Resistance & exchange Flashcards
Arterioles
Contribute more than 60% of the total resistance to flow
- Vasodilating and vasoconstricting arterioles leads to changes in resistance to blood flow, thus blood pressure
==> primarly done by 1) endothelium cels (ensure blood flows nicely and smoothly through blood vessels so the wont clot and prevent platelets from sticking to the subendothelial collagen matrix)
==> 2) smooth muscle layer that wraps around endothelial cells, can contract and relax, changing radius - Arteriolar resistance influenced by local and systemic mechanicsms
- Metabolic requirements = legs need muscle during exercise
- Sympathetic reflexes = short term of vessel diameted
- Hormonal control = hormones released from adrenal medulla that will then facilitate long term control over vessel dilation and constriction.
Regulation of arteriole smooth muscle
- In most blood vessels, the smooth muscle is a state of partial constriction at all times to maintan muscle tone (tension)
Influenced by calcium that is modulated by: (amount of calcium that’s in these cells dictates the amount of contraction or relaxation that these the smooth muscle layer undergoes.) - Neurotransmitters from systemic innervation from your sympathetic nervous system
- Hormones = long term regulation
- Paracrine signals
–> Secreted by endothelial cells or
surrounding tissues
Local control of vasoconstriction/dilation
- Produced by both the blood vessels and underlying tissues can alter arteriolar diameter and therefore resistance
factors :
1. myogenic activity = the amount of stretch that a vessel undergoes/ mechano receptors detect how much blood is going through and blood vessels resist the flow by stretching
–> increased stretch/myogenic activity == resuts in antagonistic force and vasoconstriction vice versa
2. O2/ CO2 = too much O2 and too little CO2 will cause vessels to constrict as there is too much exchange vice versa too little CO2 so increase blood flow vasodilation
3. NO = natural potent basal dilator and more nitric oxide causes vasodilation
5. sympathetic stimulation e.g. histamine + heat will cause vasodilation for heat transfer from blood/ cold cause vasoconstriction
hyperaemia
Hyperaemia is the process by which the body adjusts blood flow to meet the metabolic needs of its different tissues in health and disease.
- Active hyperaemia matches blood flow to increase metabolism
- increased tissue metabolism== arterioles dilate
- resistance drops, blood flow increases
- o2 and nutrients supply to tissue increased
- Reactive hyperaemia follows a temorary period of decreased blood flow
- tissue blood flow decreases occulsion/blockage
- arterioles dilate, but blockage prevents blood flow
- once occlusion removed, resistance drops, blood flow increases
- as vasodilators wash away, arterioles constrict and blood flow returns to normal
Hormonal control of arterioles
- Most significant are: Epinephrine and norepinephrine
==> Generally, reinforce sympathetic nervous system in most organs
Vasopressin, angiotensin II and natriuretic peptides (these are released from heart) - Important in controlling fluid balance
- Dominant role in the renal system and regulation of ion and water excretion
Sympathetic control of resistance arterioles
one of the most dominant forms of arterial radius control
Most systemic arterioles are innervated by sympathetic neurons
1. sympathetic neuron releasing noradrenaline that act alpha receptors on your smooth muscle layer
* nor/adrenaline binds to α-receptors causing vasoconstriction
== have a regular firing of these electrical signals from these neurons to release norepinephrine at a constant rate
== causes that tone or that partial constriction.
increasing/decreasing the amount of firing within these neurones and control the amount of noradrenaline released can regulate the dilation
* Tonic release
Microcirculation
- Arterioles, capillaries and venules form the microcirculation
there is no smooth muscle layer in this capillary bed == cell walled tubes of just endothelial cells
–> regulate the amount of pressure inside the actual capillaries themselves.
instead, precapillary sphincters control the amount of blood that flows into these capillary beds.
