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Physiology > Lecture 11: The vascular tree > Flashcards

Lecture 11: The vascular tree Flashcards

1
Q

What are the control points for distribution of blood into the capillary beds?

A

Arterioles

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2
Q

What is the hydraulic filter? Function?

A

Combination of elastic conduits and high resistance terminals

Converts intermittent output of the heart to steady flow to capillaries (smoothing of blood flow)

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3
Q

Importance of compliance of aorta

A

During cardiac contraction, the aorta expands

During relaxation, the aortic valve closes, allowing maintenance of pressure in the arterial tree.

Potential energy is stored in the aortic wall

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4
Q

What happens if the aortic valve leaks?

A

Regurgitation

Pressure in the arterial tree is lost

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5
Q

Aorta compliance and ageing

A

As people get older, their aorta gets stiffer and stiffer (non-expansive)

Normal natural ageing event

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6
Q

What does a rigid aorta result in?

A

Less compliance

Most stroke volume will flow through the capillaries during systole with minimal flow during diastole.

This is inefficient and makes the heart work harder

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7
Q

What is vessel compliance?

A

aka Capacitance

Distensibility of a vessel

Increase in volume caused by a given increase in pressure

compliance = delta V / delta P

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8
Q

Effect of age on vessel compliance

A

Aortic compliance decreases with age. (delta V decreases)

The heart in an elderly person is less able to eject stroke volume into the arterial system, despite normal cardiac function.

Peak arterial pressure occurs progressively later in systole.

The rapid ejection phase is significantly prolonged vs that of a younger person’s arterial system.

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9
Q

Blood pressure throughout the circulatory system

A

See figure

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10
Q

Why is the minimum pressure in the aorta higher than that of the ventricles?

A

Aortic valve saves the pressure in the aorta (conserves energy)

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11
Q

Why is there higher systolic pressure in the systemic arteries than the aorta?

A

Phenomenon referred to as systolic amplification

Causes reflectance of pressure wave

Not completely understood

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12
Q

Why is there a drop in pressure at the arterioles?

A

Arterioles have a lot of muscle, allow little blood through when constricted

Potential energy is low

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13
Q

What happens to arterial pressure during diastole?

A

falls slowly as the aorta imparts it’s stored PE back into the system.

Blood continues to flow to the capillaries during diastole.

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14
Q

Why does mean arterial pressure or MAP (dotted lines in diagram) fall by a negligible fraction in the large and medium arteries?

A

1) They have larger diameter
2) Don’t offer much resistance to flow
3) little frictional drag

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15
Q

MAP as distance from heart increases

A

MAP decreases with distance from the heart

Blood moves to capillaries down a pressure gradient

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16
Q

Pulse pressure =

A

Systolic P - diastolic P

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17
Q

MAP =

A

MAP = Pd + 1/3(Ps-Pd)

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18
Q

Table showing BP in system

A
  • note systolic amplification

See figure

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19
Q

Factors that determine MAP

A

Physical factors: Mean blood volume and Arterial compliance

Physiologic factors: cardiac output, peripheral resistance

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20
Q

What is blood volume?

A

A product of cardiac output and the rate of outflow from arteries to the resistance vessels

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21
Q

What is a pressure wave?

A

During cardiac ejection (a relatively violent event) a pressure or pulse wave
is transmitted down the aorta and its branches, with a relative velocity that is several times faster than actual blood flow.

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22
Q

What does taking the pulse at the wrist detect?

A

Detection of pressure wave and is due to the movement of the arterial wall as it absorbs the shock of cardiac systole

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23
Q

Variation of volume of pressure wave

A

Varies inversely with vascular compliance

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24
Q

How is the arterial blood pressure measured?

