Structure and Function of the Urinary Tract in Dogs

What Is the Urinary Tract?

The urinary tract is a system made up of the kidneys, ureters, urinary bladder and urethra. These organs work together to produce, transport, store and excrete urine. The urinary tract also rids the body of many fluid waste materials and products and has other vitally important functions, including controlling the volume and composition of the body fluids.

Where Is the Urinary Tract Located?

Dogs and cats have two kidneys, both located in the abdominal cavity under the backbone, close to where the last rib meets the spine. The ureters (one from each kidney) extend from the kidneys to the bladder. The bladder is located in the abdomen just in front of the pubic bone of the pelvis. The urethra, the tube that carries urine from the bladder to the outside, passes along the floor of the pelvis. It ends in the vestibule of the vagina in the female dog and at the tip of the penis in the male dog.

What Is the General Structure and Function of the Urinary Tract?

The kidneys are paired, bean-shaped organs. The indentation of the "bean" is called the hilus, which is the area where the blood vessels, nerves and ureters enter and leave the kidney.

The structural and functional unit of the kidney is the nephron. There are hundreds of these microscopic filtering units, and each has the ability to form urine by itself.
Each nephron consists of a circular, ball-shaped cluster of small blood vessels called a glomerulus, and a small tube called a renal tubule. Nephrons are responsible for removing urea, which is combined with water and other waste products to produce urine.

The urine is emptied into the pelvis of the kidney, a collecting chamber located in the middle of the kidney in the area of the hilus. Urine from the kidney passes into the ureters, which are very thin tubes that extend from the kidney to the urinary bladder. Muscles in the walls of the ureters cause the urine to travel towards the bladder where it enters in small spurts.

The bladder is a collapsible muscular organ shaped like a balloon found in the back portion of the abdomen. It provides a means for temporary urine storage. The bladder is held in place by ligaments that are attached to other organs and the pelvic bones. A circular muscle called a sphincter controls the outlet opening of the bladder, and keeps the urine from leaking out of the bladder. Once the bladder reaches a certain point of distention, nerves in the bladder wall notify the brain that it is time to empty the bladder.

When appropriate, the animal voluntarily relaxes the sphincter and allows the release of urine. The released urine enters the urethra, a tube that carries urine to the outside. The male urethra is long and thin, and terminates at the tip of the penis. The female urethra is much shorter and wider, and opens just in front of the opening of the vagina.

What Are the Functions of the Kidneys?

Although the kidneys work in tandem to perform many vital functions, dogs can live a normal healthy life with just one kidney. The kidneys and urinary tract regulate many important body functions.

The kidneys perform their most important functions by filtering and removing waste products of metabolism. The kidneys "clear" unwanted substances by excreting them in the urine while returning substances such as water and certain salts back into the blood stream. The kidneys monitor and maintain the balance of body water, ensuring that body tissues receive enough water to remain hydrated, to function properly and to remain healthy.

The kidneys help regulate the acid-base balance (the pH) of the blood and body fluids. Certain ions, such as hydrogen, are either retained or lost from the body to help maintain this balance.

By regulating salt levels through the production of an enzyme called renin (as well as other substances), the kidney ensures that blood pressure is regulated. The amount of sodium salt retained in the body greatly influences blood pressure.

The kidneys influence certain hormonal functions in the body. They secrete a hormone called erythropoietin, which stimulates the bone marrow to produce red blood cells. The kidneys also make a substance that converts vitamin D to its most active form, calcitriol.