Control Of The Circulation

Question 1

What does circulation control?

Answer 1

Maintain blood flow
Maintain arterial pressure
Distribute blood flow
Auto-regulate/homeostasis
Function normally
Prevent catastrophe!
(maladapt in disease)

Question 2

Arteries

Answer 2

Low resistance conduits
Elastic
Cushion systole
Maintain blood flow to organs during diastole

Question 3

Arterioles

Answer 3

Principal site of resistance to vascular flow

Therefore, TPR = Total Arteriolar Resistance. This controls how much blood flows through

Determined by local, neural and hormonal factors

Major role in determining arterial pressure

Major role in distributing flow to tissue/organs

Question 4

TPR- Total Peripheral Resistance

Answer 4

Vascular smooth muscle (VSM) determines radius

VSM Contracts = ↓Radius = ↑Resistance ↓Flow

VSM Relaxes = ↑Radius = ↓Resistance ↑Flow

Or Vasoconstriction and Vasodilatation

VSM never completely relaxed = myogenic tone

Question 5

Capillaries

Answer 5

40000km and large area means that there is a slow flow of of blood

Allows time for nutrient/waste exchange

Plasma or interstitial fluid flow determines the distribution
of ECF between these compartments

Flow also determined by
- Arteriolar resistance
- No. of open pre-capillary sphincters

Question 6

Veins

Answer 6

Low resistance conduits- blood flows at a low level through the lumen of veins

Capacitance vessels- veins can store a large volume of blood as they have a larger lumen

Up to 70% of blood volume but only 10mmHg

Valves aid venous return (VR) against gravity

Skeletal muscle/Respiratory pump aids return

SNS mediated vasoconstriction maintains VR/VP

Question 7

Lymphatics

Answer 7

Fluid/protein excess filtered from capillaries

Return of this interstitial fluid to CV system
-Thoracic duct; left subclavian vein

Uni-directional flow aided by
-Smooth muscle in lymphatic vessels
-Skeletal muscle pump
-Respiratory pump

Question 8

Equations

Answer 8

Stroke volume
Volume of blood ejected from each ventricle during systole
SV = end diastolic volume - end systolic volume

Mean arterial pressure
MAP = diastolic pressure + ⅓PP

Blood pressure
BP = CO X TPR

Cardiac output and how to measure this
Volume of blood each ventricle pumps as a function of time
CO = heart rate X stroke volume

Ohm’s law and vessel resistance
Flow = pressure gradient / resistance

Poiseuille and blood flow
Flow = Radius4

Pulse pressure
PP = systolic pressure - diastolic pressure

Question 9

Equations in words

Answer 9

SV increases as End-Diastolic Volume increases
Due to Length-Tension (L-T) relationship of muscle

↑EDV (end diastolic volume)= ↑Stretch = ↑Force of contraction

Cardiac muscle at rest is NOT at its optimum length
↑Venous return = ↑EDV = ↑SV = ↑CO
(even if HR constant)

Question 10

Blood volume

Answer 10

BV is a Long term moderator

BV= Na+ and H2O

Controlled by:
Renin-Angiotensin-Aldosterone system
ADH

Control takes place in the Adrenals and kidneys

Question 11

Blood pressure

Answer 11

Pressure of blood within and against the arteries

Question 12

Systolic

Answer 12

Highest, when ventricles contract (100-150mmHg)

Question 13

Diastolic

Answer 13

Lowest, when ventricles relax (not zero, due to aortic valve and aortic elasticity .. 60-90mmHg)

Question 14

Mean Arterial pressure

Answer 14

=D + 1/3(S-D)

Measured using a sphygmomanometer

Using brachial artery
-Convenient to compress
-Level of heart

Question 15

Components of BP control

Answer 15

Autoregulation
Local mediators
Humoral factors
Baroreceptors
Central (neural) control

Question 16

Neural control of circulation

Answer 16

Peripheral chemoreceptor

Arterial baroreceptors

Cardiopulmonary baroreceptor

Question 17

Peripheral chemoreceptors

Answer 17

Where?
Aortic arch
Carotid sinus

Sensitive to?
Increase in CO2
Decrease in pH
Decrease in O2

Response always?
SYMPATHETIC
Impulses to PRESSOR region of medulla
Leads to Increase in BP

Question 18

Arterial baroreceptors

Answer 18

Where?
Aortic arch
Carotid sinus

Responds to?
High Blood pressure

Mode of action?
PARASYMPATHETIC
Increase in BP means theres an increase in firing rate from the arterial baroreceptors (as more distorted baroreceptors)
This increases impulses to DEPRESSOR centre of medulla
This reduces BP

Question 19

Cardiopulmonary baroreceptors

Answer 19

Where?
Atria
Ventricles
Pulmonary artery

Responds to?
Increase in blood volume

Mode of action?
PARASYMPATHETIC
Increase in blood volume means increase in blood pressure which means more distorted baroreceptors
This causes increase in impulses to DEPRESSOR region of medulla
Leads to decrease in BP

Question 20

Local substances/ factors controlling BP

Answer 20

Vasodilators
-Hypoxia
-Decrease in pH, increase in H+, increase in CO2
-Bradykinin
-Nitric Oxide
-Prostacyclin, type of prostaglandin
-Increase in K+

Vasoconstrictors
-Endothelin 1

Question 21

Hormones and neurotransmitters that control BP

Answer 21

Vasodilators
-ACh (acts on M2 receptors)
-ANP

Vasoconstriction
-Angiotenisn II
-ADH
-NAd (noradrenalin)

Question 22

Pulmonary vessels

Answer 22

BP- 25/8

Thin walled

Hypoxia causes vasoconstriction of blood vessels to redirect blood to are of lung with more oxygen and allow greater oxygen uptake

Question 23

Systemic blood vessels

Answer 23

BP- 120/80

Thick walled

Hypoxia causes vasodilation of systemic blood vessels to allow more haemoglobin to be available to bind to whatever oxygen is available