Cardiac muscle tissues such as the sinoatrial node are self exciting. This is because they do not rely on any outside resources to stimulate the heart. This is what gives the heart the pace making rhythm and keeps the heart beating.
Answer: There are are three types of muscles in the human body. The skeletal muscle which can be controlled voluntarily, the smooth muscles in the blood vessel walls and alimentary tract that cannot be controlled voluntarily but is controlled by the autonomic nervous system and the cardiac muscle that cannot be controlled voluntarily but is capable of generating electrical impulses on its own.
The repeated electrical impulses that makes the heart beat starts its journey at the Sino-Atrial Node, composed of nodal tissue which is altered cardiac muscle tissue capable of spontaneously generating electrical impulses, situated in the right atrial wall. (The heart has four chambers, a pair of atria and a pair of ventricles.) The electrical impulse generated gets conducted across the atrial wall to reach the Atrio-ventricular node in the inter-ventricular septum that separates the two sides of the heart. From the AV node conduction of the electrical impulse occurs across a system of the main bundle which branches off into left and right bundle branches and further branches carry the impulse across the whole ventricle. At a given time the atria and ventricles are in different phases of contraction and relaxation. It is the patterned flow of this electrical electrical impulse that makes the cardiac muscle contract in such a sequence that at the time that the atria contracts the ventricles are relaxed to receive blood from the atria and when the ventricle gets filled, it starts to contract and the valve between the atria and ventricle closes to prevent return and the blood gets pumped to the pulmonary circulation from the right ventricle and to the systemic general circulation from the left ventricle, while at this time, the atria is in a relaxed stage and receiving venous blood (The left and right side of the heart are normally sealed from one another).
This process of the cardiac cycle gets repeated 72 times per minute.
Answer: It depends where you are building in the world. Steel is ready available every where but has the diadvantage of rusting over time if water gets to it. This can be slowed by painting with an industrial coating. I have seen steel walls in the ground after 25 years and all is good. It is relatively easy to repair. Steel is cheaper than polymer but polymer is a whole lot lighter and easier to work with and does not corride at all. In the USA the polymer wall is a formed product and is very good. In Australia there is not much available in polymer but Albatross Pools in Melbourne do a great steel walled pool and have been for over 30 years.
I would go with what ever is available and work within your budget. Both systems are extremely easy pools to construct and easy and much cheaper to renovate when required in the future.
Answer: The toughness is due primary to the cell wall components, pectins, hemicelluloses and cellulose which change during maturation, storage and processing. There are many factors impacting the toughness including tissue conditions, pH, enzymes, and salt concentrations. Certainly the texture-affecting reactions of pectic materials such as glycosidic hydrolysis, beta-elimination type depolymerization, demethoxylation, and complex formation occurs
The crispness of a vegetable is due to the movement of water in the plant. There are a number of factors which impact this. It is critical for the texture.
Answer: There are several types of tissues. The muscle tissue is calledmyocardium. The inner lining is an epithelial tissue calledendocardium. The outer layer has a synovial membrane associatedwith it and is called pericardium, or epicardium. Then there arenerves that transmit impulses for it to beat with a regular rhythm,and also some conducting fibers called Purkinje fibers.