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Kidneys Part I: Healthy Kidneys

One of the common age-related chronic diseases we see, especially in cats, is called chronic kidney disease (CKD). Kidneys can be acutely damaged by genetic conditions, infections, injury, cancers, exposure to a variety of toxins, or drugs; however, CKD is a slow, progressive loss of normal kidney tissue over time that usually does not have a specific inciting cause. Before delving into CKD, let’s first look at how normal kidneys function.

Healthy kidneys look like plump beans and are located in the abdomen just at or behind the last rib and up against the spine, one on the left and one on the right. Their main function is to make urine, which is carried to the bladder by muscular tubes called ureters, but what does that mean?

Each kidney is made of millions of tiny filtering units called nephrons. Each nephron is made of two parts: The glomerulus and the tubule. The glomerulus is a cluster of tiny blood vessels which filters water and molecules below a certain size out of the blood and passes this mixture onto the tubule. In healthy kidneys proteins and other larger molecules, as well as blood cells, stay in the blood vessel.

The tubule forms a loop which is surrounded by more tiny blood vessels. The glomerulus is fairly non-specific and many substances we actually need (electrolytes, minerals, nutrients, and water) are removed by this filtering process. The tubule’s job, then, is to selectively return these needed substances to the blood while sending wastes and some water on to the bladder in the form of urine via the ureter.

A very simplified diagram of a nephron.

The entire volume of blood is filtered through the kidneys several times per day. Most of the water and other needed substances are reclaimed in the tubules, but this depends on many factors such as dietary levels of various minerals, electrolytes, and how much water is drunk on any given day.

Because the kidneys are in charge of what stays and what goes, they are critical to maintaining normal levels of water, minerals (especially calcium and phosphorus), and electrolytes (especially potassium and sodium) in the body. Other organs and tissues (such as muscles, bone, and nerves) may not work properly if there is either too much or too little of these substances.

They kidneys also make hormones that help maintain normal blood pressure, tell the bone marrow when it needs to release more red blood cells, tell us when we feel thirsty, and maintain normal bone strength and structure. If they are not working properly, we can see complications like hypertension (high blood pressure), dehydration, or anemia (low numbers of red blood cells) that many of us don’t immediately associate with the kidneys.

The kidneys are complicated but this is the end of your simple primer about them. If something doesn’t make sense, please let me know!

Next time we’ll talk about what can go wrong with they kidneys and what we can do to manage pets with CKD. The good news is, while we can’t cure CKD we have options to manage it and its complications and many pets live good quality lives for a significant period of time once diagnosed.

My dog, Maisy, is living with CKD.

Karen Christopherson DVM CVA