Circulation

Question 0

What does the rate of diffusion depend on?

Answer 0

Surface area for diffusion:
Branching network of capillaries increases s.a. by increasing capillary density (metabolically active tissues have more capillaries)

Diffusion resistance:

• nature of barrier e.g. size of pores
• diffusion distance
• nature of molecule (hydrophobicity & size)

Concentration gradient between capillary blood and tissues (limiting factor: rate of blood flow):

• rate tissue uses substance
• rate of blood flow through capillary bed (perfusion bed)

Question 1

How can the circulation be described as a whole? What makes up the parts of the circulatory system?

Answer 1

2 pumps in series

Flow control = arterioles & pre-capillary sphincters
Distribution system = vessels & blood
Pump = heart
Exchange mechanism = capillaries

Question 2

What is the minimum and maximum blood flow in a 70kg male? What organs have a constant blood flow and which do not?

Answer 2

Minimum: 5.0l/min

Maximum: 25l/min

Constant blood flow: brain, kidneys

Heart & muscle blood flow increases through exercise

Gut blood flow increases through digestion

Skin blood flow increases through vasodilation

Question 3

How is perfusion to the brain maintained against gravity?

Answer 3

Increase resistance in non-essential tissues to redirect blood to brain by constricting arterioles.

Use capacitance: veins act as a variable reservoir for blood which can be returned to the heart by venoconstriction
(most blood in body is in peripheral veins)

Question 4

How is blood distributed throughout a 70kg male? In what areas is the blood flow fastest and slowest?

Answer 4

15l outside cells

6l of blood total:

• 3.9l in veins (1l spare)
• 1.2l in heart & lungs
• 0.6l in peripheral arteries
• 0.3l in capillaries

Blood flow fastest when total cross-sectional area is least

Therefore blood flow is fastest in aorta and arteries, and slowest in capillaries

Question 5

What is an end artery?

Answer 5

Terminal artery supplying most/all of the blood to a body part without significant collateral circulation

Undergo progressive branching without the development of channels connecting with other arteries

e.g. coronary artery, splenic artery, renal artery, central artery to retina, labyrinthine artery of internal ear

Question 6

What is bridging?

Answer 6

Narrowing of coronary artery due to muscle squeezing lumen during systole

Question 7

What do lymphatic capillaries do?

Answer 7

Drain away excess extracellular fluid, returning it to the blood at the junctions of the internal jugular and subclavian veins

Question 8

What is the central channel?

Answer 8

Metarteriole + thoroughfare channel allows capillary bed to be bypassed

Precapillary sphincters close off branches of capillary bed except for one channels to the post-capillary venule.

Question 9

What are the different mechanisms of transport across capillaries?

Answer 9

Direct diffusion
Diffusion through intercellular cleft
Diffusion through fenestration
Pinocytic vesicles

Question 10

What is a pericyte?

Answer 10

Cells capable of dividing into muscle cells or fibroblasts during angiogenesis, tumour growth, and wound healing.

Form branching network on the outer surface of the endothelium of continuous capillaries.

Question 11

Define flow. What is it proportional to? What is it determined by?

Answer 11

FLOW = volume of fluid passing a given point per unit time (l/min) (must be the same at all points in a vessel)

Proportional to pressure gradient

Flow for a given pressure gradient determined by resistance of vessel

Question 12

Define velocity. What is it determined by?

Answer 12

VELOCITY = rate of movement of fluid particles along a tube.

At a given flow, velocity is inversely proportional to the overall cross-sectional area

Question 13

At a constant pressure, what is the relationship between flow and velocity?

Answer 13

Flow determined by mean velocity

Mean velocity determined by cross-sectional area and viscosity

Increase cross-sectional area = increase velocity (directly proportional)
Increase viscosity = decrease velocity (inversely proportional)

          PRESSURE     =        FLOW        X       RESISTANCE

Question 14

Define viscosity. What is the difference between laminar and turbulent flow?

Answer 14

VISCOSITY = how easily layers of fluid slide over each other

LAMINAR FLOW = gradient of velocity from the middle to the edges (velocity highest in the centre, and fluid almost stationary at edges)

TURBULENT FLOW = mean velocity increases above threshold so that the velocity gradient breaks down (fluid tumbles over itself, increasing resistance)

Question 15

What is the advantage of having blood vessels in parallel rather than in series?

Answer 15

Effective resistance is lower.

Question 16

What are the approximate pressures in the circulatory system?

Answer 16

HEART —> ARTERIES —> ARTERIOLES —> CAPILLARIES —>
(100mmHg) (35mmHg)

—> VENULES —> VEINS
(10mmHg) (8mmHg —-> 3mmHg)

Question 17

What does central venous pressure depend on?

Answer 17

• venous return
• pumping of the heart
• gravity (when standing, blood pools in leg veins -> reduces venous return)
• “muscle pumping” (muscles around veins pump -> compress veins)

Question 18

What is transmural pressure?

