The urinary system's function is a complex, vital mechanism for our pets' health. In dogs and cats—as in humans—the urinary tract includes the kidneys, ureters, bladder, and urethra. The kidneys are the key organs in this system, responsible for filtering the blood to remove waste and regulating fluid and electrolyte balance.
What Are the Different Parts of the Urinary System?
The Kidneys
The kidneys are the functional organs of the urinary system. They consist of various structures.
- The capsule is a fibrous membrane that surrounds each kidney, providing physical protection against damage. It also helps maintain the kidney’s shape and structure.
- The cortex is the kidney’s outer layer, where the glomeruli—part of the nephrons—and most of the renal tubules are located. This is where the blood filtration process begins.
- The medulla is the kidney’s inner region, made up of structures called renal pyramids. This is where most of the renal tubules are found, responsible for concentrating urine and eliminating waste.
- The calyces and the renal pelvis are cavities located at the center of each kidney, collecting urine produced by the renal tubules and transporting it to the ureters.
- The hilum is the area where blood vessels enter and exit the kidney. It is a key zone for vascularization and blood supply to the kidneys, essential for their functions.
The Ureters
The ureters connect each of the two kidneys to the bladder. Their role is to transport urine from the kidneys to the bladder.
The Bladder
The bladder is a sac made up of a mucous membrane—or urothelium—and muscles. It stores urine from the ureters until the next urination.
The Urethra
The urethra is the duct connecting the bladder to the urinary meatus. It allows urine to be expelled through the urinary meatus.
How Do the Kidneys Work?
These bean-shaped organs are located in the lumbar region. They perform several key functions: blood filtration and waste elimination, blood pressure regulation, hormone production, and electrolyte balance.
The functional unit of the kidney is the nephron. Each nephron begins with a glomerulus, a structure composed notably of blood vessels that allow fluids and small molecules to pass through while retaining larger elements such as cells or albumin. Then, as it travels through the tubule, the primitive urine becomes concentrated, and useful elements are actively reabsorbed—this is the case for glucose or certain minerals—while unwanted elements like waste products—urea—or useful elements present in excessive amounts in the blood—such as water or certain minerals—remain.
In simplified terms, the kidney acts as a recovery unit through "selective sorting." When the number of functional nephrons is sufficient, the body's internal balance is maintained.
