Cardiovascular System: Principles of Flow
rate of diffusion depends on
In a circulatory system, the rate of exchande with interior celss if proportional to
rate of blood flow
To determine the flow (Q) of blood through a given resistance (R) use:
Ohm's law Q = Change (P)/ R
Mean Arterial Pressure (MAP)
Pressure on the arterial blood (typical value is 85 mmHg)
The resistance of the systemic circulation
Total Peripheral Resistance (TPR)
expressed in unit of pressure/flow or
mmHg/(ml/sec)...named peripheral resistance units (PRU)
flow of blood through the system
cardiac output (CO)
CO = MAP/TPR (note CO = SV X HR)
Two forces that fluids face when flowing through a vessel
Pressure (push) vs. Friction (slows down)
layers near wall slowly (due to friction) while layers near center quickly (due to pressure)
the resistance of a single vessel is determined by
poiseuille's law: R = 8nl/pi r^4
Factors that increase resistance
increase in vessel length (l)
increase in blood viscosity (n)
decrease in vessel radius (r)...has the most influence over resistance which is why the body manipulates it often
blood flows through each vessel in turn and must flow through all to reach the end
series resistance (has greater resistance than parallel)
blood has mucltiple possible paths it can follow to reach end
parallel resistance (has less resistance than series)
T/F: Arterioles and precapillary sphincters are the points of highest resistance int he CV system
4 factors that determine pressure
Cardiac Output (CO)
the rigidity or stretchy state of a vessel that helps determine pressure
high compliance = vessel strtchy = low pressure
low compliance = rigid vessel = high pressure
T/F: Arteries have low compliance
True...creates greater pressure
T/F: Veins have high compliance
True...hold large volumes of blood without much pressure (most of blood in body lies in systemic veins)
Capillary pressure (Pc) will be closer to venous pressure (Pv) when:
Rpost < Rpre...because most of the pressure drops during Rpre
Capillary pressure (Pc) will be closer to arterial pressure (Pa) when:
Rpost > Rpre...because there's less relative pressure drop at Rpre
If capillaries have smallest radius, why are arterioles the main site of resistance in the CV system?
because there is an increased number of capillaries which leads to greater conductances
what two key mechanisms of veins helps drive blood back to the heart?
1.venomotor tone: causes veins to contract (actually move forward through muscle pump)
2. venous valves: prevent blood from moving backwards
Why do veins need reinforcement in their walls if blood pressure is so low?
because of wall tension (T)...where T is the force that the vessel must exert to resist splitting open
use Laplace's law: T = change P x r
veins have about 140 times more wall tension than capillaries, therefor need thicker walls
why does blood flow at lower velocity in capillaries that the rest of the vascular system?
becuase of gas exchange...capillaries increase their cross sectional area in order to decrease blood velocity in order to exchange material with tissues (flow (Q) remains the same)
what is one key result of 65% of blood volume being in the systemic veins?
serves as a blood volume reserve in case theres a sudden increased need for cardiac output
T/F: blood pressue drops continuously as it flows from arteris to capillaries to veins
the process where large proteins are actively transported across cells is known as:
transcytosis (requires a lot of energy)
the tightly sealed separtation between blood and brains tissue is:
T/F: exchange of solute in capillaries occrus mostly by diffusion
the main goal of the CV system is to maximize the exchange between blood and tissues (Ex)...what are two ways to Maximize Ex?
1. maximal difference between blood and tissue concentrations (keep nutrients high and wastes low in the blood)
2. maximal flow (Q)
What are two ways that the body maximizes cross-sectional area of flow?
short term: increase number of open capillaries
long term: grow new capillaries
what are the four forces (capillary pressure, interstitial fluid pressure, osmotic pressure of capillaries, and osmotic pressure of interstitial fluid) known as?
the Starling forces
what do the combination of all Starline forces yield?
Net Filtration Pressure (NFP)
NFP = (P(c)+Pi(if)) - (P(if)+Pi(c))
If NFP is positve, net pressure is outward and if NFP is negative, net pressure is inward
What are typical values for NFP?
arterial end= (38+0)-(1+25) equals 12 mmHG outward
venous end= (16+0)-(1+25) equals -10 mmHg inward
In regard to exchange of solutes between blood and tissue what are the forces of osmotic pressure and pressure on capillary and interstitial fluid
(Pi)c is pressure pulling water into the capillary and (Pi)if is pressure pushing water out of the capillary
Pc is pressure pushing water out fo the capillary and Pif is pressure pulling water into the capillary
the accumulate of water in lower legs and feet when standing for long periods of time is known as
a type of venous blockage caused by liver cirrhosis is known as
decreased blood protein can lead to increased water flow into tissues...this is known as