Microcirculation

Question 1

What does every organ have

Answer 1

Its own microcirculation

Question 2

What is meant by the microcirculation

Answer 2

It is the branch of the arterial system that enters the tissues

Question 3

Describe the arrangement of the microcirculation

Answer 3

▪ The blood flows through the arteries which branch off into the first order arterioles → terminal arterioles → capillaries.
▪ Capillaries carry out exchange functions and then the blood flows → post-capillary venules (pericytic) → venules → veins.

Question 4

What is the overall aim of the CVS

Answer 4

Overall aim of CVS = Adequate blood flow

through the capillaries

Question 5

What is meant by blood flow rate

Answer 5

Blood flow rate – Volume of blood passing through a vessel per unit time

Question 6

How do we calculate pressure differences in the microcirculation

Answer 6

Pressure A (start of arteriole) - Pressure B (leaving the terminal arteriole)
Greater difference= Greater flow rate

Question 7

What is meant by resistance in the blood vessels

Answer 7

‘Hindrance to blood flow due to friction between moving fluid and stationary vascular walls’

Question 8

How is Cardiac Output related to MAP and TPR

Answer 8

CO= MAP/TPR

Question 9

What is meant by TPR

Answer 9

▪ TPR is the sum of resistance of all arterioles in the body.

Question 10

What is the resistance dependant on

Answer 10

o Blood viscosity. o Vessel length. o Vessel radius (MOST IMPORTANT – Poiseuille’s Law).
Halving the radius decreases the flow 16 times

Question 11

What is the effect of increasing blood pressure

Answer 11

Increases change in pressure

Therefore increasing flow

Question 12

What is the effect of arteriolar vasoconstriction

Answer 12

Increased resistance

Therefore decreased flow

Question 13

What is the velocity of blood flow inversely proportional to

Answer 13

The velocity of blood flow is inversely related to the total cross-sectional area. The branching nature of the circulatory system means that the total cross-sectional area of the capillaries is much greater than that of arterioles. This substantially reduces the velocity in the capillaries, allowing diffusion to take place.

Question 14

In a single vascular bed, what is blood flow determined by. How does this compare to that of the whole vasculature

Answer 14

In a single vascular bed, the blood flow is determined by the arterial pressure - venous pressure (perfusion pressure) divided by the resistance to flow in the vascular bed.
As a whole, total blood flow= CO
And C0=MAP/TPR

Question 15

What is the major determinant of blood flow in the body

Answer 15

This usually means the major determinant of the blood flow in the body is the resistance of the arterioles in the organ (radii). Arteriolar radius is controlled by numerous factors.

Question 16

Describe the differences in resistance of the capillaries and arterioles

Answer 16

Total resistance in the vasculature is greatest in the arterioles, through a combination of their length and reduced radius without a significant change in their cross-sectional area.
Capillary resistance is less than arteriolar resistance because capillaries are shorter, large numbers of capillaries occur in parallel, and they have single-file flow as opposed to laminar flow. However, due to their smaller radius, a given length of capillary will have a greater resistance for the same length of arteriole.

Question 17

What is the effect of blood viscosity on resistance

Answer 17

According to Poiseuille’s Law, resistance is proportional to viscosity.
Viscosity is influenced by plasma and cellular components of blood, but the haematocrit (percentage of red blood cells in the blood volume) is the main determinant

Question 18

Describe the effect of the haematocrit on viscosity

Answer 18

An elevated haematocrit increases the carriage of oxygen, but increases viscosity (more friction of red cells with blood vessel wall) impeding flow and increasing cardiac work. Polycythaemia describes an increased haematocrit and it can be caused by a physiological adaptation to chronic hypoxia or a myeloproliferative disease increasing red cell production. Sever dehydration where blood volume is reduced without any changes in red cell number is also a cause

Question 19

Describe plasma viscosity

Answer 19

Plasma viscosity is determined by the level of plasma proteins and is increased in conditions such as myeloma, in which there is increased production of immunoglobulin. In addition to increasing plasma viscosity, this increase in plasma proteins also promotes the aggregation of red blood cells, further increasing viscosity. The vessel diameter and rate of flow also influences red cell aggregation, and as a result, viscosity is different at different points of the vascular system.

Question 20

What is pressure A and pressure B normally

Answer 20

▪ Blood entering the arterioles has a Mean Arterial Pressure (MAP) of 93mmHg. ▪ Blood leaving the arterioles has a MAP of 37mmHg (but depends on tissue bed)

Question 21

When talking about flow to an organ, why do we assume that the change in pressure is equal to the MAP

Answer 21

Average pressure in arteries is MAP and in veins is ~0 so change in pressure is regarded as MAP.

