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This page has three major sections, listed below. Use this list to jump to whichever section you want to go to, by clicking on the opening curtains!
Where is the heart?An image of the position of the heart within the thorax can be seen:
In this textbook picture, you can see how the heart lies between the lungs in a region called the mediastinum. You can also see that the heart is wrapped by some membranes that also hold the heart in its position relative to the diaphragm and lungs.
The PericardiumThe pericardium is the set of membranes around the heart. It is actually composed of three layers of membranes. As we continue this semester, you will see that the thoracic, abdominal, and pelvic organs are all surrounded by membranes, and that each organ's membrane is actually composed of two membranes. Every organ has a membrane that tightly enwraps it, and this membrane is called the visceral membrane. Every organ also has a membrane outside of the visceral one, that connects it to the body somehow, so that the organ isn't just sloshing around in your body cavity. This outer membrane is the parietal membrane. Well the pericardium has three membranes! The innermost is the visceral pericardium, the middle is the parietal pericardium, and the outer one is the extra one, called the fibrous pericardium. The inner two (visceral and parietal) are rather thin and delicate. The outer one, the fibrous pericardium, is tough. It is made out of fibrous connective tissue, so dense that it even appears white (almost like tendons and ligaments!). This outer tough membrane is the one that in the picture above from your book appears to make all the tough attachments, like to the diaphragm and to the major blood vessels emerging from the heart. The visceral pericardium lies tightly against the heart tissue
itself. There's a tiny space between the visceral pericardium and the parietal
pericardium. This space is called the pericardial cavity. The
parietal pericardium is directly attached to the fibrous pericardium, so it just seems
like the inner lining of the fibrous pericardium. The relationship among these
layers of pericardia is shown in this next figure from your book... by looking at the wall
of the heart in more detail. What is the space (the pericardial cavity) for? Well, the visceral and parietal pericardia are actually secretory membranes, secreting serous fluid. You will see that many membranes secrete serous fluid. Serous fluid simply acts like a lubricant. The heart has to move a lot as it contracts to pump blood around... so it will be easier for the heart to do all that movement in the absence of friction-- in other words, lubrication makes movement easier. And we certainly want to make everything as easy as possible for our hearts, right?!
Heart TissueThe heart is mainly composed of cardiac muscle. I know that you learned a bit about cardiac muscle before (in Unit 2 and again as we compared it with skeletal muscle in Unit 8, I think). You know some basics about cardiac muscle, but are missing out on many of the ideas you need to really understand how the heart works. Also, there is another membrane on the inner side of the cardiac muscle to learn about, called the endocardium. The endocardium is a very smooth membrane-- it has to be, because the heart has to move all that blood around with as little interference as possible. The thing about cardiac muscle is that although it is made up of individual cells, these cells are connected together in order to work together. You know how you learned about the intercalated disks of cardiac muscle? The intercalated disks are found at the borders between cells, remember? Well, the intercalated disks appear dark when cardiac tissue is stained because there is so much happening at those borders... there are so many proteins there. Let me explain. All the cardiac muscle cells have to stick together and communicate with one another if they are to contract in unison. So, at the borders where cells come together there are intercellular junctions holding them together and allowing them to communicate with one another. Intercellular junctions can be seen in Figure 3.8 of your textbook. There are different types. I think that the one that helps to hold the cardiac muscle cells together is the desmosome. But there is only one type that allows for cells to communicate with one another, and that type is the gap junction. You should now understand that the borders where one cardiac muscle cell abuts another are filled with the intercellular junction proteins to hold these cells together and allow for communication. That's why these borders can be made to stain so darkly and why they then are revealed as what we call "intercalated disks." The cardiac muscle cells are connected together through gap
junctions to allow them to be active at the same time. In fact, there are two sets
of connected muscle cells-- one for the atria and one for the ventricles. That way,
when one atrial cardiac muscle cell gets excited, that excitation spreads to all the
atrial cardiac muscle cells and both atria end up contracting. But not the
ventricles. The ventricles are a separate unit. But once one ventricular
cardiac muscle cell is excited, the excitation spreads to all the ventricular cardiac
muscle cells and both ventricles contract. A set of cardiac muscle cells that act
together is called a syncytium, and the plural form of this word (since
there are 2 of these) is There are some cells within the heart muscle tissue that do not function to contract, but instead function to rhythmically activate ther regular cardiac muscle cells. These cells are pacemaker cells, and we'll come back to those when we go over the cardiac conduction system. |
© 2006 STCC Foundation Press |
in the following picture from your book.
