BIOL125 Lecture Notes - Lecture 9: Autoregulation, Nitric Oxide, Sympathetic Nervous System
BIOL week 9 LC Control of Blood Flow. Exercise-induced changes to CVS function
Three Cardiovascular Pressures
- Blood flows in one direction through body due to the pumping action of the heart
and pressure differences in vascular system
- BP = arterial pressure; high pressure gradually drops the rest of the way through the
vascular system from 100 mmHg at aorta to 35 mmHg at arterial end of capillary bed
- Capillary Hydrostatic pressure force exerted by fluid (blood) pressing against
capillary wall; drops from 35 mmHg at arterial end of capillary bed to 18 mmHg at
venous end
oHigh pressure near arterial end and low pressure near venous end
oCauses some substances to filter out into extracellular fluid
- Venous Pressure pressure within venous system; typically very low from 18 mmHg
in capillaries to about 2 mmHg in vena cavae i.e. 16 mmHg pressure drop
oHigher pressure allows exchange of materials across the capillary wall
Capillary Exchange
- Three types of exchange across capillary walls diffusion, filtration and
reabsorption; provides solutes to cells and removes waste products
- Diffusion moves dissolved gases, water, ions, small organic molecules eg. Glucose,
urea and amino acids between adjacent endothelial cells or at fenestrated capillaries
(wider walls)
- Many ions also move through protein channels and lipid soluble molecules diffuse
across endothelial membrane
- Plasma proteins usually only cross at sinusoids, unless inflammation present
(because they are too large)
oSinusoidal capillaries (eg. Liver) are the most permeable and therefore
diffusion can occur
- Filtration movement of solutes and water by pressure at arterial end of capillary
(capillary hydrostatic pressure)
oDependent on pressure gradient
oHigher pressure at arterial end (capillary hydrostatic pressure)
- Reabsorption occurs at venous end due to Starling forces. Oncotic pressure of the
capillary supports/facilitates/promotes reabsorption
oAllows exchange of materials across capillary wall
oOsmotic Pressure = the force of osmotic water movement; the pressure that
must be applied to prevent osmosis across a membrane
oOncotic pressure = the colloid osmotic pressure of body fluids
oColloid = solution containing large organic molecules in suspension
Pressure and Resistance
- Interplay between Filtration and Reabsorption
1. Ensures that plasma and interstitial fluid are in constant communication and to
facilitate mutual exchange
2. Accelerates distribution of:
oNutrients, hormones and dissolved gases throughout tissues
- Net hydrostatic pressure forces water out of solution
- Net osmotic pressure forces water into solution
- Both control filtration and reabsorption through capillaries
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Factors that contribute to Net Hydrostatic Pressure:
1. Capillary hydrostatic pressure (CHP)
2. Interstitial fluid hydrostatic pressure (IHP)
- Net capillary hydrostatic pressure tends to push
water and solutes:
oOut of capillaries
oInto interstitial fluid
Net Capillary Colloid Osmotic Pressure
- Is the difference between:
1. Blood colloid osmotic pressure (BCOP)
exerted by proteins contained in the blood
2. Interstitial fluid colloid osmotic pressure (ICOP)
exerted by proteins contained in the
interstitial fluid
oBCOP is usually higher than ICOP because
there is hardly any protein in the interstitial
fluid
- Pulls water and solutes:
oInto a capillary
oFrom interstitial fluid
Net Filtration Pressure (NFP)
- The difference between:
oNet hydrostatic pressure
oNet osmotic pressure
oNFP = (CHP – IHP) – (BCOP – ICOP)
CHP = capillary hydrostatic pressure
IHP = interstitial hydrostatic pressure
Capillary Exchange
- At arterial end of capillary:
oFluid moves out of capillary
oInto interstitial fluid
- At venous end of capillary:
oFluid moves into capillary
oOut of interstitial fluid
- Transition point between filtration and reabsorption (is closer to venous end than
arterial end)
- Capillaries filter more than they reabsorb (excess fluid enters lymphatic vessels)
- When there is an imbalance in the system, lymphatic system will pick up any excess
fluid in interstitial space an accumulation of this results in swelling and edema
- Two ways cancer can spread: blood vessels and lymphatic vessels
Forces Across Capillary Walls
- Net capillary hydrostatic pressure tends to push water and solutes out of capillaries
- Net hydrostatic pressure generated from capillary hydrostatic pressure (35 18
mmHg) and ISF hydrostatic pressure (average 0 mmHg)
- Plasma proteins in blood create colloid osmotic pressure (BCOP); tends to pull water
and solutes into a capillary
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- BCOP about 25 mmHg and ISF colloid osmotic pressure 0-5 mmHg (almost nil as few
proteins in ISF)
- Net filtration pressure = net hydrostatic pressure – net colloid osmotic pressure
- Any condition which affects hydrostatic or osmotic pressures in tissues or blood
shifts the balance (ie. Affects capillary exchange) eg. In dehydration BCOP increases
due to water loss increased reabsorption of fluids
- Excess fluid not reabsorbed at venous end is taken into lymphatic vessels; cleaned
and returned to blood at subclavian veins:
Cardiovascular Regulation
- Tissue perfusion
oBlood flow through the tissues
oCarries O2 and nutrients to tissues and organs
oCarries CO2 and wastes away
oIs affected by:
Cardiac output
Peripheral resistance
Blood pressure
Cardiovascular Regulation Changes Blood Flow to a Specific Area
1. At an appropriate time
2. In the right area
3. Without changing blood pressure and blood flow to vital organs
Controlling Cardiac Output and Blood Pressure
- Autoregulation
oCauses immediate, localised homeostatic adjustments
oBrain and kidneys can control local blood flow through autoregulatory
mechanism
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Document Summary
Biol week 9 lc control of blood flow. Blood flows in one direction through body due to the pumping action of the heart and pressure differences in vascular system. Bp = arterial pressure; high pressure gradually drops the rest of the way through the vascular system from 100 mmhg at aorta to 35 mmhg at arterial end of capillary bed. Venous pressure pressure within venous system; typically very low from 18 mmhg in capillaries to about 2 mmhg in vena cavae i. e. 16 mmhg pressure drop: higher pressure allows exchange of materials across the capillary wall. Three types of exchange across capillary walls diffusion, filtration and reabsorption; provides solutes to cells and removes waste products. Diffusion moves dissolved gases, water, ions, small organic molecules eg. glucose, urea and amino acids between adjacent endothelial cells or at fenestrated capillaries (wider walls) Many ions also move through protein channels and lipid soluble molecules diffuse across endothelial membrane.
