

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7
Q

What effect does increasing blood pressure have on blood flow?

A

Increases pressure gradient

Increases flow

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8
Q

What effect does arteriolar vasoconstriction have on blood flow?

A

Decreases radius of arterioles supplying tissue
Increases resistance
Decreases flow

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9
Q

What is usually the pressure at the start of the arteriole?

A

MAP ~ 93 mmHg

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10
Q

What is usually the pressure at the end of the arteriole?

A

Changes depending on which tissue bed,

but often ~37 mmHg

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11
Q

Why can change in blood pressure be substituted by MAP in the flow equation for an organ?

A

Arterial BP always MAP
BP in veins is usually ~0 mm Hg
so the change in BP through a capillary bed is ~ MAP.

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12
Q

What is the normal state of vascular smooth muscle? Why?

A

Normally in a state of partial vascular constriction (vascular tone)
Allows further constriction or dilation

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13
Q

What are the 2 controls of vessel radius?

A 
Intrinsic controls (independent of nervous/ endocrine stimulation): match blood flow to metabolic needs of specific tissue
Extrinsic controls (via nerves/ blood, centrally coordinated): regulate systemic arterial BP
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14
Q

Describe how vessel radius responds to the chemical environment.

A

When tissues are highly metabolically active they will produce a lot of ATP and use up a lot of O2.
Increased uptake of O2 is detected by the tissues, which sends a message to the arteriolar smooth muscle to dilate.
= active hyperaemia.

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15
Q

Describe how vessel radius responds to the physical environment.

A

Decrease in blood temperature causes vascular smooth muscle to constrict so that less blood reaches the surface and so less heat is radiated away.

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16
Q

Describe myogenic autoregulation

A

Start exercising BP increases
Increases pressure gradient across every tissue bed
Can’t happen as not enough blood to perfuse every tissue in the body
(Prioritise muscles, brain, heart. Divert away from gut)
So you get myogenic autoregulation in less prioritised places e.g. gut
Initially causes increased flow to tissue
Sensed by stretch receptors in arterioles
Signal to arterioles to constrict to decrease blood flow back to normal levels

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17
Q

How can the flow rate, pressure gradient and resistance equation be applied to the entire circulation?

A

Flow rate is CO
Pressure gradient is MAP
Resistance is TPR
CO = MAP / TPR

18
Q

What are the 2 pathways controlling arterial blood pressure?

A

Neural

Hormonal

19
Q

Where is the centre that regulates arterial blood pressure found?

A

In the medulla:

cardiovascular control centre

20
Q

What are the main vasoconstrictor hormones?

A

Vasopressin
Angiotensin II
Adrenaline
Noradrenaline

21
Q

What is capillary exchange?

A

Delivery of metabolic substrates to the cells of an organism

22
Q

What tissues have a high capillary density?

A

Skeletal muscle
Myocardium
Brain
Lungs

23
Q

Why is capillary density important?

A

Need to ensure every cell is close to a capillary
Ideally suited to enhance diffusion:
Minimise diffusion distance
Maximise SA and time for diffusion

24
Q

What is the function of pre-capillary sphincters and where are they found?

A

Skeletal muscle
Allow certain capillaries to be ‘shut down’ at rest, otherwise vast amounts of blood would flow to skeletal muscle unneccesarily

25
Q

What are the 3 main types of capillary and how do they differ?

A

CONTINUOUS: small water filled gap junctions that allow the passage of electrolytes and small molecules
FENESTRATED: slightly bigger gaps allowing slightly larger molecules to pass through
DISCONTINUOUS: large gaps in the capillary

26
Q

What is the most common type of capillary?

A

Continuous

27
Q

Where are fenestrated capillaries found and why?

A

Glomerulus

need to filter molecules of a certain size

28
Q

Where are discontinuous capillaries found and why?

A

Bone marrow: WBCs need to exit bone marrow tissue into blood

Liver: deals with metabolism, need to allow substances to access liver tissue to be metabolised

29
Q

How is the blood brain barrier different to other capillaries and why?

A

Blood brain barrier is a continuous capillary structure with tight junctions instead of gap junctions, giving brain much tighter control over what accesses brain tissue
If very lipid soluble, will access brain anyway
If water soluble, will need a protein transporter
Brain is more protected

30
Q

What is the difference between hydrostatic and oncotic pressure in capillaries?

A 
Hydrostatic =  ‘pushing’ force
Oncotic = ‘pulling’ force
31
Q

Why is oncotic force needed?

A

BP generated by the heart forces blood through capillary at relatively high speed
Would cause fluid to be squeezed out into tissues and BP would decrease

32
Q

Starlings forces

A

Must be a balance between the hydrostatic pressure of the blood in the capillaries and the osmotic attraction of the blood for the surrounding fluids

33
Q

Describe the pressure changes across a capillary

A

Pressure drops across capillary
So hydrostatic pressure is much higher at beginning than end
No protein outside capillaries, but lots in, so oncotic pressure remains constant
As blood enters capillaries, net force pushes fluid out
At end, net force pushes fluid back in

34
Q

Ultrafiltration

A

If pressure inside the capillary > in the IF

35
Q

Reabsorption

A

If inward driving pressures > outward pressures across the capillary

36
Q

What is the significance of the fact that ultrafiltration is more effective than reabsorption?

A

Always losing fluid from blood system, if there wasn’t a mechanism to draw fluid back in BP would just keep dropping
necessitates lymphatic system

37
Q

Describe 5 characteristics of the lymphatic system.

A

Consists of blind-ended lymphatic capillaries
Next to blood capillaries
Valves - prevent backflow
All but the right upper quadrant (RUQ) of the body drains via the thoracic duct into the left subclavian vein
RUQ drains into the left subclavian vein

38
Q

What are the key roles of the lymphatic system?

A

Return excess fluid back into blood
Immune surveillance system: If infection present in fluid in lymphatic capillaries, detected when passes through lymph nodes, activates production of lymphocytes

39
Q

What ensues if rate of production (amount of fluid lost into tissues) > rate of drainage?

A

Oedema

40
Q

Volume of lymph fluid produced per day

A

3L

Cardiovascular System (16 decks)

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