CVS 3 - arteries, veins and peripheral vascular resistance Flashcards
1
Q
What are the roles of the vascular system?
A
distribute blood flow to tissues and regulate blood pressure
2
Q
What are the components of the vascular / circulatory system?
A
arteries, arterioles, capillaries, venules, veins (medium to great veins)
3
Q
What are the 2 ways of calculating mean arterial pressure?
A
MAP = DBP + 1/3 pulse pressure
MAP = CO x TPR
4
Q
Define pulse pressure
A
the force the heart generates with each contraction to overcome arterial resistance (to maintain perfusion of tissues)
5
Q
Define afterload
A
the force (pressure) against which the heart must contract to eject blood into the arteries
6
Q
What parameters affect pulse pressure?
A
stroke volume, ejection velocity of stroke volume, arterial compliance
7
Q
What change to the stroke volume will increase pulse pressure?
A
an increase in stroke volume will increase pulse pressure
8
Q
What change to the ejection velocity of the stroke volume will increase pulse pressure?
A
an increase in ejection velocity will increase pulse pressure
9
Q
What change to arterial compliance will increase pulse pressure?
A
decreased arterial compliance will increase pulse pressure
10
Q
Function of arteries
A
transport blood from heart to tissues and act as a pressure reservoir to maintain blood flow during diastole
11
Q
How do arteries act as a pressure reservoir?
A
they contain a small amount of blood at a high pressure
12
Q
Describe the branching of the aorta
A
has major branches (subclavian, common carotid and iliac) and medium / muscular branches (coronary and renal arteries) which branch into small arteries and further into arterioles
13
Q
Diameter of small arteries
A
<2mm
14
Q
Diameter of arterioles
A
20-100um
15
Q
In which part of the circulatory system is most of the arterial BP dissipated?
A
arteriolar system
16
Q
Diameter of capillaries
A
5-10um (erythrocytes flow in single file)
17
Q
Approximately how thick are capillary walls?
A
20 um thick (one squamous epithelial cell for short diffusion distance)
18
Q
What features of capillaries allow for gas and nutrient exchange between blood and tissues?
A
fenestrations (pores) and junctions (between two endothelial cells)
19
Q
How does the structure of a capillary wall differ from other vessel walls?
A
capillaries lack tunica media and tunica adventitia / externa
20
Q
Function of veins
A
transport blood back to the heart from tissues. Act as a collecting system and volume reservoir
21
Q
What percentage of the total blood volume is carried in veins?
A
70% at low pressure
22
Q
What feature of veins prevent the backflow of blood?
A
venous valves
23
Q
What are the possible consequences of venous walls or valves losing their elasticity?
A
turbulent blood flow in vessel, development of varicose veins as the vessel wall becomes distended
24
Q
What is the general 3 layer structure of blood vessel walls?
A
tunica intima, tunica media, tunica adventitia/externa
25
What is the tunica intima composed of?
endothelial cells attached to a basement membrane with an underlying layer of extracellular matrix
26
What separates the tunica intima from the tunica media?
an internal elastic lamina
27
What is the tunica media composed of?
layers of elastin fibres and smooth muscle cells
28
What does the proportion of elastin fibres and smooth muscle cells in the tunica media depend on?
vessel function - whether a muscular or elastic artery
29
What is the function of a high elastin content in the tunica media?
enables vessel wall expansion during systole and recoil during diastole
30
What structure often separates the tunica media from the tunica adventitia/externa?
external elastic lamina
31
What is the tunica adventitia/externa composed of?
thick connective tissue (elastic and collagen fibres) and network of nerve fibres, lymphatics and vasa vasorum (in great vessels)
32
What is the name of the small arterioles that perfuse the tunica adventitia/externa of great vessels?
vasa vasorum
33
How does the composition of an arterial wall differ from a venous wall?
arteries have a thicker tunica media (muscular and elastic layer)
34
Define vascular compliance
the ability of a blood vessel wall to passively expand and recoil in response to changes in pressure (buffering function of vessel)
35
How is vascular compliance calculated?
