Lesson 6: The Circulatory System

Blood Vessels

If you could join all the blood vessels in your body in a straight line, it would be about 100,000km long!

Our blood vessels are not one long tube but a complex network of tubes that branch and rebranch.

Arteries & Arterioles: blood vessels that carries

blood away from the heart and towards the body tissues

walls of the arteries have three layers of tissue – connective tissues, smooth muscle and endothelium

when the ventricles of the heart contract to pump blood around the body, the arteries expand slightly to accommodate the increased pressure of the blood within them. (The outer layer of the arteries include elastin fibres giving the vessel elasticity.)

Arteries & Arterioles:

when the ventricle relax, the walls of the arteries return to their original size, pushing the blood farther into the downstream vessels

when the artery expands from contractions of the heart, it is felt as a pulse

when arteries branch and rebranch, the smallest artery is called the arterioles

Controlling Blood Flow in Arterioles:

Because arterioles have smooth muscle, they can be controlled by the nervous system.

Signals from the nerves can regulate the diameter of the arterioles and control blood flow to certain parts of the body.

Vasodilation: an increase in the diameter (dilation) of arterioles that increases the blood flow to tissues. This can be used by the body as a cooling strategy: warm blood close to the skin loses thermal energy to the surrounding environment. Eg (minoxidil, nitroglycerin, asperin,  sildenafil, alcohol

Vasoconstriction: a decrease in the diameter of arterioles that decreases the blood flow to tissues. This can be used if the body is cold. Restricting blood flow to the skin prevents thermal energy loss to the environment. Also, vasoconstriction ensures that the 5L of blood in your body is distributed where it is needed. (eg. Epinephrine, Pseudoephedrine, Caffeine, Nicotine)

Capillaries: arteries branch into

arterioles and when those reach the tissue of the body, they branch further into smaller blood vessels called capillaries

capillaries form networks of blood vessels that supply oxygen and nutrients to every cell throughout the body tissues

capillary walls are only a single cell layer thick which allows easy diffusion of oxygen and nutrients into the tissues as well as carbon dioxide and other waste products to diffuse from the tissue back into the capillary

There is no smooth muscle in the walls of the capillaries so the diameter of the capillaries cannot be controlled by the nervous system. Instead, pre-capillary sphincters contract and relax to regulate blood flow. When the sphincters are relaxed, blood flow to a tissue is increased.

Veins & Venules:

capillary networks have an arteriole side and an venule side; (more pressure on the arterial side)

venules and veins carry deoxygenated blood containing carbon dioxide and other waste products; blood is returned to the heart

smooth muscles in veins is not as thick as in arteries and the walls are not as elastic; this makes the diameter of veins greater than arteries which reduces the pressure

blood pressure in veins is lower than blood pressure in arteries; how does blood return to the heart? (muscles and the valves)

Questions

1. Compare and contrast Arteries and Veins

2. Describe the differences between vasodilators and vasocontrictors and give an example of each.

3. If you sustained a near mortal wound while being attacked by zombies, name a drug you would reach for.