3.3 Urinary System

Overview

The urinary system is responsible for cleansing the blood and removing wastes from the body. It also plays a role in regulating pH, blood pressure, and the concentration of electrolytes in the blood. Controlled by the nervous system, the urinary system also stores urine until a convenient time for disposal, then provides the structures for transporting liquid waste from the body, thus maintaining homeostasis.

The Path of Urine

Blood from the heart enters each kidney via the renal artery. The blood is then filtered by the nephrons (the functional units of the kidney), and waste products are removed which forms urine. The filtered blood then leaves the kidneys via the renal veins. The urine is drained from the kidneys into the ureters, which transport the urine to the urinary bladder (Fowler et al., 2013). This is where urine is stored until it is excreted from the body through the urethra.

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Structures of the Urinary System

Kidneys: Paired organs in the abdomen that are attached to the dorsal wall on either side of the spine. Kidneys are imperative in maintaining homeostasis and contain nephrons that filter the blood. Mammals have two kidneys, but they are shaped differently in different species. 

 

Urinary bladder: A hollow, muscular organ that collects urine from both ureters and holds it until it is released from the body via the urethra. The size of the bladder depends on the amount of urine in it. 

Urethra: A tube that transports urine from the bladder to outside the body.  

 

Common Pathological Conditions of the Urinary System

Cystitis: Inflammation of the urinary bladder

Feline lower urinary tract disease (FLUTD): A common disease in cats; any disorder affecting the bladder or urethra of cats

Urethral obstruction: A condition in which the urethra is blocked, which prevents urination. This is most common in male cats (and male goats) and is also referred to as “blocked cat” in companion animal practice. It is a condition requiring emergency treatment.

 

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Kidney failure (Renal failure): Inability of the kidneys to function that occurs when the kidneys lose all, or some, of their ability to produce urine and filter the blood. This can present as an increase or decrease in urination.

Clinical Insight

There is both chronic renal failure (chronic kidney disease) and acute renal failure (acute kidney injury). Generally, the acute condition has a faster onset, whereas the chronic condition develops more slowly over time. An example of acute renal failure would be when a cat eats lilies—these flowers are toxic to cats and very dangerous to their kidneys! 

Inappropriate urination: Eliminating urine at the wrong time or in the wrong place (e.g., in cats, urinating outside the litterbox)

Incontinence: The loss of the ability to control the release of urine from the bladder

Urolith: A stone in the urinary tract; also called a calculus

 

Example

Urinary stones can be a life-threatening condition if left untreated. When taking a history from the client, they may indicate that their animal is urinating blood (hematuria) and/or straining to urinate (dysuria). Figure 3.7 below shows urinary stones (uroliths or calculi) removed from a dog.

 

Urinary tract infection (UTI): An infection anywhere in the urinary tract

Clinical Insight

Just like female humans, female dogs are more likely to get UTIs than their male counterparts.

Common Procedures

Urinalysis

A urinalysis can be performed in a clinic, or a urine sample can be sent to an off-site lab for analysis. The first step of the analysis is the gross examination, in which the urine is examined by the naked eye for observation of the colour, turbidity (transparency), and odour. The second step is a chemical analysis in which drops of urine are placed on a reagent strip (Figure 3.6) to observe the reaction of the substances in the urine. Some of the chemical reactions determine pH, glucose, ketones, and bilirubin levels in the urine. To measure how concentrated the urine is, a drop is placed on a refractometer to check the urine’s specific gravity, which tells us how concentrated or dilute the urine is—an indicator of how well the kidneys are filtering. The last step is a microscopic examination in which the urine is viewed under a microscope. In this step, the urine is viewed at a cellular level and the presence of blood, bacteria, and crystals are observed (Burton & Lalande, 2021). For more laboratory details, see 5.4. Laboratory Terminology.

 

Urinary catheterization: Placing a catheter (a hollow tube) inside the urethra to collect urine

References

Burton, E., & Lalande, A. (2021). Clinical veterinary diagnostic laboratory. University of Minnesota. https://pressbooks.umn.edu/cvdl, licensed under CC BY-NC 4.0

Fowler, S., Roush R., Wise, J., Reeves, N., DeSaix, J., Kuehner, B., Leady, B., Boggs, L., Broverman, S., Byres, D., Marcus, B., Mhlanga, F., Mignone, M., Nash, E., Newton, M., Oliveras, D., Piperberg, J., Reisenauer, A., Rumfelt, L., Belk, M. … Zoubina, E. (2013). Concepts of biology. OpenStax. https://openstax.org/details/books/concepts-biology, licensed under CC BY 4.0

Image Credits
(images are listed in order of appearance)

Keogh, L., Kilroy, D., & Bhattacharjee, S. (2020). The struggle to equilibrate outer and inner milieus: Renal evolution revisited. Annals of Anatomy – Anatomischer Anzeiger, 233, 151610. http://dx.doi.org/10.1016/j.aanat.2020.151610, licensed under CC BY 4.0

Dog Bladder (26990477411) by National Human Genome Research Institute (NHGRI), CC BY 2.0

Urinary bladder stones by mirkosajkov, Pixabay licence

Special urine test during urine analysis by Ajay Kumar Chaurasiya, CC BY-SA 4.0