change in volume/change in pressure
36
Why does the arterial wall need to expand during systole?
to accommodate the ventricular stroke volume
37
Function of arterial compliance
allow large arteries to act as pressure reservoir (to maintain high pressure and pulsatile flow to meet metabolic demands)
38
Why does the arterial wall recoil during diastole?
to drive blood flow within artery
39
How does arterial compliance change with age?
arterial compliance declines with age
40
How does a decrease in arterial compliance (e.g. with age) affect pulse pressure?
a decline in arterial compliance can increase pulse pressure
41
Define arteriosclerosis
age related arterial stiffness due to calcification of elastin, collagen and the extracellular matrix
42
Why can arteriosclerosis increase pulse pressure?
the heart must pump against more resistance
43
Where are endothelial cells located?
inner lining of blood vessels (tunica intima) and the heart
44
Functions of endothelial cells
local blood pressure control, minimise shear stress, regulate permeability of BVs, regulate platelet function and fibrinolysis, promote angiogenesis
45
How do endothelial cells minimise shear stress?
minimise friction on surface so blood flow is less turbulent as possible
46
Where is the vascular smooth muscle located?
in tunica media
47
Function of the vascular smooth muscle
controls total peripheral resistance (TPR), arterial and venous tone, and distribution of blood flow
48
What is meant by the tone of a blood vessel?
the degree of constriction
49
What type of cell makes up the majority of the tunica media?
smooth muscle cell
50
Describe the structure of a smooth muscle cell
mononucleated, spindle-shaped, non-striated myocyte
51
How are vessels given elastic properties?
smooth muscle cells secrete an extracellular matric which contains elastin
52
How does smooth muscle alter vascular tone?
vasoconstriction (vascular SM contracts to narrow lumen and reduce radius) and vasodilation (vascular SM relaxes to widen lumen and increase radius)
53
How does central control of BP occur?
via baroreceptors and altering blood volume
54
What type of mechanism does local control of BP refer to?
altering small artery and arteriolar resistance in organs and tissues
55
What substances are involved in the local control of BP?
hormones, vasoactive substances (produced by endothelial cells), vasodilators, vasoconstrictors
56
Which hormones are involved in the local control of BP?
adrenaline, atrial natriuretic peptide (ANP), angiotensin II
57
What effect does adrenaline have on blood vessels?
dual role - can cause vasodilation or vasoconstriction
58
Where is adrenaline released from?
adrenal medulla
59
When adrenaline is present in the circulation at high concentrations, what is the effect produced?
contraction of arteriolar smooth muscle causing vasoconstriction
60
Which receptor causes vasodilation when adrenaline binds to it?
B2 adrenoreceptors
61
What happens when adrenaline binds to B2 adrenoreceptors?
increase in cAMP and reduced Ca sensitivity for SMC contraction leading to vasodilation
62
Which receptors does adrenaline bind to when present in a high concentration to cause vasoconstriction?
a1 adrenoreceptors on arteriolar smooth muscle causing vasoconstriction
63
What happens when adrenaline binds to a1 adrenoreceptors?
contraction of arteriolar smooth muscle causing vasoconstriction
64
Function of atrial natriuretic peptide (ANP)
potent vasodilator and acts to decrease BP by increasing Na and water excretion (decreases blood volume)
65
Function of angiotensin II
constricts arterioles, important part of RAAS
66
What is the trigger for the renin angiotensin aldosterone system?
decreased blood flow to kidney
67
Which receptors detect a reduced blood flow to the kidney to initiate RAAS?
receptors in the juxtaglomerular apparatus
68
What happens when the juxtaglomerular apparatus receptors detect a reduced blood flow to the kidneys?
juxtaglomerular cells release renin
69
What is the action of renin in RAAS?
renin converts circulating angiotensinogen into angiotensin I
70
Where is angiotensinogen produced?
in the liver
71
What happens to angiotensin I?
angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE)
72
Where is angiotensin I converted to angiotensin II by ACE?
in lungs and kidney
73
Where is angiotensin converting enzyme released from?