Capillaries
- Thin-walled, small-radius, extensively branched
- Single layer of endothelial cells with diameter of ~ 1 RBC wide
- Sites of exchange between blood and surrounding tissue cells
== Maximised surface area and minimised diffusion distance
Cell junctions between endothelial cells determine the “leakiness” of the capillary (from less leaky to more leaky)
* Continuous capillaries = make up the majority of the capillary beds that exist within our body/ - majority of organs
* Fenestrated capillaries = have that have tiny openings, or pores found in GI system (absorption), kidneys (renal exchange/filtration)
* Sinusoids = liver ( liver is the primary source of protein production so needs to be leaky for plasma prtoeins to leave), bone marrow (our bone cell production factories, RBC and platelets to leave and enter circulation)
Capillary blood flow
- The rate of blood flow through capillaries effects the efficiency of exchange between blood and interstitial fluid
==> at smaller, thinly walled capillaries, that resistance or that change in arterial capillary diameter, that’s really going to drastically slow the rate of blood flow = allow this exchange process to happen between blood and tissue
==> diameter is large in capillaries compared to aorta for more amount of blood flow - Primary determinant of velocity of blood flow across capillaries is the total cross sectional area of all capillaries
Capillary exchange
Capillary exchange can occur by three main mechanisms:
* Diffusion
* Transcytosis (vesicular transport)
* Bulk flow
Diffusion rates are determined by concentration gradients
Capillary exchange: Bulk flow
- Bulk flow = mass movement of water and dissolved solutes between blood
and the interstitial fluid as a result of hydrostatic or osmotic pressure
bulk flow has both filtration (irection of flow that goes from the blood into the tissue so out of the capillary) and absorption (from the tissue back into the capillary) - Filtration = flow out of the capillary
- Reabsorption = flow into the capillary
movement of water and solutes can move in both directions
* Most capillaries show Net Filtration at the arterial end and Net
Reabsorption at the venous end
osmotic and hydrostatic pressure
hydrostatic :always driving stuff out of the capillary
net filtration at the start as hydrostatic is greater than osmotic pressure
net reabsorption at the venous end, osmotic pressure is greater hydrostatic
osmotic pressure
Due to a solute difference between the plasma and the interstitial fluid
The solute difference is due to plasma proteins
* Known as colloid osmotic pressure (COP)
The COP is generally higher in the plasma and causes the osmotic movement of water into the capillary
* Other solutes are permeable
==> DO not contribute to the COP
*** COP is the same along the length of the capillary ** just the hydrostatic pressure changing across capillary
Hydrostatic pressure
- Result of fluid flowing through the capillaries
- Decreases along the length of the capillary due to energy loss caused by friction
- Forces fluid out of the capillaries through pores
Net pressure =
Net pressure = PH - 𝝅P
PH – Capillary hydrostatic (blood) pressure (outward pressure)
𝝅P – Plasma-colloid osmotic pressure (inward pressure)
Net fluid flow across the capillary is determined by the differences between the hydrostatic and the colloid osmotic pressures
Positive net exchange = filtration
Negative net exchange = reabsorption
Lymphatic system
Interacts with cardiovascular, digestive and immune systems within the body
role : make sure we recycle blood, don’t decrease total blood volume and adversely affect things like blood pressure and blood flow.
Functions:
* Return of fluid and proteins filtered out of capillaries into circulatory system
* Picks up absorbed fat from the small intestine to transfer to circulatory system
* Filter, capture and destroys foreign pathogens
Allows one-way movement of interstitial fluid from the tissues to the circulation
1. Lymph = Interstitial fluid that leaves that particular tissue and then enters a lymphatic vessel
2. liquid enters a lymphatic capillary bed of its own == Initial lymphatics (lymph capillaries)
* Small, blind-ended terminal lymph vessels
* Permeate almost every tissue of the body
3. Lymph ends up at lymph nodes
* Convergence with larger lymphatic vessels
* Eventually empty into venous system at thoracic duct near where blood enters right atrium
* One-way valves like veins spaced at intervals direct flow of lymph toward venous outlet in chest