Question 22

Why is this pressure difference important

Answer 22

Without this pressure difference blood would not reach tissue capillary beds

Question 23

What is the consequence of the change in blood pressure being equal to MAP

Answer 23

Blood flow to any tissue depends on the resistance of that organ (as blood pressure is the same)

Question 24

What are the effects of arteriolar vasoconstriction

Answer 24

Decreased radius
Increased resistance
Decreased flow

Question 25

What are the effects of arteriolar vasodilation

Answer 25

Increased radius Decreased resistance Increased flow

Question 26

What causes vasoconstriction and vasodilation

Answer 26

Vasoconstriction- Smooth muscle contraction | Vasodilation- Smooth muscle relaxation

Question 27

What is the 'resting state' of the arterioles

Answer 27

Arteriolar smooth muscle normally displays a state of partial constriction – this is called vascular tone This allows the smooth muscle to both increase and decrease blood flow accordingly, it needs to be in a 'halfway' state.

Question 28

Describe the functions of the changes in radii of the arterioles

Answer 28

Coordinated constriction or dilatation of a large proportion of arterioles will alter the TPR and thus arterial blood pressure Constriction or dilatation of arterioles in single organs allows the distribution of the cardiac output to the different organs to be regulated. Dilatation of the arterioles in a particular organ will decrease resistance and increase blood flow to that organ. Constriction or dilatation of arterioles influences the hydrostatic forces in the capillaries and thus has an effect on fluid filtration. Constriction of arterioles increases resistance and causes a greater pressure drop, reducing the pressure in the distal vessels, including the capillaries.

Question 29

Describe how matching blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs) is regulated

Answer 29

Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation This regulation may be chemically driven, or physically driven.

Question 30

Describe how regulating systemic arterial blood pressure is regulated

Answer 30

Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated This regulation can be neural or hormonal

Question 31

Describe how vasodilation of the arterioles can be regulated chemically

Answer 31

The muscle becomes more metabolically active. Increased production of metabolites and increased oxygen usage. These metabolites diffuse into the tunica media of the arterioles and cause relaxation of the vascular smooth muscle and thus vasodilation. vasodilation occurs to to INTRINSIC responses. This is known as ACTIVE HYPERAEMIA. To increase blood flow to that tissue bed, vasodilation of arteries and arterioles upstream also needs to occur. This is achieved by flow-mediated vasodilation in which the shear force on the endothelial cells stimulates them to release factors that cause relaxation of adjacent smooth muscle cells (increased flow increases shear force)

Question 32

What is meant by active hyperaemia

Answer 32

Active Hyperaemia – an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue.

Question 33

Describe the physical factors that can lead to a change in radii of arterioles to alter the blood flow to that tissue to match metabolic needs

Answer 33

the tissue responds to local changes in temperature → vasoconstriction so less heat is lost in the blood flow (as less blood is flowing past the cold area). The increased blood pressure increases the stretch (distension) of the blood vessel wall. This is called myogenic autoregulation.

Question 34

What is meant by myogenic autoregulation

Answer 34

raised BP increases stretch on cells which stimulates myogenic vasoconstriction This decreases flow and increases resistance. This occurs in response to increased perfusion pressure when the tissues don't require an increased flow.

Question 35

Describe the neural regulation of vasoconstriction and vasodilation

Answer 35

Sympathetic vasoconstrictor nerves innervate the smooth muscle of arterioles and veins. In arterioles, a basal activity of these nerves is responsible for vessel tone at rest. The neurotransmitter involved is noradrenalin which acts on alpha 1 receptors causing vasoconstriction. A decrease in sympathetic nervous activity has the opposite effect, causing vasodilation

