pressure and flow in arteries and veins

Question 1

describe how you would measure arterial blood pressure using a sphygmomanometer & stethoscope

Answer 1

pump the cuff to above systolic - around 160mmHg- no blood flow, then slowly (about 4-5mmHg at a time) reduce the pressure of the cuff.
when blood starts spurting through (tapping sounds) - systolic bp
when you can no longer hear any sounds - diastolic bp

Question 2

describe how arteries respond to contraction and relaxation

Answer 2

During contraction- ventricles contract, semilunar valves open, aorta and arteries absorb the pressure in their elastic walls
During relaxation- ventricles relax, pressure gets released from elastic walls and some of this causes the semilunar valves to shut- which pushes the blood forward through the circulatory system

Question 3

what do elastic arteries act as?

Answer 3

they act as a pressure reservoir, dampening any pressure variations

Question 4

what are the 4 factors which affect the pressure wave?

Answer 4

-STROKE VOLUME - the higher the volume of blood ejected, the higher the pressure
-VELOCITY OF EJECTION - the slower the blood is ejected, the more time the artery walls have to absorb the pressure, whereas the faster the blood is ejected the less pressure will be absorbed which causes an increase in BP
-ELASTICITY IN ARTERIES- the more elastic the arteries are, the more pressure they can store in their walls = higher diastolic pressure
Less elastic = higher systolic
-TPR - if resistance decreases, pressure falls too

Question 5

what is pulse pressure?

Answer 5

systolic- diastolic blood pressure

Question 6

describe the changes in pressure throughout the vascular tree

Answer 6

• from elastic arteries to muscular arteries slight drop (95-90mmHg)
• from muscular arteries to arterioles big drop (90-40mmHg)
• from arterioles to capillaries slight drop, which is convenient as capillaries require low pressure (40-20mmHg)
• from capillaries to veins (20-5mmHg) - systemic filling pressure (venous return is determined by the pressure difference between RA and systemic filling pressure)

Question 7

describe the changes in pressure throughout the vascular tree

Answer 7

• from elastic arteries to muscular arteries slight drop (95-90mmHg)
• from muscular arteries to arterioles big drop (90-40mmHg)
• from arterioles to capillaries slight drop, which is convenient as capillaries require low pressure (40-20mmHg)
• from capillaries to veins (20-5mmHg) - systemic filling pressure which pushes blood back to heart (venous return is determined by the pressure difference between RA and systemic filling pressure)

Question 8

what can be drawn from the velocity equation?

Answer 8

velocity is inversely proportional to cross-sectional area- the bigger the area, the slower the blood flow i.e. capillaries
the smaller the area i.e. vena cava, aorta, the faster the blood flow

Question 9

why can veins be heavily influenced by outside influences?

Answer 9

because they have a very thin wall which is easily distensible and collapsible

Question 10

what are the 5 factors which affect venous pressure?

Answer 10

gravity, skeletal muscle pump, respiratory pump, venomotor tone & systemic filling pressure

Question 11

describe the effect of gravity on pressure and flow in veins

Answer 11

when someone is lying down, all the forces on the thorax are equal. However standing up causes a column of blood to go from thorax down to lower limbs and feet and causes the pooling of blood in the deep veins of the lower limbs and feet.

Question 12

describe the effect of gravity on pressure and flow in veins

Answer 12

When someone is lying down, all the forces on the thorax are equal. However standing up causes blood to accumulate in the lower extremities. This pooling of blood causes distension of the veins & therefore venous pressure & volume to increase. This causes reduced CVP, reduced venous return, reduced EDV & preload, reduced CO and reduced MAP.

• This however does not affect driving pressure from arteries to veins as arterial pressure increases too, so gradient is maintained
• does cause venous distension in legs which can lead to conditions such as DVT/varicose veins
• does cause venous collapse in neck due to the pressure of veins in neck becoming sub atmospheric

Question 13

describe the effect of the skeletal muscle pump on veins

Answer 13

when muscles contract they compress veins which pushes blood upwards past valves and back to the heart, which increases venous return to the heart
rhythmic vs static exercise
rhythmic - period contractions - increase EDV & maintain preload on the heart= good for the heart
static - sustained contraction- bad for the heart as the pump can’t work as well

Question 14

describe the effect of the respiratory pump on veins

Answer 14

has 2 effects:
-effect on pressure of RA
-effect on diameter of cardiac chambers and vena caveae
Lowering pressure of RA facilitates venous return.
During inspiration, volume of thoracic cavity increases, pressure decreases, & lungs, cardiac chambers (RA&RV), SVC & IVC expand. This expansion causes intracardiac and intravascular pressures to fall, so increased venous return, increased preload and SV.
The harder and the more you breathe in, the more blood you suck back to the heart.

Question 15

describe the effect of venomotor tone on veins

Answer 15

This is the effect of motor nerves on the smooth muscle surrounding the walls of veins, causing contraction, which pushes more blood back towards the heart and increases EDV etc

Question 16

describe the effect of systemic filling pressure on veins

Answer 16

systemic filling pressure is the pressure created by the ventricles & transmitted all the way through to the veins.. Difference between RA pressure & systemic filling pressure determines venous return