Overview
Function:
- to serve the needs of the body tissue
- to transport
- nutrients to the body tissue
- waste products away
- hormones from one area of the body to another one
- to maintain an appropriate environment
- optimal function of the cells = survival
- blood flow rate is controlled as a function of the actual needs
- additional function (e.g.: the kidney):
- blood flow rate is >> of effective metabolic requirements
- excretory functions require large volumes to be filtered each minute
What are the mechanisms for controlling blood volume and flow?
Physical characteristics
Pulmonary circulation
Systemic circulation (greater/peripheral)
- circular closed loop
- made of several circuits in parallel
- DIFFERENT types of vessels are organised in SERIES
- SIMILAR types of vessels are found in PARALLEL


Arteries: pressure reservoir
- transport blood under high pressure
- strong vascular wall
- high blood velocity (fast route)
Arterioles: act as control conduits
- release blood to capillaries
- muscular wall (contraction/dilation regulates blood flow)
Capillaries: exchange fluids/metabolites/hormones/gas etc.
- at the tissue level between blood and the interstitial fluid
- thin wall
- capillary pores (semi-permeable)
Venules: collect blood from the capillaries
Veins: conduits of transport of blood back to the heart
- blood (volume) reservoir
- low pressure -> thin wall, large lumen, no elastic layer


Amount of blood volume
- Pulmonary circulation: 9%
- Heart: 7%
- Systemic circulation: 84%
- Arteries: 13%
- Arterioles and capillaries: 7%
- Veins, venules, and venous sinuses: 64%
Cross sectional area
- Arteries: 0.5
- Arterioles: 10
- Capillaries: 59
- Venules: 29
- Veins: 1,5
Blood velocity
Principle of conservation of mass
F = v x A —> v = F/A
Arteries: 33cm/sec, Capillaries: 0.33 mm/sec (1/1000)
- While each branching originates smaller vessels, the sum of cross-sectional area progressively increases
- Velocity of blood flow depends on the total cross-sectional area
Regulation mechanisms
Three basic principles
- Blood flow controlled according to the tissue need
- Rest vs. Exercise: tissues may need up to 20-30 fold increase of blood flow
- however CO can increase up to 4-7 folds
- Microvessels act directly on the local blood flow (dilation/contraction)
- Nervous system plays a role through nervous control
- CO = Sum of all the local flows —> the heart pumps what it “receives” ..however CO can increase up to 4-7 folds
- Arterial blood pressure is independent of local blood flow and/or CO
- if arterial pressure falls down nervous reflexes (within sec)
- a) ++heart force pumping
- b) ++veins contraction -> ++blood volume back to the heart
- c) ++arterioles contraction -> ++blood storage in the large arteries -> ++arterial pressure
The Ohm’s Law
- Pressure, Flow, and Resistance
- Flow depends on
- Pressure difference
- Impediment to flow (vascular resistance)
- F = (Pin – Pout)/R = ΔP/R
- Note: pressure gradient is not absolute pressure


- blood flows according to gradient
- pressure decreases because of vascular resistance
- shear stress: friction of blood elements against the vascular wall
- main pressure drop occurs in arterioles
- progressive switch from pulsatile to steady state (compliace)
- P ≈ 0 in the vena cava and right atrium
- depicted is average pressure
- see P drop in the pulmonary circuit


What is pressure?
- blood pressure is the force exerted by the blood against unit of area of the vessel wall
- P = F/A
- P = 50 mmHg —> the force needed to push a column of Hg against gravity up to a level of 50 mm
What is resistance?
- is the impediment to blood flow in a vessel
- Remember: blood flows according to P gradient, P decreases because of vascular R, shear stress is the friction of blood elements against the vascular wall
- If ++R —> –ΔP —> –F
- In systemic circulation: CO ≈ 100 mL/sec and ΔP from the arteries to the veins is 100mmHg
- R = ΔP/F —> 100mmHg / 100 mL/sec —> R = 1 PRU
- Vasconstriction —> ++R = 4 PRU (max)
- Dilation —> – R = 0.2 PRU (min)
- In Pulmonary circulation CO ≈ 100 mL/sec and ΔP from the pulmonary artery to the left atrium is 16-2 =14mmHg
- R = ΔP/F —> 14mmHg / 100 mL/sec —> R = 0.14 PRU
The Poiseuille’s Law
F = (Pin – Pout)/R = ΔP/R = (πΔP x r^4)/(8ηL)
R = 8ηL/πr^4
- depends on geometry (L, r) and viscosity (η)
- increases linearly with L and η
- decreases with 4th power of r


Flow characteristics
- Laminar
- Parabolic velocity profile
- friction progressively decreases from the vascular wall to the center of the vessel
- the fluid touching the wall moves slowly because of friction but the next layer slips over the previous one and so on
- Turbolent
- Disordered velocity profile
- the blood flows along the vessel and crosswise in the vessel (spirals/eddy currents)
- obstruction (sudden ++velocity), sharp turn (directional change, bifurcation), sudden change in vessel diameter, pulsatility
- Reynolds number —> Re = (v x D x ρ)/η
- 200<Re —> Laminar (small vessels (arterioles, capillaries))
- 200<Re<400 —> transitional
- Re>400 —> turbulent (large vessels (large arteries: Re >10^3)
Autoregulation
- how pressure influences resistance and flow) ex: arterial vasoconstriction
- Vasconstriction
- —> ++P —> ++F
- —> ++R —> –F
- Vascular DISTENSIBILITY allows storing large amount of blood (that can be called into use when necessary)
- Veins are much more distensible than arteries (8x)
- Compliance = Increase in Vol / Increase in P
- i.e., ability of a vessel to expand when the internal pressure varies
- Immediate elastic distention is followed by stress-relaxation Effect of smooth muscle cells
- Attenuation of pulsatility
- it is an effect of vascular compliance of the arterial tree that progressively attenuates pressure pulsations
- the greater the compliance the smaller the velocity of propagation

