What Powers Muscle Contraction

In order for muscle fibers to contract, a lot of ATP must be used (Figure 2.8). Some of the energy released from ATP is used to power the contraction. Interestingly, ATP is also necessary for a contracted muscle cell to "relax" as well. When the muscle is no longer being stimulated, ATP helps the thick and thin filaments to dissociate from each other so

Thick filaments ¡n center of a sarcomere

Thin filaments at the "bookends" of a sarcomere

Glucose

Mitochondria

Fat and glucose

Figure 2.8 Muscle cell contraction is powered by adenosine triphosphate (ATP).

The energy released by ATP allows myosin to pull actin filaments towards the center of the sarcomere. The net effect of all the sarcomere contraction is a shortening of the entire muscle cell. Carbohydrate and fat are mostly used to regenerate the ATP as it is being used.

Glucose

Mitochondria

Fat and glucose

Figure 2.8 Muscle cell contraction is powered by adenosine triphosphate (ATP).

The energy released by ATP allows myosin to pull actin filaments towards the center of the sarcomere. The net effect of all the sarcomere contraction is a shortening of the entire muscle cell. Carbohydrate and fat are mostly used to regenerate the ATP as it is being used.

that each sarcomere can return to a relaxed (or unstimulated) position. In addition, ATP is necessary to pump calcium out of intracellular fluid of the muscle fiber. Calcium is either pumped out of the cell or more likely into sarcoplasmic reticulum organelles.

If ATP is deficient, muscle fibers become locked in a contracted state called rigor. Rigor mortis occurs when the human body dies as the integrity of muscle cell membranes decrease. This allows calcium to leak into the contracting regions of muscle fibers from the extracellular fluid and from within the sarcoplasmic reticulum. As a result, calcium bathes myofibrils and contraction is invoked. Usually there is enough ATP in these dying cells to power the contraction. The dying cell then remains locked in a contracted state.

The Heart and Circulation Are a Delivery System What Is the Heart and Circulation?

Some ancient philosophers believed that the heart was the foundation of our soul. Today we recognize the heart for its true function, that of a muscular pump. The adult heart is about the size of its carrier's fist and weighs about one-half pound (Figure 2.9). It serves to pump blood through thousands of miles of blood vessels to all regions of our body. Blood leaves the heart through arteries on route to tissue throughout the

Pulmonary trunk

Right atrium

Right ventricle

Pulmonary trunk

Right atrium

Right ventricle

Aorta

Left atrium

Left ventricle

Muscular heart wall

Figure 2.9 The anatomy of our heart. There are four chambers (right and left atria and ventricles).

Aorta

Left atrium

Left ventricle

Muscular heart wall

Figure 2.9 The anatomy of our heart. There are four chambers (right and left atria and ventricles).

body. Arteries feed into smaller arterioles and subsequently tiny capillaries, which then thoroughly infiltrate tissue. Most blood vessel mileage is attributable to capillaries. These blood vessels are so numerous in tissue that nearly every cell in our body will have a capillary right next to it or very close. This is like having one river (artery) flowing into town that branches to the extent whereby every house has its own little stream (capillary).

Our heart is made up of muscle, nerves, and connective tissue and can beat more than two billion times during a lifetime.

As blood reaches the end of the capillaries and the tissue has been properly served, the blood will then drain into larger venules. The venules will eventually drain into larger veins, which ultimately return blood to the heart. This is like the streams draining into larger steams, which then drain back into the larger river. Quite simply, our blood serves as a delivery system. It delivers oxygen, nutrients, and other substances to cells throughout our body. At the same time, blood also serves to remove the waste products of cell metabolism, such as carbon dioxide and heat from our tissue. Capillaries are the actual sites of exchange of substances between our cells and the blood.

Our heart consists of four chambers (two atria and two ventricles), left and right. The left half, consisting of the left atrium and ventricle, serves to receive oxygen-rich blood returning from the lungs and to pump it to all the tissues throughout the body. The right half of the heart, consisting of the right atrium and ventricle, serves to receive oxygen-poor blood returning from tissue throughout our body and to pump it to the lungs. Therefore, our heart functions as a relay station for moving blood throughout our body in one large loop, hence the term circulation.

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