CVS 3 - arteries, veins and peripheral vascular resistance Flashcards
What are the roles of the vascular system?
distribute blood flow to tissues and regulate blood pressure
What are the components of the vascular / circulatory system?
arteries, arterioles, capillaries, venules, veins (medium to great veins)
What are the 2 ways of calculating mean arterial pressure?
MAP = DBP + 1/3 pulse pressure
MAP = CO x TPR
Define pulse pressure
the force the heart generates with each contraction to overcome arterial resistance (to maintain perfusion of tissues)
Define afterload
the force (pressure) against which the heart must contract to eject blood into the arteries
What parameters affect pulse pressure?
stroke volume, ejection velocity of stroke volume, arterial compliance
What change to the stroke volume will increase pulse pressure?
an increase in stroke volume will increase pulse pressure
What change to the ejection velocity of the stroke volume will increase pulse pressure?
an increase in ejection velocity will increase pulse pressure
What change to arterial compliance will increase pulse pressure?
decreased arterial compliance will increase pulse pressure
Function of arteries
transport blood from heart to tissues and act as a pressure reservoir to maintain blood flow during diastole
How do arteries act as a pressure reservoir?
they contain a small amount of blood at a high pressure
Describe the branching of the aorta
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
Diameter of small arteries
<2mm
Diameter of arterioles
20-100um
In which part of the circulatory system is most of the arterial BP dissipated?
arteriolar system
Diameter of capillaries
5-10um (erythrocytes flow in single file)
Approximately how thick are capillary walls?
20 um thick (one squamous epithelial cell for short diffusion distance)
What features of capillaries allow for gas and nutrient exchange between blood and tissues?
fenestrations (pores) and junctions (between two endothelial cells)
How does the structure of a capillary wall differ from other vessel walls?
capillaries lack tunica media and tunica adventitia / externa
Function of veins
transport blood back to the heart from tissues. Act as a collecting system and volume reservoir
What percentage of the total blood volume is carried in veins?
70% at low pressure
What feature of veins prevent the backflow of blood?
venous valves
What are the possible consequences of venous walls or valves losing their elasticity?
turbulent blood flow in vessel, development of varicose veins as the vessel wall becomes distended
What is the general 3 layer structure of blood vessel walls?
tunica intima, tunica media, tunica adventitia/externa
What is the tunica intima composed of?
endothelial cells attached to a basement membrane with an underlying layer of extracellular matrix
What separates the tunica intima from the tunica media?
an internal elastic lamina
What is the tunica media composed of?
layers of elastin fibres and smooth muscle cells
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
What is the function of a high elastin content in the tunica media?
enables vessel wall expansion during systole and recoil during diastole
What structure often separates the tunica media from the tunica adventitia/externa?
external elastic lamina
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)
What is the name of the small arterioles that perfuse the tunica adventitia/externa of great vessels?
vasa vasorum
How does the composition of an arterial wall differ from a venous wall?
arteries have a thicker tunica media (muscular and elastic layer)
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)
How is vascular compliance calculated?
change in volume/change in pressure
Why does the arterial wall need to expand during systole?
to accommodate the ventricular stroke volume
Function of arterial compliance
allow large arteries to act as pressure reservoir (to maintain high pressure and pulsatile flow to meet metabolic demands)
Why does the arterial wall recoil during diastole?
to drive blood flow within artery
How does arterial compliance change with age?
arterial compliance declines with age
How does a decrease in arterial compliance (e.g. with age) affect pulse pressure?
a decline in arterial compliance can increase pulse pressure
Define arteriosclerosis
age related arterial stiffness due to calcification of elastin, collagen and the extracellular matrix
Why can arteriosclerosis increase pulse pressure?
the heart must pump against more resistance
Where are endothelial cells located?
inner lining of blood vessels (tunica intima) and the heart
Functions of endothelial cells
local blood pressure control, minimise shear stress, regulate permeability of BVs, regulate platelet function and fibrinolysis, promote angiogenesis
How do endothelial cells minimise shear stress?
