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The Cardiovascular System
LO1 - List and explain the factors that influence mean arterial pressure
(MAP), and describe how MAP is regulated
Blood flow
Volume of blood flowing through a vessel/organ/entire circulation in a given period - ml/min
Blood flow is equal to cardiac output (CO) in regard to the whole cardiovascular system
When resting CO is constant
Blood flow through each individual organ may very widely depending on their immediate
needs
Blood pressure
The force per unit area exerted on a vessel wall by the contained blood - mm Hg
(millimetres of mercury)
Means the systemic arterial blood pressure in the largest arteries in the heart unless stated
otherwise
Pressure gradient, the differences in blood pressure in the vascular system, provides the
driving force that keeps blood moving - flow from higher pressure to lower pressure
Resistance
Opposition to flow and is a measure of the amount of friction that blood encounters as it
passes through the vessels
Most friction is encountered in the peripheral - systemic - circulation, it is termed peripheral
resistance
Blood viscosity
• Internal resistance to flow that is in all fluids is viscosity
• Related to the thickness of a fluid
• Greater the viscosity, the harder it is for molecules to slide past one another and keep
the fluid moving
• Blood contains formed elements and plasma proteins, making it more viscous than water
• Blood viscosity is fairly constant, but a lot or too little red blood cells can change this
Total blood vessel length
• The longer the vessel the greater the resistance
• Child’s blood vessel length increases with age, as will peripheral resistance and blood
pressure
Blood vessel diameter
• Influence of these factors can be consistent in health people due to unchanging blood
viscosity and vessel length
• Changes in blood vessel diameter lead to a change in peripheral resistance
• Fluid along the wall of the blood vessel experiences more friction than that in the middle
of the blood vessel
• If a blood vessel is wider in diameter there is more blood going through the middle
encountering less friction and flowing faster
• If a blood vessel in thinner in diameter, more blood is in contact with the blood vessel
wall, hence encountering more friction and flowing slower
• Resistance varies inversely with the fourth port of the vessel radius 1/r4
• Because arteries close to the heart don't dramatically change in diameter they do not
contribute much to peripheral resistance
• The smaller-diameter arteriole which can enlarge or constrict in response to chemical
and neural controls, are the main determinants of peripheral resistance
• Laminar flow/streamlining is when in a tube of a given size the relative speed and
position of a fluid in the different regions of the vessel’s cross sectional remain constant
• When faced with vessel diameter change or rough/protruding areas of the tube wall,
smooth laminar blood flow is replaced by turbulent flow - irregular fluid motion where
blood from different laminae mixes
• Turbulence drastically increases resistance
Blood flow (F) is directly proportional to difference in blood pressure (∆P) between two
points in the circulation - the bigger the change in pressure, the faster the blood flows
Blood flow is inversely proportionate to the peripheral resistance (R) in systemic circulation
- is R increases, blood flow decreases
F=∆P/R
R is most important variable - it is easily changed by a change in blood vessel diameter
Systemic blood pressure
• Blood flows through the blood vessels on a pressure gradient - from higher to lower
• Systemic blood pressure is highest in the aorta and declines on the way around the body
until it reaches 0 mm Hg
• Steepest drop in blood flow occurs in the arterioles as they offer a resistance to blood
flow
• Arterial blood pressure
- Reflects how much the elastic arteries close to the heart can stretch - compliance or
distensibility, and the volume of blood forced into them at any time
- Blood pressure is pulsatile, it rises and falls in a regular fashion in the elastic arteries
near the heart, meaning the amount of blood entering and leaving the elastic arteries
is not equal
- As the left ventricle contracts is imparts kinetic energy into the blood, which then
allows it to stretch the elastic aortas aortic pressure reaches its peak
- Systolic pressure - the pressure peak generated by ventricular contraction, averages
120 mm Hg