There are a couple major reasons why we have a circulatory system. One is to get the nutrients from the area of the small intestine and the oxygen from the lungs to all the cells in the body. Our cells also need a way to get rid of their wastes. As we look closer at the human circulatory system, you'll see that there are some other tasks accomplished with the help of our "closed circulatory system" (also known as the Cardiovascular System).

There are three types of vessels in a closed circulatory system.

1. Arteries carry blood away from the heart toward the cells and tissues of the body.
2. Veins return the blood to the heart.
3. Capillaries connect the arteries and veins. They are tiny and pass by all the cells.
The Mammalian Cardiovascular System. Let's take a trip down the blood stream. We'll start in the right ventricle of the heart. (See 23.4a in your text)

Ventricular contractions pump us toward the lungs via the pulmonary artery. There's low oxygen in the blood here. We arrive at the lungs where we pass through the tiny capillaries. The blood picks up oxygen, dumps carbon dioxide.

We float on down toward the heart again, this time in the pulmonary vein. We enter the left atrium of the heart and get pumped into the left ventricle. The ventricular contracts and sends us out the aorta. We have a choice now, we can tour the body where ever we want. We choose the ascending aorta and go up to the upper part of the body.

We squeeze through some capillary beds and eventually make our way to the superior vena cava, a large vein that collects blood from the upper body and returns it to the right atrium. As soon as we hit the right atrium we get pumped into the right ventricle. Things are starting to look familiar...

Blood Vessel Structure

Arteries and veins can be viewed as a series of concentric tubes. (See 23.5) Arteries have an outer tube of connective tissue for structural support. The next layer is smooth muscle tissue. These smooth muscle tissue layers allow us to change the diameter of the arteries. If we constrict all of the arteries, the total space for our blood is lessened, and the blood pressure rises. This is how we make short term adjustments to blood pressure (e.g. when we go from a sitting position to a standing position). It is also possible to close some arteries down (or open them) independently of others. This is how the smooth muscle cells that surround the blood vessels regulate blood flow to the various parts of the body. At any given moment, only about 5-10% of the capillaries in the body actually have blood flowing in them. The smooth muscles of the blood vessels may close the entire bed or just the branches, allowing blood to pass right through. (23.11) Why would we want to shut down some areas and open other? I explained a few examples in class. The innermost layer of an artery is epithelial tissue. It's smooth and keeps the blood cells from being damaged.

Capillaries don't have thick outer layers, just epithelium (that "basement membrane" mentioned in your text is produced, in part, by the epithelial cells of the capillary. You don't need to worry about that.) Some larger capillaries may have connective tissue or smooth muscle tissue. It is important that the wall of the capillary be thin, because we want things to diffuse into and out of capillaries.

Veins have a connective tissue outer layer and an epithelial tissue inner layer, but not much in the way of muscle (fig 23.5 makes it look like there's a lot of muscle there but there's not). After the blood has been pumped all the way out to the capillary beds of the body it may have trouble making it back up to the heart. All the friction caused by the blood squishing through the tinier and tinier vessels really takes the force out of the blood. That's why veins have valves in them. (23.9) The skeletal muscles of the body squeeze the veins whenever they bulge during muscular contractions. This squeezing, combined with the one-way action of the valves, helps return the blood to the heart.

What is Blood?

There's plasma, a liquid that makes up 55% of the blood, and there's cellular components.

PLASMA: Water, salts, all kinds of stuff that is transported by the blood like glucose and other nutrients, hormones, waste products, clotting agents, oxygen and carbon dioxide. Not much gas transport is done by the plasma - that's the territory of the RBCs. Hey, what about these clotting factors? They circulate throughout your body, ready to be activated and participate in the process of keeping you from bleeding to death if you get damaged. They need to be everywhere because you never know where damage to the blood vessels might occur. (One more function of the circulatory system)


Erythrocytes - These are the Red Blood Cells and they are for oxygen and carbon dioxide transport. Hemoglobin is a protein found in the erythrocytes that binds to oxygen. Erythrocytes are the most numerous of the blood cells, with about 25 trillion in the body.

