42 Urinary Levels of Organization
Learning Objectives
- Explain the main steps of the urinary system
- Explain the movement of fluid throughout the body.
The primary function of the urinary system is to maintain homeostasis of fluid and small molecules within the body. We will start with a molecular characterization of urine. Then we will follow the movement of material from the bloodstream, through the functional units of the kidney through the other organs of the urinary system, and out of the body.
Molecular Level – The Components of Urine
Our input of water varies greatly from day to day. Despite major differences in fluid intake, the total volume of fluid within the body remains the relatively constant. The body’s fluid homeostasis is achieved largely due to the kidneys’ ability to regulate how much water is excreted in urine. If a person with healthy kidneys drinks a large volume of fluids, the kidneys will produce a large volume of urine. When a person with healthy kidneys does not drink enough fluids or experiences significant fluid loss, the kidneys will produce a small amount of concentrated urine to conserve water.
Urine is composed of urea, chloride, sodium, potassium, creatinine and other compounds (ions, inorganic and organic). Urea is an organic breakdown product of nitrogenous materials.
| Characteristic | Normal Value/Nature |
|---|---|
| Color/transparency | Yellow/clear |
| Odor | Varies from slightly aromatic to ammonia-like; food and beverages can change the odor |
| pH | 4.5 to 8.0 (average: 6.0) |
| Specific gravity (the density of urine compared with the density of pure water) | 1.001 to 1.035 |
| Water content | 95 to 97 percent |
| Volume | 1 to 2 liters/ day (quarts/day) |
