Circulation Flashcards by Isobel Ritchie

What does the rate of diffusion depend on?

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)

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

2 pumps in series

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

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

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

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How is perfusion to the brain maintained against gravity?

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)

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

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

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What is an end artery?

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

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What is bridging?

Narrowing of coronary artery due to muscle squeezing lumen during systole

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What do lymphatic capillaries do?

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

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What is the central channel?

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.

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What are the different mechanisms of transport across capillaries?

Direct diffusion
Diffusion through intercellular cleft
Diffusion through fenestration
Pinocytic vesicles

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What is a pericyte?

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.

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

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

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Define velocity. What is it determined by?

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

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

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

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

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

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)

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

Effective resistance is lower.

What are the approximate pressures in the circulatory system?

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

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

What does central venous pressure depend on?

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)

What is transmural pressure?

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

What factors affect systolic pressure?

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)

What factors affect diastolic pressure?

systolic pressure

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

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

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

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

What is reactive hyperaemia?

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)

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

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)

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

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)

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

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

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

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)

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?

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

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

ABOVE = overestimation (blood pressure increases to pump against gravity) BELOW = underestimation (blood pressure decreases due to gravity)

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

Pressure INCREASES -> flow resistance decreases Pressure DECREASES -> flow resistance increases -> flow stops Smooth muscle contracts -> critical closure at increased pressure (reduced capacitance)

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.

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

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

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)

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

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)