The Vascular System

Define vaso and veno constriction.

• Vasoconstriction: Constriction of the arteries and arterioles.
• Venoconstriction: Constriction of the veins and venules.

Describe how elastic arteries maintain blood flow during diastole.

Just a quick reminder: elasticity is not a measure of how much something can be stretched- it's a measure of how well something can "snap back" to normal size after being stretched.

When blood is pumped into an elastic artery, the artery expands. When the artery recoils during diastole, the blood is squeezed along, maintaining blood flow and pressure. The artery is able to recoil because of its elastic tissue.

Define resistance and total peripheral resistance.
Describe how changes in cardiac output and total peripheral resistance alter mean arterial pressure.

This guy really loves definitions, doesn't he?

• Resistance- A measure of the opposition to blood flow. Directly proportional to the viscosity of the blood, directly proportional to vessel length and inversely proportional to vessel radius. (Radius is the most important factor: R ∝ (ηL)/r^4, where R = resistance, η (eta) = viscosity, L = length, r = radius. As r is raised to the power of 4, any small changes will affect resistance greatly.)
• Total peripheral resistance- The total resistance from all of the systemic peripheral vessels together. Mostly caused by arteriolar resistance.

Two important equations that you might want to know are:

Flow = ΔP/R, where ΔP is the difference in pressure between the beginning and end of a vessel, and R is the resistance. (You may also see "flow" written as a Q with a dot on top. Why Q? I don't know.)

Mean arterial pressure (MAP) = cardiac output * total resistance
(also written as MAP = CO*TPR)

The second is actually the first one rearranged- in this case ΔP is the mean arterial pressure. The average pressure of blood leaving the left ventricle is around the mean arterial pressure, and the blood re-entering the right atrium has a pressure of around 0mmHg, so overall ΔP = MAP. Resistance here is the total resistance, because if we're looking at the change in pressure around the whole systemic circuit, we need to look at resistance around the whole systemic circuit too. Flow is equal to the cardiac output because all of the blood pumped out into the body per minute also has to make it back into the heart at the same rate, or blood would accumulate somewhere.

An important point to make is that from this equation, you can see that an increase in cardiac output and/or total resistance will also increase mean arterial pressure. This also means that factors affecting cardiac output (stroke volume and heart rate) and total resistance (blood viscosity, vessel length, radius) will all affect mean arterial pressure.

Describe the role of arterioles in the regulation of total peripheral resistance.

As I mentioned before, arteries are pretty crucial when it comes to total peripheral resistance. This is because they have smooth muscle that can contract or relax to reduce or increase radius, which has a roll-on effect on resistance, as I mentioned earlier. Constriction of the arterioles is known as vasoconstriction, as also mentioned earlier.

Describe the actions of sympathetic tone on arterioles.

The sympathetic nervous system causes arteries and arterioles to constrict. This decreases the radius and thus increases resistance, decreasing blood flow (since Flow = ΔP/R, increasing R decreases flow). This means that arterioles can effectively control where the blood goes.

This may sound really unintuitive- the sympathetic nervous system is there for "fight or flight," but surely you wouldn't want blood flow to decrease when you're in danger, right? Not to worry: I'll cover this in a later post.

Define capacitance.

Capacitance is, to my understanding, the ability of blood vessels to store blood.

Describe the role of veins as capacitance vessels.

As mentioned in an earlier post, veins are often known as capacitance vessels because they can hold a lot of blood (at any one time, ~60% of your blood will be in your veins). The amount of blood that the veins can hold is regulated by venous constriction, or venoconstriction, which decreases the diameter. As veins have a very low resistance, venoconstriction has little effect on total peripheral resistance. (Contrast this with arterioles, which do have a large effect on total peripheral resistance when constricted, but have a relatively limited capacity for holding blood.)

Describe how the sympathetic tone alters venous capacitance and venous return.

Just like in the arteries and veins, sympathetic tone causes constriction of the veins. This decreases capacitance, causing more blood to move back to the heart (i.e. increasing venous return).

From here you should be able to see that as venous return increases, cardiac output increases (Frank-Starling law: "within physiological limits the heart pumps out all blood that it receives") which in turn increases mean arterial pressure (MAP = CO*TPR).

Describe the function of the venous valves.

Valves stop backflow of blood due to gravity, as mentioned previously. They close off to stop blood from flowing away from the heart. Contraction of skeletal muscle helps to squeeze the veins so that blood can flow forward. (This is why people are at risk of deep vein thrombosis, or DVT, on long flights: when sitting still for long periods of time, the skeletal muscles aren't contracting so blood isn't being pushed by the muscles towards the heart.)

Know typical values for venous pressure and pulmonary arterial pressure.

Venous pressure is typically very low- less than 10mmHg. Pulmonary arterial pressure is higher than venous pressure, but still much lower than systemic arterial pressure: 22/8 mmHg.