A

By a sphygmomanometer

Rubber bad is wrapped around upper arm over the brachial artery

One tube connects the inside of the bag with a manometer containing mercury

Another tube connects the inside of the bad to a hand operated pump and a release valve

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25
Q

How to use a sphygmanometer

A

Air is pumped into the rubber bag until the pressure in the cuff is greater than the pressure in the artery during systole

Artery is now closed during systole AND diastole

By releasing the valve, the pressure in the cuff is gradually reduced until maximum pressure in the artery just overcomes pressure in the cuff. Some blood begins to spurt through during systole, but artery is still closed during diastole

At this point, faint, rhythmic tapping sounds begin to be heard through the stethoscope

The height of mercury in millimetres is taken as the systolic blood pressure

Pressure in the cuff is reduced still further until it is just less than the lowest pressure in the artery towards the end of diastole. Blood flow is unimpeded during systole and diastole and the sounds stop. The height of the mercury indicates the diastolic blood pressure.

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26
Q

What are capillaries?

A

tubes of endothelial cells that branch from the arterioles

sites of O2 and nutrient exchange between the blood and cells.

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27
Q

Abundance of capillaries

A

Very abundant

Offer large surface area

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28
Q

How does exchange occur in the capillaries?

A

By diffusion

Enhanced by large surface area and increased time for diffusion (low velocity)

RBCs slowly squeeze through the thin-walled capillaries

See figure

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29
Q

Resistance of flow in capillaries vs arterioles

A

Resistance of flow is low in capillaries

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30
Q

Adjustment of capillary and arteriole calibre

A

Capillaries cannot adjust flow

Rely on arterioles for adjustments

31
Q

What do capillary pores allow?

A

enable passage of small, water-soluble substances that can’t cross endothelial cell walls

Ions, glucose and amino acids permeate the pores

32
Q

Where are the pores located in capillaries?

A

Pores are present at cellular junctions between endothelial cells.

See figure

33
Q

How do lipid soluble substances cross the capillaries?

A

Dissolve through the lipid bilayer

34
Q

Capillary pore size

A

differs with tissue type

Liver capillary pores are large

Brain capillary pores are non-existent.

35
Q

Do proteins cross through the capillary pores?

A

Proteins are generally excluded

Except in the liver capillaries

36
Q

What substance enlarges capillary pores?

A

Histamine

37
Q

What structures are in between arterioles and capillaries?

A

Metarterioles

38
Q

Where are the pre capillary sphincters? What do they do?

A

Surround capillaries

Contraction of sphincters reduces blood flowing into capillary bed

Myogenic tone is high (capillaries do not carry blood during resting conditions)

See figure

39
Q

What is the pre capillary sphincter muscle sensitive to?

A

Metabolic changes

40
Q

What is the interstitial fluid? How is it a site of exchange?

A

The interstitial fluid (IF) is an intermediary between the blood and tissue cells.

Exchange between RBC’s and target cells is via the IF

41
Q

How much of the ECF is interstitial fluid?

A

80 %

Other 20% is blood plasma

42
Q

Type of exchange between interstitial fluid and plasma membranes

A

Active: carrier-mediated

Passive: diffusion down concentration gradients

43
Q

What is bulk flow? What substances cross the capillary wall this way?

A

Constituents in a fluid move through the wall in bulk (no discrete movement of single components)

Fluid is pushed through the wall from higher pressure to lower pressure

Na, K, glucose, amino acids

44
Q

What is ultrafiltration?

A

fluid (ultrafiltrate) moves from the capillary lumen (site of higher hydrostatic pressure = p) to the interstitial fluid (IF) outside the capillary (pi-p

See figure

45
Q

What happens to plasma proteins during ultrafiltration?

A

Plasma proteins remain in the capillary, unable to pass through the pores

46
Q

What is reabsorption?

A

Reverse bulk flow (IF to capillary lumen): inward- driving pressure exceeds an outward opposing pressure across the capillary wall.

47
Q

What are the driving forces for bulk flow?

A

Hydrostatic (fluid) pressure

Colloid osmotic pressure (oncotic pressure)

48
Q

What is the significance of bulk flow?

A

It regulates the distribution of ECF between plasma and IF

49
Q

What is the outward pressure at the arteriolar end of the capillary?

A

High

50
Q

What is the inward pressure at the venular end of the capillary?

A

Greater than the outward pressure

See figure

51
Q

Importance of lymphatic system

A

There is more fluid ultrafiltrated than fluid reabsorbed, so there is extra fluid, which is removed by the lymphatics

52
Q

What is lymph?

