Microcirculation

Question 1

What is microcirculation and describe its arrangement?

Answer 1

The blood supply for each individual tissue

1st order arteriole -> terminal arteriole -> capillary -> post capillary venule -> venule

Question 2

What is the overall aim of the CVS?

Answer 2

To provide adequate blood flow through the capillaries

Question 3

What is blood flow rate? How is it calculated?

Answer 3

The volume of blood passing through a vessel per unit time

F = delta P/ R
F - flow rate, P - pressure gradient (pressure between start and end of vessel), R - vascular resistance (hindrance to blood flow due to friction between moving fluid and stationary vascular walls)

Question 4

If the pressure gradient is higher, what will happen to flow rate?

Answer 4

It will increase

Question 5

What factors affect the resistance ?

Answer 5

blood viscosity, vessel length and vessel radius

Question 6

What is the relationship between resistance and vessel radius?

Answer 6

R is directly proportional to 1/r^4

Question 7

If blood pressure increases, what happens to delta P, R and F?

Answer 7

delta P = increases
R =
F = increases

Question 8

If arteriolar vasoconstriction occurs, what happens to delta P, R and F?

Answer 8

delta P =
R = increases
F = decreases

Question 9

What are the major resistance vessels and hence major determinant of blood flow?

Answer 9

arterioles

Question 10

What is the equation for blood flow rate in an organ and how is it derived?

Answer 10

Forgan = delta P (=MAP)/Rorgan

A arterial pressure is around MAP, pressure going out is around 0 (pressure at end of capillaries is usually venous pressure), delta P is usually MAP
Without this pressure difference the blood would not reach capillary beds

Question 11

What is vascular tone and its importance?

Answer 11

Arterial smooth muscle is in a state of partial constriction - vascular tone.
This allow a way to change blood flow by increasing or decreasing (constriction or dilation) radius.

Question 12

What is the importance of arterioles in causing pressure changes?

Answer 12

They are major resistance vessels.
BP doesn’t change much through arteries (in any artery it is around MAP = 93mmHg) so in arteioles also around 93. In venous system (e.g. venules) it is around 0mmHg. The biggest change occurs across arterioles - pressure drops the most here. In different tissues the pressure at the end of the arterioles differs but the pressure at the start of the arteriole is constant.

Question 13

What are the two functions for which the radii of arterioles are adjusted, and how are they regulated?

Answer 13

1. To match blood flow to the metabolic needs of tissue
   It uses local intrinsic control -> tissue determines it not hormones or nerves. Examples are physical (stretch) and chemical (metabolic activity)
2. To regulate arterial blood pressure
   Controlled by extrinsic controls e.g. the brain and hormones

Question 14

What is active hyperemia?

Answer 14

When muscles become more active they consume more oxygen and so metabolise more. This is detected by local tissues and signals are sent to the arterioles to dilate. This is known as active hyperemia. It is a response to chemical changes in the environment. It is one of the reasons arterioles adjust their radii to meet metabolic demands.

Question 15

How do tissues respond to a change in temperature?

Answer 15

If blood temperature decreases, microcirculation detects this change. The arteriole smooth muscle contracts and so less blood reaches the surface and less is radiated away. This is vasoconstriction and a response to physical change in the environment.

Question 16

How does high blood pressure during exercise for example lead to a myogenic vasoconstriction?

Answer 16

BP increases when doing physical activity in order to get more blood to muscles a. Blood cannot just distribute to all tissues, it only goes to the ones that require it. Blood vessels stretch and the tissues detect this. Tissue that don’t need more blood vasoconstrict - this process is called autoregulation. It is a response to physical change - stretch. It is one of the reasons arterioles adjust their radii to match blood flow to metabolic needs of tissues.

Question 17

How can F=delta P/R be applied to the whole circulation?

Answer 17

CO = MAP/TPR

Question 18

How does neural regulation control arterial blood pressure?

Important receptor

Answer 18

BP is controlled by cardiovascular control centre in the medulla. To increases BP, vasconstriction occurs and so TPR increases. Blood flow is sacrificed to some tissues to maintain blood pressure and ensure brain is perfused.

Release of neurotransmitters from nerve to alpha adrenoreceptors which respond to noradrenaline (sympathetic) and they bind to vessel and lead to constriction.

Question 19

How does hormonal regulation affect blood pressure?

Answer 19

The brain can stimulate the release of hormones affecting BP. Stimulating vasopressin and angiotensin 2 release lead to vasoconstriction.

Question 20

How are capillaries suited to exchange?

Answer 20

very narrow walls - 1 micrometer thick

extensive branching - no capillary far from cells

Question 21

Which 3 things are important to maximise exchange?

Answer 21

• minimise diffusion distance
• maximise SA
• maximise diffusion time

Question 22

Why is capillary density important?

Give examples of tissues with different capillary densities

Answer 22

The more metabolically demanding a tissue in, the greater the capillary density and more branching.

Lungs - high to maximise gas exchange

heart/brain/muscle - high density due to metabolic activity

adipose tissue - low density as doesn’t require lots of rapid responses to occur

Question 23

What do skeletal muscles use at rest?

Answer 23

At rest skeletal muscles have lots of shut off vessels - pre capillary sphincters shut off arterioles

Question 24

What are the three types of capillaries, describe them and where they may be found?

Answer 24

1) continuous - lots of capillaries have this, small water filled gap junctions between endothelial cells allowing electrolytes/small molecules to move through
2) fenestrated - leakier, bigger holes called fenestrae so larger molecules can cross. Found in kidneys
3) discontinous - large holes in capillaries, imp in bone marrow to allow WBCs into blood

Question 25

What is the blood brain barrier and its importance?

Answer 25

Really tight gap junctions between endothelial cells so access to the brain is tightly restricted. Substances must cross the endothelial cells, protects the brain. Some areas of the brain have a discontinuous blood brain barrier- normal water filled gap junctions

Question 26

How does fluid move across a capillary?

Answer 26

Volume of protein free plasma filters out of the capillary into surrounding fluid and is reabsorbed - BULK FLOW

Question 27

What forced affect bulk flow (starling’s)?

Answer 27

hydrostatic pressure, oncotic pressure

Question 28

What is the process of bulk flow?

Answer 28

formation of tissue fluid

Oncotic pressure is constant as plasma concentration doesn’t change

Question 29

The structure of the lymphatic system and movement of lymph.

Answer 29

The lymphatic system is not a closed loop. The lymphatic capillaries are blind ended. Valves in the vessels means no backward flow into the tissues. The movement of lymph driven by lymphatic pressure (muscle pressure, lungs inflating and deflating, no heart). Lymphatics have immune role

Question 30

Where does lymph drain?

Answer 30

The thoracic duct drains the rest of the body into the left subclavian vein
Right lymphatic duct drains upper right side of body into right subclavian

Question 31

How much lymph is returned to the circulation each day?

Answer 31

3L