minimise friction on surface so blood flow is less turbulent as possible
Where is the vascular smooth muscle located?
in tunica media
Function of the vascular smooth muscle
controls total peripheral resistance (TPR), arterial and venous tone, and distribution of blood flow
What is meant by the tone of a blood vessel?
the degree of constriction
What type of cell makes up the majority of the tunica media?
smooth muscle cell
Describe the structure of a smooth muscle cell
mononucleated, spindle-shaped, non-striated myocyte
How are vessels given elastic properties?
smooth muscle cells secrete an extracellular matric which contains elastin
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)
How does central control of BP occur?
via baroreceptors and altering blood volume
What type of mechanism does local control of BP refer to?
altering small artery and arteriolar resistance in organs and tissues
What substances are involved in the local control of BP?
hormones, vasoactive substances (produced by endothelial cells), vasodilators, vasoconstrictors
Which hormones are involved in the local control of BP?
adrenaline, atrial natriuretic peptide (ANP), angiotensin II
What effect does adrenaline have on blood vessels?
dual role - can cause vasodilation or vasoconstriction
Where is adrenaline released from?
adrenal medulla
When adrenaline is present in the circulation at high concentrations, what is the effect produced?
contraction of arteriolar smooth muscle causing vasoconstriction
Which receptor causes vasodilation when adrenaline binds to it?
B2 adrenoreceptors
What happens when adrenaline binds to B2 adrenoreceptors?
increase in cAMP and reduced Ca sensitivity for SMC contraction leading to vasodilation
Which receptors does adrenaline bind to when present in a high concentration to cause vasoconstriction?
a1 adrenoreceptors on arteriolar smooth muscle causing vasoconstriction
What happens when adrenaline binds to a1 adrenoreceptors?
contraction of arteriolar smooth muscle causing vasoconstriction
Function of atrial natriuretic peptide (ANP)
potent vasodilator and acts to decrease BP by increasing Na and water excretion (decreases blood volume)
Function of angiotensin II
constricts arterioles, important part of RAAS
What is the trigger for the renin angiotensin aldosterone system?
decreased blood flow to kidney
Which receptors detect a reduced blood flow to the kidney to initiate RAAS?
receptors in the juxtaglomerular apparatus
What happens when the juxtaglomerular apparatus receptors detect a reduced blood flow to the kidneys?
juxtaglomerular cells release renin
What is the action of renin in RAAS?
renin converts circulating angiotensinogen into angiotensin I
Where is angiotensinogen produced?
in the liver
What happens to angiotensin I?
angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE)
Where is angiotensin I converted to angiotensin II by ACE?
in lungs and kidney
Where is angiotensin converting enzyme released from?
endothelial cells (in lungs and kidneys)
Function of angiotensin II
potent vasoconstrictor, stimulates water and Na reabsorption, release of aldosterone and anti-diuretic hormone
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
Where is aldosterone released from?
adrenal cortex
Where is ADH released from?
posterior pituitary gland (neurohypophysis)
Function of aldosterone
increases Na and water retention in DCT and collecting duct which increases the intravascular volume (can increase BP)
Function of ADH
inserts aquaporins into DCT and collecting duct causing increased water reabsorption, and causes vasoconstriction via V1 receptors on vascular SMCs
Which receptors does ADH bind to in order to cause vasoconstriction?
V1 receptors on SMCs (in tunica media)
What mechanism is a frequent target for antihypertensives?
renin angiotensin aldosterone system (RAAS)
Example of antihypertensives that act of the RAAS
ACE inhibitors (Captopril, Enalapril) and angiotensin II receptor blockers (ARBs e.g. Losartan)
How do ACE inhibitors lower blood pressure?
prevent the conversion of angiotensin I into angiotensin II
How do angiotensin II receptor blockers (ARBs) lower blood pressure?
prevent the action of angiotensin II
