Muscle activity reduces amount of pooled blood in the legs , this reduces venous pressure

Systolic pressure is determined by the aortic compliance and cardiac output ( CO )

Diastolic pressure is affected , in part , by total peripheral resistance ( TPR )

$\text{Resistance}\propto \frac{1}{{\text{radius}}^4}$$\text{Resistance} \propto \frac{1}{\text{radius}^4}$

• therefore , small changes in vessel radius have enormous affect on resistance

Flow Trend = $Q=\frac{\mathrm{\Delta }P}{R}$$Q = \frac{\Delta P}{R}$

• therefore , higher pressure differences between aorta and veins increases the blood flow rate

Velocity = $\frac{Q}{A}$$\frac{Q}{A}$

• therefore , the larger the cross-sectional area , the smaller the velocity

$MAP=CO\ast TPR$$MAP = CO * TPR$

• TPR is mainly determined by the arterioles

• Increasing TPR increases blood pressure

Why does blood velocity increase in the increases as merge venous system?

• vessel diameter increases as venules merge into larger veins

  • this reduces recistance , and allows for faster flow back to the heart

Why does velocity decrease from arteries to capillaries ?

• the total cross-sectional area of the capillaries is much greater than arteries

• this slows down the flow

• allows for better nutrient and gas exchange

What happens to resistance ?

• it increases as blood moves from arteries to arterioles and to capillaries

  • because the vessel diameter keeps decreasing

• once in the veins , resistance starts to decrease

How does this relate to the Poiseuille equation and Ohm's law ?

• via Poiseuille : $\text{Resistance}\propto \frac{1}{{\text{radius}}^4}$$\text{Resistance} \propto \frac{1}{\text{radius}^4}$

  • smaller capillaries and arterioles have a higher resistance

• via Ohm's Law : pressure gradient and resistance determine flow

  • this explains why changing resistance is necessary to maintain efficient circulation

Capillaries have the highest cross-sectional area in the vasculature

• This is why even though an individual capillary will have a higher resistance than an individual arteriole ,

  • in TOTAL the highest resistance in the vasculature is at the level of the arterioles.

Vasodilators :

• prostacyclin

• nitric oxide

Vasoconstrictors :

• endothelium

• vasoconstrictors

Arterioles are the main regulator of flow

• major determinant of TPR