Peripheral vascular physiology Flashcards
What is the rate of cerebral blood flow?
750ml per minute
What percentage of the cardiac output does the brain receive per minute?
15-20%
In what 3 ways is the cerebral circulation adapted for it’s specialised nature?
Arterial anastamoses
BBB
Autoregulatory mechanisms
Why is the circle of Willis important?
Because this anastomosis provides collateral blood supply which is protective against cerebral ischaemia
Which 3 paired vessels are the main constituents of the circle of willis?
Anterior referral arteries
Internal carotid arteries
Posterior cerebral arteries
Which vessels connect the 3 paired vessels together to complete the circle of willis?
Anterior and posterior communicating arteries
What is the blood brain barrier?
A high selective permeable barrier that separates blood from the brain ECF in the CNS
Which 2 properties of the brain endothelium allow the BBB to be effective?
Tight junctions that prevent paracellular movement of molecules
AND
No transendothelial pathways (i.e. intracellular vesicles)
What is the BB permeable to?
Water
Non-ionised lipid soluble substances (barbiturates, ethanol, caffeine)
What is the BBB impermeable to?
Proteins
Protein-bound substances (drugs, hormones)
Strongly hydrophilic substances (Na/ K)
Most bacteria, antibodies and antibiotics (too large to cross BBB)
What allows molecules with low lipid solubility to move across the BBB?
Carrier-mediated transport.
How does glucose cross the BBB and enter the brain?
Enters the brain via the GLUT-1 transport protein.
What are the main auto regulatory mechanisms that help to regulate cerebral blood flow?
Myogenic and metabolic autoregulation
What is myogenic auto regulation?
A mechanism that regulates local blood flow to the brain by allowing changes in the diameter of blood vessels in response to changes in BP.
What does a fall in blood pressure do to vessels in the brain?
Compensatory vasodilation of blood vessels = increased blood flow
At what blood pressure does myogenic auto regulation fail?
50mmHg (blood vessels cannot dilate any further at this point)
What is metabolic auto regulation?
A mechanism that helps to maintain local blood flow to the brain by allowing compensatory changes in blood vessel diameter in response to changes in the partial pressure of arterial CO2.
What does metabolic auto regulation respond to?
CO2
What does myogenic auto regulation respond to?
Blood pressure
What does hypercapnia cause to happen to vessels in the brain?
Vasodilation
**Hypercapnia is indicative of high metabolic demand = increased blood flow needed
What does hypocapnia cause to happen to vessels in the brain?
Vasoconstriction
An increase in which ion causes vasodilation in the cerebral circulation?
Potassium
How is the pulmonary circulation adapted to facilitate consistent gas exchange for high volumes of blood?
Anatomical adaptations
Short diffusion distance
Hypoxic pulmonary vasoconstriction
Give the 3 anatomical adaptations of pulmonary circulation that allow for effective gas exchange.
Low pressure system
Low resistance system
Large surface area for gas exchange
Why are pressures within pulmonary circulation less than pressures within systemic circulation?
Because pulmonary arteries have thin vascular walls and high compliance
What is the MAP within pulmonary circulation?
5-15mmHg
Give the 3 reasons as to why pulmonary circulation has low resistance.
Pulmonary vessels are shorter and wider
Pulmonary capillaries run in parallel, not in series
Relatively little smooth muscles in arterials = reduced arterial tone
What allows for there to be a large surface area for gas exchange in the lungs?
The branching structure of the tracheobronchial tree
What is the combined thickness of the alveolar and capillary endothelium?
0.3 micrometres
What is the main intrinsic regulator of pulmonary blood flow?
The local partial pressure of alveolar oxygen
What does a low alveolar partial pressure of O2 cause?
Vasoconstriction of arterioles
What is the hypoxic pulmonary vasoconstriction (HPV) reflex?
Vasoconstriction of arterioles at low alveoli partial pressures of oxygen
What does the HPV reflex facilitate?
Diversion of blood flow away from poorly ventilated alveoli and towards well-ventilated alveoli to maximise gaseous exchange
When does maximal hypoxic vasoconstriction occur?
At 70mmHg (at a normal pH)
What percentage of total body weight does skeletal muscle make up?
50%
What percentage of cardiac output at rest is directed towards skeletal muscle?
15-20%
What percentage of cardiac output can skeletal muscle receive during exercise?
Can rise to over 80%
Blood flow is tightly matched to what in skeletal muscle?
Metabolic rate
What causes most skeletal capillaries to be closed off and therefore not perfused during rest?
Contraction of pre-capillary sphincters
What does contraction of pre-capillary sphincters and lack of capillary perfusion cause?
Higher vascular tone and constriction of vessels
What are capillaries recruited by during exercise?
Metabolic hyperaemia
What is metabolic hyperaemia caused by?
Release of K+, CO2 and adenosine
Which type of muscle plays an important part in helping venous blood return to the heart?
Skeletal muscle
What structural aspect of veins helps in return of blood to the heart?
