In this chapter... To investigate how the kidneys filter our blood, we will need to review most types of membrane transport. Since membrane transport is a crucial part of cellular biology, the review will help these concepts sink in.

The urinary system is part of what you might know as the “excretory system.” In short, it cleans up the internal mess that results from living life. Just like going about your daily life results in messes that create housework, our bodies need to “take out the trash” several times each day by making urine.

Why this is important…Our urinary system cleans our blood. It also helps us keep a proper water balance and pH level in our body. While malfunctions here are not as immediately life-threatening as problems with the heart or lungs, they are very serious and often require immediate medical attention.

Foundations

Our urinary system works with a few other systems (respiratory, digestive and integumentary) to get rid of metabolic wastes. This is not a large system, but it does a surprisingly large and complex job.

Diagram of human urinary system in female.

FIGURE 7-1. Urinary system anatomy. ("Urinary System (female)" by BruceBlaus is licensed under CC BY-SA 4.0)

Details

Kidneys are the workhorse of the urinary system. They don’t look complicated from the outside, but they have a complex internal structure that allows them to filter waste from the bloodstream. An amazing 1.2 liters of blood flow through the kidneys every minute. The wastes extracted from the blood are diverted into the urine, which flows into the urinary bladder and eventually out of the body.

The main job of the kidneys is to filter waste products from the bloodstream. These waste products are the byproducts of all the chemical reactions in the cells. While the lungs take care of the carbon dioxide, the kidneys take care of the nitrogen-rich byproducts that result from breaking down proteins. These quickly become toxic if they build up in the bloodstream.

Other important tasks performed by the kidneys include:

Regulate water balance by getting rid of excess water (this also regulates blood volume)
Regulate blood chemistry, especially pH and electrolyte levels
Excrete hormones and drugs from the bloodstream (often referred to as “clearing”)

Getting rid of nitrogenous wastes will be our focus in this chapter. When proteins are broken down into amino acids, these can be used to create more proteins or to power the cell. If they are used for energy, the amino group (NH₃) needs to be removed from the amino acid first. This becomes ammonia, which raises the pH of the blood and is generally toxic to the body. The liver comes to the rescue and neutralizes ammonia by combining it with carbon dioxide to produce a much safer chemical – urea. If you have been around dirty diapers (or dirty restrooms) you know that urea breaks down into ammonia again, creating very strong odors. Kidneys need to keep filtering out urea before this breakdown happens in the blood.

Learning objectives

Name and identify the major organs of the urinary system
Describe the basic functions of the urinary system