A

Interstitial fluid that enters a lymphatic vessel

Contains escaped plasma proteins and bacteria that are not reclaimed by the blood plasma

53
Q

How is lymph propelled into larger lymphatic vessels?

A

Skeletal muscle and smooth muscle propels lymph into larger lymphatic vessels

Lymph is eventually combined with venous blood near the heart

54
Q

Functions of the lymphatic system

A

Defense against disease by the lymph nodes

Transport of absorbed fat from the digestive tract

55
Q

What is edema?

A

When there is excessive interstitial fluid in tissues?

56
Q

Causes of edema?

A

Reduced concentration of plasma proteins (starvation)

Increased permeability of the capillary walls… allows more plasma proteins to pass from blood fluid to interstitial vessel

Increased venous pressure (blood dams up in the venous side, increases venular capillary hydrostatic pressure = less reabsorption)

Blockage of lymphatic vessels (filariasis is a parasite that invades the lymph and causes elephantiasis)

57
Q

Role of veins

A

Blood flows from the capillaries to the veins.

Veins collect blood for its return to the heart.

58
Q

Resistance in veins

A

Veins have large radii and offer low resistance to the flow of blood

59
Q

Velocity of blood flow in veins vs capillaries

A

The velocity of blood flow increases in the veins (vs capillaries), as cross-sectional area is relatively low.

60
Q

Thickness of vein walls

A

Thinner than arteries

Can be distended

Capacitance vessels, serve as blood reservoir (hold 60% of blood volume)

61
Q

What is venous return?

A

volume of blood entering each atrium per minute.

62
Q

What is the mean pressure of the venous circulation?

A

17 mm Hg

63
Q

What are the three main components of the venous system?

A

Sphlanic, cutaneous and muscle veins

64
Q

How are capacity and pressure of veins adjusted?

A

Adjusted by contraction of smooth muscle within their walls

Splanchnic veins are particularly active in this function

Control central blood volume

See figure

65
Q

What is the central blood volume?

A

Effective regulation of blood volume in the right heart, pulmonary vessels and left heart

Controlled by systemic veins

66
Q

Compliance of veins vs arteries

A

Arterial walls are thicker and less distensible vs those of systemic veins.

Veins are, on average, more distensible than arteries by a factor of 8.

67
Q

What does compliance of veins allow them to do?

A

Veins are important as blood reservoirs

They readily alter their net volume capacity with only slight alterations in intraluminal pressure.

68
Q

Five major factors for enhancement of venous return?

A
  1. Increased sympathetic stimulation of the veins
  2. Increased skeletal muscle contraction
  3. Valves inside veins prevent return of blood to point of constriction by muscle pump - blood does not flow backward
  4. Cardiac suction
  5. Respiration (breathing)

See figure

69
Q

How does increased sympathetic stimulation of the veins enhance venous return? What mediates this?

A

contraction of the smooth muscle in the wall of the veins = venoconstriction.

Stored blood is mobilized to the heart.

a-adrenoceptors mediate smooth muscle cell contraction.

70
Q

How does increased skeletal muscle contraction enhance venous return?

A

Skeletal muscle contraction compresses veins and increases venous pressure/venous return.

NB in exercise and to counter the effect of gravity.

Gravity contributes a great deal to absolute pressure

71
Q

How does cardiac suction enhance venous return?

A

Heart creates suction by expansion of the ventricular chamber due to the presence of “internal resistor” within the cardiac extracellular matrix.

72
Q

How does respiration (breathing) enhance venous return?

A

Creates subatmospheric pressure during inspiration in the chest cavity (5 mm Hg less), drawing blood toward the heart.

73
Q

Influence of gravity on pressure

A

Pascal’s 3rd law for hydrostatic fluid at rest

p = sigma x g x h

sigma = density (g/ml)

g = force produced by acceleration of gravity

h = height of column (cm)

Pressure in the arteries below heart level is higher by a factor of (sigma)gh (gravitational potential)

potential energy is essentially = pressure.

See figure

74
Q

Where is arterial pressure usually measured?

A

Measured at a point at the same level as the heart
(arm if sitting)

Because gravity has such a strong effect

Physiology (19 decks)

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