7. Microcirculation

Question 1

Microcirculation

Answer 1

Every organ in body has its own microcirculation

Question 2

Why do we regulate blood flow to each microcirculation?

Answer 2

Different amounts of blood go to different tissues dependent on need

Question 3

What equation connects blood flow, pressure gradient and resistance?

Answer 3

Flow = pressure gradient/ resistance

Question 4

What factors affect vascular resistance?

Answer 4

Vessel radius
Vessel length
Blood viscosity

Question 5

What is the relationship between resistance and radius?

Answer 5

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

Question 6

What are the major resistance vessels?

Answer 6

Arterioles

Question 7

What effect does increasing blood pressure have on blood flow?

Answer 7

Increases pressure gradient

Increases flow

Question 8

What effect does arteriolar vasoconstriction have on blood flow?

Answer 8

Decreases radius of arterioles supplying tissue
Increases resistance
Decreases flow

Question 9

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

Answer 9

MAP ~ 93 mmHg

Question 10

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

Answer 10

Changes depending on which tissue bed,

but often ~37 mmHg

Question 11

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

Answer 11

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

Question 12

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

Answer 12

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

Question 13

What are the 2 controls of vessel radius?

Answer 13

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

Question 14

Describe how vessel radius responds to the chemical environment.

Answer 14

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.

Question 15

Describe how vessel radius responds to the physical environment.

Answer 15

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

Question 16

Describe myogenic autoregulation

Answer 16

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

Question 17

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

Answer 17

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

Question 18

What are the 2 pathways controlling arterial blood pressure?

Answer 18

Neural

Hormonal

Question 19

Where is the centre that regulates arterial blood pressure found?

Answer 19

In the medulla:

cardiovascular control centre

Question 20

What are the main vasoconstrictor hormones?

Answer 20

Vasopressin
Angiotensin II
Adrenaline
Noradrenaline

Question 21

What is capillary exchange?

Answer 21

Delivery of metabolic substrates to the cells of an organism

Question 22

What tissues have a high capillary density?

Answer 22

Skeletal muscle
Myocardium
Brain
Lungs

Question 23

Why is capillary density important?

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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

Question 26

What is the most common type of capillary?

Answer 26

Continuous

Question 27

Where are fenestrated capillaries found and why?

Answer 27

Glomerulus | need to filter molecules of a certain size

Question 28

Where are discontinuous capillaries found and why?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

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

Answer 30

``` Hydrostatic = ‘pushing’ force Oncotic = ‘pulling’ force ```

Question 31

Why is oncotic force needed?

Answer 31

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

Question 32

Starlings forces

Answer 32

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

Question 33

Describe the pressure changes across a capillary

Answer 33

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

Question 34

Ultrafiltration

Answer 34

If pressure inside the capillary > in the IF

Question 35

Reabsorption

Answer 35

If inward driving pressures > outward pressures across the capillary

Question 36

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

Answer 36

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

Question 37

Describe 5 characteristics of the lymphatic system.

Answer 37

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

Question 38

What are the key roles of the lymphatic system?

Answer 38

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

Question 39

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

Answer 39

Oedema

Question 40

Volume of lymph fluid produced per day

Answer 40

3L