I'm assuming from the context of your questions that you are looking
for the purpose of chloride ion in the human body.
Chloride and sodium work together to maintain water balance in animal
(including human) cells. So, Cl- balances out Na+. This helps to
regulate not only how much water is inside vs. outside cells, but also
the acid/base balance of your body. Here are some websites:
KidsHealth for Parents: Electrolytes
"Chloride is an element in the blood and body fluid that functions
like sodium to maintain fluid balance. Table salt is composed of a
combination of sodium and chloride. Disporportionate loss of chloride
can lead to the body's environment becoming more acidic."
Medlineplus Medical Encyclopedia: Chloride in diet
"Chloride is essential in the maintenance of the body's acid-base and
fluid balance. It is an essential component of the gastric (digestive)
Increased intake of chloride in the form of salt can cause fluid
retention, but this is primarily because of the sodium in the salt,
not the chloride.
A deficiency of chloride can be caused by fluid loss as a result of
excessive sweating, vomiting, or diarrhea. Use of medications such as
diuretics can also cause a deficiency. This loss of chloride can
result in excessive alkalinity of the body fluids (alkalosis), low
fluid volume (dehydration), and loss of potassium in the urine."
UCSF Mission Bay: Spotlighting Science
"Cells also have channels that control the flow of the positively
charged ions sodium and calcium, and channels for negatively charged
chloride. Similar to the way that electric power companies convert
energy from one form to another before delivering it to our homes,
cells use chemical energy to pump ions across their membrane and
establish ion concentration gradients, and then allow ions to move
through specialized channels, making every cell negatively charged
compared to its surroundings, and thereby creating an electrical
potential that serves as an energy source well suited to performing
certain cellular tasks such as the generation of action potentials."
Just for fun, this time I searched MedlinePlus (www.medlineplus.gov)
for sites and information about chloride. I also did the following
search on Google: "chloride sodium pump human body site:.edu"
As always, please let me know if you need more information!