Leukocytes - White Blood Cells for defense and immunity. There are a few types. Some leukocytes release chemicals to fight foreign organisms and some phagocytize ("eat") foreign cells and bacteria. We'll take a closer look at these when we cover defense and immunity. For now, at least recognize that here we have another function of the circulatory system - to provide access to the body for these defensive cells and the chemicals they produce.

 Platelets - They are fragments of cells used to help you stop bleeding.

It is important to note that the white blood cells that defend the body against infections and cancers and the red blood cells which house the hemoglobin and transport gasses and the platelets all arise from a single cell type called the stem cell. Stem cells differentiate into these other types of cells. Sometimes, if a person has something wrong with his/her blood, as in the case with leukemia (the cells that make white blood cells are cancerous), a marrow transplant can be done. Bone marrow is where the blood cells originate, so getting new bone marrow means getting new cells that produce blood cells. It means also getting a lot of cells that will grow up to become immune cells and recognize the recipient's body as foreign. The recipient of this type of treatment has to take immune system suppressing drugs, leaving the patient open to infections. What would be great is to get a bunch of stem cells and none of the other stuff in the marrow. They're working on it.

Hemostasis, is the stoppage of bleeding. It involves factors in plasma, substances released by platelets, and substances released by damaged tissue cells. There are three basic mechanisms to reduce blood loss: vascular spasm, platelet plug formation, and clotting, or coagulation.

A Vascular spasm - constriction of the damaged blood vessel.

1 works for a few minutes to a few hours
2 triggered by:
a) damage to smooth muscle
b) chemicals released by cells lining blood vessel
c) chemicals released by platelets
B Platelet Plug Formation
1 1st step is platelet adhesion - they stick to the area near the damage.
2 Platelets become activated
a) grow projections
b) activate nearby platelets, enhance vascular spasm.
3 Platelet aggregation. The platelets become sticky.
C Coagulation - Blood goes from a liquid to a gel. Clotting too easily leads to something called thrombosis, which is clotting in an unbroken vessel. Too slow to clot may lead to hemorrhage. Clotting involves about a dozen clotting factors including Ca++, inactive enzymes from the liver, stuff from platelets, and stuff from damaged tissue.
Clotting involves the conversion of Prothrombin to Thrombin. Prothrombin is one of those "factors", an inactive enzyme secreted by the liver. (23.16)  After that, thrombin converts fibrinogen to fibrin threads. Fibrinogen is a soluble plasma protein made by the liver, the fibrin threads that it turns into are insoluble. The threads glue platelets together and trap other formed elements.  Thrombin also activates factors that further stabilize the clot.  Eventually agents are released to dissolve the clot.

The Heart

The heart pumps blood by rhythmic contraction and relaxation (and the valves). The heart is made up mostly of contractile cells (muscle cells) but there are some cells called pacemaker cells which generate signals similar to those made by nerves. It is these signals that maintain the heart's rhythm and rate. (If you remove a heart from a living human it still beats). You've heard of "pacemakers". These man-made devices that are implanted into a person deliver an electrical signal to trigger the contractions. Obviously, the brain has some input as well (by means of nerves). This enables us to respond to our environment.

Heart attacks

Think about heart muscle. Its cells are like all the rest of the cells in your body in that they use ATP to do work. Heart muscle cells do a lot of work - they use up ATP rapidly. What is the cell's source of ATP? Cellular respiration. O2 + glucose (food) ------> CO2 + ATP

And how do O2 and glucose (and other nutrients) get to the heart muscle cells? By way of the blood vessels that serve them. We call these particular vessels the coronary arteries (23.8). A blockage in one of the coronary arteries will lead to a lack of O2 in the cells served "downstream" of the blockage and those cells will die. Dead heart muscle is a bad thing. What causes the blockages? The arteries can be gradually closed by a buildup of lipids along the inner lining (atherosclerosis) or clots may break free and block an artery. A "coronary bypass" is just what it sounds like. Blood vessels (from the leg usually) are sewn in to bypass the blocked area.

This is all we're going to cover for the circulatory system.