Answer 18

Pressure within vessel generates pressure across walls of vessel (between inside and outside) which stretches the tube

Increase pressure -> vessel stretches -> more blood transiently flows in than out (capacitance)

Decrease pressure -> vessel collapses -> hypovolaemic shock

Question 19

What factors affect systolic pressure?

Answer 19

• force of contraction
• total peripheral resistance (inversely proportional to body’s need for blood flow)
• compliance (stretchiness) of vessels

note: most tissues automatically adjust their perfusion to match pressure changes (except the heart & brain)

Question 20

What factors affect diastolic pressure?

Answer 20

• systolic pressure

- total peripheral resistance (inversely proportional to body’s need for blood flow)

Question 21

What is the pulse pressure and what is the average pressure?

Answer 21

PULSE PRESSURE = difference between systolic and diastolic pressure (~40mmHg)

AVERAGE PRESSURE = diastolic pressure + 1/3 of pulse pressure
(as systole is shorter than diastole)

Question 22

What is reactive hyperaemia?

Answer 22

Circulation cut off (for a few min.), causing an accumulation of vascular metabolites (or form of autoregulation in response to increased metabolism or blood flow)

When blood flow is restored, the flow is very high due to the vasodilator metabolites (H+, K+, adenosine) lowering the resistance

Blood flow dilutes vasodilator metabolites

Normal vasomotor tone resumes

note: amount of blood that resumes flow is equal to amount that was prevented from reaching tissues (same flow volume)

Question 23

What is the effect of changing the total peripheral resistance when cardiac output is constant?

Answer 23

Decrease total peripheral resistance -> decrease arterial pressure and increase venous pressure (easier for blood to get into veins)

Increase total peripheral resistance -> increase arterial pressure and decrease venous pressure (harder for blood to get into veins)

Question 24

What is the effect of changing the cardiac output when there is a constant total peripheral resistance?

Answer 24

Decrease cardiac output -> decrease arterial pressure & increase venous pressure (removing less blood from veins)

Increase cardiac output -> increase arterial pressure & decrease venous pressure (removing more blood from veins)

Question 25

How do changes in demand for blood in certain organs alter the circulation?

Answer 25

e.g. eat

local vasodilators -> dilate arterioles -> reduces total peripheral resistance -> reduces arterial pressure & increases venous pressure

• > increases heart rate -> increases cardiac output -> increases arterial pressure & reduces venous pressure (stabilises)
  note: no long term change in mean circulatory pressure

Question 26

Where is the best place, in principle, to measure arterial pressure?

Answer 26

The aorta (gives best estimation of the pressure directly after exiting heart)

BUT there is little difference in pressure along the arteries, so the brachial artery is a good enough estimation (and is at the same level as the heart)

Question 27

How does the estimate of arterial pressure when measured in the lower leg differ when measured in a person standing up opposed to sitting down?

Answer 27

Standing up -> increased hydrostatic pressure (act of gravity on a column of fluid)

Question 28

How is the arterial pressure estimate affected when the arm is raised or lowered above the level of the heart?

Answer 28

ABOVE = overestimation (blood pressure increases to pump against gravity)

BELOW = underestimation (blood pressure decreases due to gravity)

Question 29

What happens to the resistance of a distensible vessel (vein) when the pressure changes? What happens when smooth muscles in distensible vessels contract?

Answer 29

Pressure INCREASES -> flow resistance decreases

Pressure DECREASES -> flow resistance increases -> flow stops

Smooth muscle contracts -> critical closure at increased pressure (reduced capacitance)

Question 30

What is the relationship between central venous pressure and end-diastolic volume? Give some examples of what could increase and decrease the central venous pressure.

Answer 30

Increase central venous pressure -> increase end-diastolic volume

CVP: 1-10mmHg (depends on total blood in circulation and distribution of blood)

Increase CVP e.g. venoconstriction, transfusion
Decrease CVP e.g. orthostasis, haemorrhage, dehydration

Question 31

What happens to baroreceptors if there is a sustained increase in arterial blood pressure? What does an increase in arterial pressure cause?

Answer 31

Baroreceptors reset at a higher arterial pressure (poor control of long term hypertension)

note: carotid sinus has baroreceptors

Increase in arterial blood pressure -> increase in parasympathetic activity (opposite to increase in atrial blood pressure)

Question 32

Where are the high pressure baroreceptors found? How do these regulate blood pressure (baroreceptor reflex)?

Answer 32

Carotid sinus & arch of aorta

Blood pressure increases -> baroreceptors stimulated by stretch -> depolarisation of glossopharangeal nerve (carotid sinus)/vagus nerve (aortic arch) -> parasympathetic stimulation -> negative inotropy and chronotropy & reduction in total peripheral resistance

Blood pressure decreases -> reduced depolarisation -> reduced parasympathetic stimulation & increased sympathetic stimulation -> positive inotropy and chronotropy & vasoconstriction (increased venous return)