endothelial cells (in lungs and kidneys)
74
Function of angiotensin II
potent vasoconstrictor, stimulates water and Na reabsorption, release of aldosterone and anti-diuretic hormone
75
How does angiotensin II affect mean arterial pressure (MAP)?
vasoconstriction due to angiotensin II increases TPR. As MAP = CO x TPR, MAP increases as TPR increases
76
Where is aldosterone released from?
adrenal cortex
77
Where is ADH released from?
posterior pituitary gland (neurohypophysis)
78
Function of aldosterone
increases Na and water retention in DCT and collecting duct which increases the intravascular volume (can increase BP)
79
Function of ADH
inserts aquaporins into DCT and collecting duct causing increased water reabsorption, and causes vasoconstriction via V1 receptors on vascular SMCs
80
Which receptors does ADH bind to in order to cause vasoconstriction?
V1 receptors on SMCs (in tunica media)
81
What mechanism is a frequent target for antihypertensives?
renin angiotensin aldosterone system (RAAS)
82
Example of antihypertensives that act of the RAAS
ACE inhibitors (Captopril, Enalapril) and angiotensin II receptor blockers (ARBs e.g. Losartan)
83
How do ACE inhibitors lower blood pressure?
prevent the conversion of angiotensin I into angiotensin II
84
How do angiotensin II receptor blockers (ARBs) lower blood pressure?
prevent the action of angiotensin II
85
What is the function of endothelial cells in local control of BP?
endothelial cells produce vasoactive substances (e.g. NO, prostacyclin, endothelin) which regulate vascular tone
86
Name 2 vasodilators
nitric oxide (NO), prostaglandin I2 (prostacyclin)
87
How is nitric oxide (NO) produced?
by nitric oxide synthase (NOS) from L-arginine in vascular endothelial cell
88
Where is nitric oxide (NO) produced?
in vascular endothelial cell
89
What triggers the release of nitric oxide (NO)?
shear stress or binding of endothelium dependent vasodilators to receptors on endothelial cell membrane
90
Examples of endothelial dependent vasodilators that can trigger the release of nitric oxide
acetylcholine, ATP, bradykinin
91
How does nitric oxide (NO) cause vasodilation?
NO diffuses into SMC (in tunica media) causing cGMP activation of guanylate cyclase which induces relaxation
92
Where is prostaglandin I2 (prostacyclin) produced?
in endothelial cells
93
What family of lipid molecules is prostaglandin I2 (prostacyclin) a part of?
eicosanoid
94
Describe the action of prostaglandin I2 (prostacyclin)
prostaglandin I2 activates adenylate cyclase which increases cAMP production, activating protein kinase A (PKA). Leads to vasodilation
95
Name 2 vasoconstrictors
endothelin -1 (ET-1) and thromboxane A2
96
What family of molecules does endothelin-1 (ET-1) belong to?
endothelin family of peptide agents
97
Where is endothelin secreted from?
endothelial cells
98
Which stimuli trigger the release of endothelin-1 (ET-1)?
pulsatile stretch, shear stress, neurohormones and cytokines
99
Describe the action of endothelin-1 (ET-1)
acts on ETa receptor on vascular SMCs to initiate vasoconstriction
100
What family of molecules is thromboxane A2 a part of?
eicosanoid
101
What triggers the activation of thromboxane A2?
tissue injury and inflammation
102
Example of vasoconstrictor ligand that can trigger the production of endothelin-1 and thromboxane A2
angiotensin II
103
How do endothelin-1 and thromboxane A2 cause SMC contraction?
by increasing Ca2+ concentration in the SMC (influx and intracellular release)
104
When does arterial pressure fall?
when blood circulates from the LV to the RV (systemic circulation) and from the RV to the LV (pulmonary circulation). Arterial pressure rises and falls during cardiac cycle
105
Define mean arterial pressure (MAP)
the average pressure pushing blood around the circulatory system
106
Describe the difference in pressures between the systemic and pulmonary circulations
systemic circulation has a much greater pressure in arteries (MAP=95) than the pulmonary circulation (MAP=15). However; systemic circulation has a much greater rate of pressure decrease as blood enters the venous system. Pressure in great veins of both systems are similar (3/5)
107
Define shear stress
force that blood flow exerts on the endothelial layer of the blood vessel wall
108
What causes shear stress?