Question 36

Describe the parasympathetic vasodilator nerves

Answer 36

``` Innervate the blood vessels of the: Skin Genitalia Salivary glands Pancreas GI mucosa The effect of these nerves on TPR is small because of their limited innervation Under normal circumstances in most vascular beds, it is decreases sympathetic activity acting on alpha 1 adrenoreceptors that brings about vasodilation ```

Question 37

Where are these neurons found

Answer 37

Cardiovascular control centre in the medulla

Question 38

When is it important to vasoconstrict

Answer 38

In times of blood loss, vasoconstrict to increase TPR and thus MAP, to maintain MAP. Prolonged vasoconstriction has negative consequences by reducing flow to the tissues (especially the brain).

Question 39

Describe the hormonal control on MAP

Answer 39

the brain can mimic the SNS to control the heart by stimulating production of A and NE. i. Different hormones such as vasopressin, angiotensin II, adrenaline and noradrenaline can all help regulate arterial BP, by increasing vasoconstriction. NE and A bind to alpha 1 receptors on smooth muscle, resulting in vasoconstriction.

Question 40

What is the priority of the hormonal system in times of need

Answer 40

To preserve flow to the brain

Question 41

What is the purpose of capillary exchange

Answer 41

The purpose of capillary exchange is the delivery of metabolic substrates to the cells of the organism [which is the ultimate function of the CVS

Question 42

Describe the dimensions of the capillaries

Answer 42

The capillaries are around 7 microm wide and the wall is 1 microm thick.

Question 43

Why is it important that the capillaries are branches

Answer 43

To ensure that every cell in the body is relatively close to a capillary to deliver metabolites to every cell to allow them to function. Why capillary density is important.

Question 44

Why do highly metabolically active cells have

Answer 44

A denser capillary network. Skeletal muscle = 100cm2/g Myocardium/brain = 500cm2/g Lung = 3500cm2/g

Question 45

Describe Fick's law

Answer 45

▪ The capillaries are around 7m wide and the wall is 1m thick. ▪ The capillaries are ideally suited to enhance diffusion as described by Fick’s Law (minimise diffusion distance and time, maximise surface area).

Question 46

Describe pre-capillary sphincters

Answer 46

Blood flow through these capillaries is controlled by small rings of smooth muscle called 'pre-capillary' sphincters. Important in skeletal muscle to prevent large volumes of blood being wasted. Skeletal muscle shuts off the majority of its capillaries via the precapillary sphincters (~10% flow) in absence of exercise

Question 47

Describe continuous capillaries

Answer 47

Endothelial cells arranged in a line, separated by small water filled gap junctions. The majority of capillaries: ▪ Small water-filled gap junctions to allow passage of electrolytes. ▪ Most substances move THROUGH cells.

Question 48

Describe fenestrated capillaries

Answer 48

Have holes called fenestrae (~80nM wide). Allows filtering of the blood. Bigger things than normal to leave.

Question 49

Describe discontinuous capillaries

Answer 49

Large holes which is used in places like the bone marrow for passage of leukocytes. Also in liver, to allow easy access for metabolites.

Question 50

Describe the blood brain barrier.

Answer 50

In the brain, there is a continuous formation with VERY tight gap junctions. Allows for a much tighter control regarding what enters the brain tissue.

Question 51

Describe the role of P-Glycoprotein

Answer 51

Transporter protein that removes invading substances that enter the brain, such as drugs. Provides more protection.

Question 52

What do certain parts of the brain have

Answer 52

Not all capillaries in the brain have a tight junction, these have a normal continuous structure, need to access the vasculature more easily.

Question 53

What is meant by bulk flow

Answer 53

A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed

Question 54

Describe the starling forces that regulate fluid movement in the capillaries

Answer 54

o Hydrostatic Pressure – from the heart. o Oncotic Pressure – from the osmotic pressure

Question 55

Describe Starling's hypothesis

Answer 55

"...there 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. " " ...and whereas capillary pressure determines transudation, the osmotic pressure of the proteins of the serum determines absorption."

Question 56

When does ultrafiltration occur

Answer 56

If pressure inside the capillary > in the IF - Ultrafiltration. Net ultrafiltration occurs at the arterial end

Question 57

When does reabsorption occur

Answer 57

If inward driving pressures > outward pressures across the capillary – Reabsorption Net reabsorption occurs at the venous end

Question 58

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

Answer 58

∆1 – Net loss Role of Lymphatic system To return this fluid lost, otherwise blood volume and hence BP would decrease throughout the day

Question 59

Describe the components of lymph vessels

Answer 59

▪ Lymphatic vessels are BLIND ENDED (not a closed loop). ▪ Valves exist to prevent backflow.

Question 60

What is the function of the lymphatic system

Answer 60

Fluid, proteins and fat globules not reabsorbed into capillaries are brought back into the vascular system through the lymphatic system. ▪ 3L of fluid is returned to the circulatory system/day.

Question 61

Describe the network of the lymphatic system

Answer 61

There is a network of lymphatic capillaries, ducts and lymph nodes which unit to form the thoracic duct, which drains the collected fluid into the left subclavian vein. Lymph drains into the: o Thoracic Duct. o Right Lymphatic Duct. o Right Subclavian Vein. o Left Subclavian Vein.

Question 62

Describe the mechanics of the lymphatic system

Answer 62

The is no pump (heart) to induce flow! Movement of fluid is driven by lymphatic pressures such as muscle pressures and thoracic cavity pressures, as well as the intestinal peristalsis. Extrinsic propulsions.

Question 63

What drives the IF into the lymph capillaries

Answer 63

The hydrostatic pressure of the IF

Question 64

Describe oedema

Answer 64

If rate of production > rate of drainage then OEDEMA ensures This can be caused by parasitic blockage of the lymph nodes or removal of the lymph nodes. o E.G. Elephantiasis Tissue damage can make the capillaries leakier.

Question 65

Describe how substances can be transported across the capillaries

Answer 65

Lipid soluble- endothelium Small, water soluble- gap junction Big, water soluble- vesicular transport (active)