The Microcirculation (31) Flashcards
What is the passage of microcirculation?
arteriole–>capillaries–>venule
What is the overall aim of the cardiovascular system?
ensuring adequate blood flow through the capillaries
What is blood flow rate?
- volume of blood passing through a vessel per unit time
- Q (flow rate) = Pressure gradient / Resistance
What is the pressure gradient in terms of blood flow to the capillaries?
pressure at the start of the arteriole - pressure at the end of the arteriole
(pressure A- pressure B)
N.B. inc. pressure gradient–> inc. flow rate
What is resistance in terms of blood flow?
hindrance to blood flow due to friction between moving fluid and stationary vascular walls
R= 8Ln/ pi x r^4
What happens to blood flow rate when you increase resistance?
blood flow rate decreases
What vessels have the biggest influence on resistance?
arterioles
bc biggest pressure gradient
Why is blood flow determined only by resistance?
as pressure gradient is basically just mean arteriole pressure, as venule pressure is so low–>0
so flow= MAP/resistance
Why is it important that vessels are normally partially constricted?
‘vascular tone’
so that it can contract further OR dilate
(if it was totally dilated or totally contracted, you would remove a method of controlling blood flow)
What happens to resistance and blood flow in vasoconstriction?
dec. radius
inc. resistance
dec. blood flow
What happens to resistance and blood flow in vasodilation?
inc. radius
dec. resistance
inc. blood flow
What are the 2 distinct functions of the arterioles?
- matching blood flow to the metabolic needs of specific tissues via local/intrinsic controls (momentary needs)
- helping regulating systemic arterial blood pressure via extrinsic controls (nerves or blood), centrally coordinated
How does active hyperaemia work?
- chemoreceptors in tissues detect inc. CO2 etc…–> directly causes smooth muscle contraction/relaxation
- vasodilation increases blood flow to active tissue
e. g. in skeletal muscle
How does myogenic auto regulation work?
if tissue doesn’t need inc. blood flow–> detects inc. stretch (due to inc. BP) —> causes vasoconstriction of arterioles ensuring that blood doesn’t overly flow into particular tissue
What happens to arterioles when blood temperature decreases?
vasoconstriction to preserve temp.
What is an equation for blood pressure across the whole circulation?
BP= Q X TPR
blood pressure= cardiac output X total peripheral resistance
How does the brain regulate arterial blood pressure via neurones?
e. g. if there is blood loss detected
- -> cardiovascular control centre in medulla
- -> vasoconstriction
- -> dec. blood flow to specific organs (brain always takes priority, so blood flow preserved here)
How does the brain regulate arterial blood pressure via hormones?
vasopressin/ADH, angiotensin 2, adrenaline/noradrenaline
–> all cause vasoconstriction
What is the purpose of capillary exchange?
to deliver metabolic substrates to the cells of the organism
- highly branched (max. SA)
- thin cell width (min. diffusion distance
^ enhance diffusion - Fick’s Law
Why is capillary density important?
- depends on how active tissue is
- dec. diffusion distance and inc. SA
- highly metabolically active tissues have denser capillary networks
e. g. brain, skeletal muscle and lungs (to enable proximity to alveoli)
What are the types of capillaries?
- continuous (small, water-filled gap junctions)
- fenestrated (larger holes)
- discontinuous (less common, but found in liver)
What is bulk flow?
a certain volume of protein-free plasma filters out of the capillary (bc hydrostatic pushing force), mixes w/ surrounding interstitial fluid and is reabsorbed (bc oncotic pulling force of proteins)
What is Starling’s hypothesis in terms of hydrostatic/oncotic forces?
there is a balance between hydrostatic pressure pushing it out and oncotic pressure drawing it back in
How do the pressure changes in capillaries affect fluid movement?
- if pressure inside capillary > pressure in interstitial fluid–> ultrafiltration
- if inward driving pressure > outward pressures across the capillary–> reabsorption (bc oncotic pressure is constant but hydrostatic dec.)
Why do we need the lymphatic system?
net loss of fluid out of capillaries bc ultrafiltration is more effective than reabsorption
What type of capillary structure is the blood brain barrier?
continuous
allows brain to have tight control over what enters