layers of blood travelling at different velocities
109
Examples of physiologic alterations in homeostatic conditions
exercise, pregnancy, growth
110
Examples of pathologic alterations in homeostatic conditions
hypertension, flow reduction, flow overload
111
What is the primary site for fluid, gas and electrolyte exchange?
capillary system
112
What are the junctions that separate endothelial cells of the capillary wall called?
intercellular clefts
113
What are the 3 structural classifications of capillaries?
continuous, fenestrated, discontinuous
114
Where are continuous capillaries located?
muscle, skin, pulmonary system, CNS
115
Describe the structure of continuous capillaries
have a continuous basement membrane with tight intercellular clefts
116
Where are fenestrated capillaries located?
in exocrine glands, renal glomeruli
117
Describe the structure of fenestrated capillaries
have a continuous basement membrane and endothelium with fenestrations
118
Where are discontinuous capillaries found?
in liver (sinusoids)
119
Describe the structure of discontinuous capillaries
have large intercellular clefts and gaps in the basement membrane
120
Describe the order of capillaries from most to least permeable
discontinuous (extremely high permeability), fenestrated (relatively high permeability), continuous (lowest permeability)
121
What are metarterioles?
terminal arterioles that have smooth muscle fibres encircling the vessel at intermittent points, instead of a continuous smooth muscle layer
122
Where are metarterioles found?
connecting arterioles and venules across the capillary bed (branches off into capillaries)
123
Describe the manner at which blood flows through capillaries
flows intermittently (not continuously) due to vasomotion (oscillating tone)
124
What are pre-capillary sphincters?
a ring of smooth muscle surrounding the branch site of capillaries from metarterioles. Contract and relax in response to local metabolic factors
125
Which two pressures influence fluid exchange in capillaries?
hydrostatic pressure and osmotic pressure
126
Function of hydrostatic pressure
force fluid and dissolved substances through capillary intercellular spaces (intercellular clefts, fenestrations) to interstitial spaces
127
Function of osmotic pressure
force fluid movement from interstitial space into blood via capillary intercellular spaces (clefts, fenestrations)
128
What is the cause of (colloid) osmotic pressure?
plasma proteins (too big to leave the capillary)
129
What happens at the arterial end of capillaries?
there is a positive net filtration pressure (HP>OP) so there is a net fluid filtration across the capillaries (fluid out)
130
What happens at the venous end of capillaries?
negative net filtration pressure (OP>HP) so there is a net fluid absorption from interstitial spaces into capillaries
131
What is the lymphatic system?
a network of small lymph nodes and lymphatic vessels through which lymph flows
132
Function of lymphatic system
an overflow mechanism that returns excess fluid volume from tissue spaces to the circulation
133
What is lymph derived from?
excess interstitial fluid that is not reabsorbed into the capillaries
134
What would result if fluid built up in tissues?
oedema
135
What is the name of the lymph vessels that absorb lipids from small intestine?
lacteals
136
What factors are under the control of the lymphatic system?
concentration of proteins in interstitial fluids, volume of interstitial fluids, interstitial fluid pressure
137
What is the effect of increased colloid osmotic pressure in interstitial fluid?
shifts the balance of forces at the capillary membrane in favour of fluid filtration
138
What is the effect of increasing interstitial fluid pressure?
increases the rate of lymph flow (carries excess interstitial fluid volume and accumulating protein)
139
What is the difference between pulmonary and systemic hydrostatic pressures?
pulmonary HP is lower than systemic HP
140
What is the difference between pulmonary and systemic colloid osmotic pressures?
no difference (equivalent)
141
What is a pulmonary oedema?
excess fluid in lungs that collects in alveoli
142
What prevents pulmonary oedema?
net fluid transfer at both ends of capillary
143
How can pulmonary oedemas be induced?
heart failure, high altitude, lung damage due to severe infection, adult respiratory distress syndrome, major injury
