What are the exchange mechanisms of sodium & potassium into and out of the cell during cellular excitation? what is the role of water when sodium moves into and out of a cell? what is the role of calcium in this process as well?

The water and electrolyte balances in the human body are vital for survival. Through a series of activities, either facilitated by energy, the use of certain carrier proteins and gateways in cells, or simply by the size and charge of an ion, this is the way that living cells are able to perform their activities every single second.

In a normal tissue, a cell has an outer an inner environment, that, aside from water, which is the medium that sustains all tissues, also counts with certain molecules and ions that ensure activity and survival. Thus, the human body counts with: sodium, potassium, calcium, chloride, magnesium and phosphate. These all play a central role. However, none as important as sodium, potassium, calcium and water.

In order for a cell to perform certain activities, like for example, a mucle to move, there is the need for something called the action potential, and this is regulated by the influx and exit of sodium and potassium, as well as calcium. When a signal for depolarization arrives at a cell, like a muscle, then, the channels that are on a cell´s membrane will open up, allowing sodium to enter and potassium to leave (usually the are stable, Sodium outside on the extracellular environment, and potassium in the cytoplasm). The influx of positive sodium, changes the membranes electrical charge. Calcium, especially in muscle cells, will follow sodium, but mostly, it will be released from its storage inside the sarcoplasmic reticulum. While Sodium and potassium will ensure that the cell is electrically charged to fire, calcium will ensure that in a muscle cell, and in other types of cells, activities like muscle fiber contraction, and excitation, happens.

Water plays an important role as it is the medium on which both electrolytes and cells are suspended. It will always be equalized, but it tends to move to where there is a higher concentration of particles like sodium. Water does not play a role in action potential, but it will leak in and out of the cells during this process as the gateways open and close for its passage. It will later on help to take out unnecessary waste, and excess ion particles from the cell and into the bloodstream.

between the two muscle layers is the myenteric or auerbach's plexus, which controls peristalsis. in the colon, the muscularis externa is much thicker because the feces are large and heavy, requiring more force to push along. the stomach has a third layer of muscularis externa: the inner oblique layer.