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Water
As we have mentioned, water is the main constituent of the human body.
Let’s take a closer look at water.

Physical Characteristics of Water
Water has several important physical characteristics that make it extremely useful to the body.

Firstly, it is a fluid. Therefore, it has the capacity to flow.

Secondly, it is often called the "universal solvent." This refers to its ability
to dissolve so many substances within itself. Thus, water is an excellent vehicle for the circulatory systems.

Thirdly, water is very useful in the temperature control mechanisms of the body.
This is due to its heat-carrying capacity and its tendency to remove large numbers
of calories during evaporation.


Water Sources to the Body
Water thirst and water satiation is controlled by special centers in the hypothalamus of the brain.

The human body obtains water in two primary ways:

Most items that humans eat or drink consist largely of water.

A second source of water is metabolic oxidation. Water is one of the main by-products as various food substances are oxidized within the individual cell. This water is referred to as metabolic water.

Water Losses from the Body
The human body loses water in the following ways:

Perspiration: Water is continuously lost from the body in the form of
perspiration or sweat. With high surrounding temperatures and/or vigorous exercise,
the sweat is obvious. This is called sensible perspiration. Otherwise, the sweat is
usually not obvious, and there is a low level of water loss. This is called insensible perspiration.

Respiration: The surfaces of the lungs must be moist to ensure the passage of gases to and from the blood. Air is moistened within the respiratory
passages and the alveoli of the lungs. Thus, moisture passes out of the body along
with the exhaled breath.

Urination: Water is also lost from the body during the excretion of urine. Urine
carries nitrogenous wastes of protein metabolism, dissolved in the water.

Vomiting and diarrhea. During vomiting and diarrhea, the body loses large quantities of water and dissolved electrolytes. In infants and the elderly, this loss of water and electrolytes can be very dangerous. Sometimes, even death may result.

Dissolved Substances in the Body
As mentioned before, one of the characteristics of water that makes it so
useful is its capacity to dissolve almost anything ("universal solvent").

In the human body water is used to carry:

Gases: Oxygen and carbon dioxide are exchanged between air in the lungs
and the blood. They are also exchanged between the blood and the individual cells of
the body. At least in part, these gases are carried as dissolved substances in the water of the blood.

Nutrients: Nutrients including vitamins and minerals are carried from the digestive system,to the rest of the body in the blood. As they are dissolved in the water of the blood, they can be distributed to the individual cells of the body.

Wastes: Wastes which result from the metabolic processes of the body are picked up from the individual cells by the blood. They are then delivered dissolved in
the water to the excretory organs of the body, such as the kidneys.

Hormones: Hormones are carried from the endocrine glands to specific target organs while dissolved in the water of the blood.


The Tissue Fluid Cycle
The extracellular fluid found among the cells is called the tissue fluid, or interstitial fluid.

Tissue fluid originates primarily with a fluid portion of the blood that escapes into the tissues from the capillaries. Part of this escaped fluid enters the beginning of the venous vessels.

However, a large percentage of the tissue fluid is picked up by another circulatory system, the lymphatic system. Thus, there is a continuous flow of these fluids throughout the body.

In addition, the intracellular fluid and the immediate extracellular fluid are continually being exchanged.