One-way valves that act as pumps during muscle activity
**This forces blood that would otherwise pool in lower limbs back to the heart
What is cutaneous circulation involved in?
Bloods supply of the skin
What is the primary function of the cutaneous circulation?
Thermoregulation
Which aspect of cutaneous circulation is involved in temperature regulation?
Arteriovenous anastomoses (AVAs)
What are arteriovenous anastomoses?
Short vessels that connect small arteries and veins.
What happens when AVAs are open?
They provide a low resistance connection between arteries and veins = blood is shunted into superficial venous complexes of limbs
What are AVAs innervated by?
Adrenergic fibres from the hypothalamus (temperature regulation centre)
What happens to AVAs at normal core temperatures?
High sympathetic output = vasoconstriction of AVAs = blood flows through capillary networks and deep plexuses
What happens to AVAs at raised core temperatures?
Reduced sympathetic output = AVA vasodilation = shunting of blood to superficial venous plexus = loss of heat via radiation to the environment
Where are AVAs found in the body?
In high numbers in peripheral regions:
Fingers
Palms
Soles
Lips
Pinna of the ear
The exchange of fluids across the capillary membrane is mainly governed by what?
Oncotic and hydrostatic pressures
What are the 4 pressure componenet stat effect exchange of fluids across the capillary membrane?
Interstitial hydrostatic pressure
Capillary hydrostatic pressure
Interstitial osmotic pressure
Capillary osmotic pressure
What happens to osmotic pressures along the course of the capillary?
They are consistent
What is the overall effect of oncotic pressure along the capillary?
It drives water into the capillary
What happens to hydrostatic pressure along the course of the capillary?
Direction of new flow changes due to the change in capillary hydrostatic pressure from arterials to venous end.
What is interstitial hydrostatic pressure?
Negligible (close to 0)
What is capillary hydrostatic pressure at the arterial end?
35mmHg
What is capillary hydrostatic pressure at the venous end?
15mmHg
At the arterial end of a capillary, what is the net movement of fluid?
Movement of water out of the capillary
At the venous end of a capillary, what is the net movement of fluid?
Absorption of fluid into capillary
In what direction does water flow under oncotic pressure?
Low to high
In what direction does water flow under hydrostatic pressure?
High to low
Which protein mainly creates oncotic pressure?
Albumin
Why does oncotic pressure cause water to move down the gradient?
Because proteins displace water, creating a relative water deficit, drawing water towards them
What is hydrostatic pressure?
Pressure of blood against vessel walls.
How does fluid filtered by microcirculation return to the blood circulation?
Via lymphatics
What are lymphatic capillaries?
Blind ending bulbous tubes that permit entry of fluid, proteins and bacteria but do not allow their exit
How is movement of lymph facilitated in vessels?
Via valves that prevent back flow
AND
Via smooth muscle that contracts to push lymph through the lymphatic system
How is fluid returned to circulation from lymph nodes?
Via efferent vessels and then the thoracic duct.
What process happens in microcirculation?
Exchange between blood and tissues
How is flow through microcirculation regulated?
By vasoconstriction of arterioles
Which molecules are able to pass through capillary walls easily along their diffusion gradient?
Lipophillic molecules (oxygen, CO2)
What is the least permeable type of capillary?
Continuous
Where are continuous capillaries found?
Skin
Fat
Muscle
Nervous tissue (incl. BBB)
Where are fenestrated capillaries found?
GI tract
Joints
Kidneys
What are the most permeable type of capillaries?
Sinusoidal (discontinuous)
Where are sinusoidal (discontinuous) capillaries found?
Bone marrow
Liver
Spleen
**locations where red cells need to cross the capillary wall
Most vasoconstrictors bind to what protein?
G-protein
What is G-protein?
An important mediator in vasoconstriction
What does the G-protein mediator cause?
A rise in intracellular calcium ions = smooth muscle contraction.
What does a rise in intracellular calcium ions cause?
Smooth muscle contraction
Name 3 important vasoconstrictors.
Endothelin
NAD
Angiotensin II
Name the 2 endothelium derived vasodilators.
NO
Prostacyclin
Name the 2 endothelium derived vasoconstrictors.
Endothelin-1
Thromboxane A2
Give the 5 mechanisms by which endothelial release of NO is triggered.
Bradykinin
Histamine
Serotonin
Substance P
Increased blood flow (shear stress)
How do vasodilators work?
By lowering concentrations of intracellular calcium ions
How is lowering intracellular calcium levels normally achieved?
Stimulation of SERCA and PMCA
AND
Hyperpolarisation the cell membrane to inhibit L-type Ca2+ channels
How does the smooth endoplasmic reticulum Ca2+ ATPase low intracellular calcium levels?
Sequesters calcium ions into the smooth ER
How does the plasma membrane CA2+ ATPase lower intracellular calcium levels?
Pumps calcium out of the cell
How are SERCA and PMCA stimulated?
Via secondary chemical messengers such as cGMP and cAMP.
Endothelin-1 release from the endothelium can be stimulated by which 4 factors?
Angiotensin II
ADH
NAD
Hypoxia
