What is the structure and function of the organs of the urinary system?

Written by Cliantha Marielle S. Asonan

Reviewed by Dr. Reuben J C. los Baños, Ph.D.

What is the structure and function of the organs of the urinary system? The urinary system is an amazing part of the body. It includes bean-shaped kidneys, tubes called ureters and urethra, and the shapeshifting bladder. Each of these organs works together to filter blood and produce urine.

Have you ever wondered how the water you drink leaves your body? How about the way the body regulates blood pressure? And has it ever come across your mind what stimulates the creation of blood in the body?

What if I told you that all the functions mentioned are the work of the urinary system? That’s right – the urinary system does more than its name suggests. It does more than produce urine, and this article will tell you more about it.

As mentioned, the urinary system includes four different organs. These are the kidneys, the ureters, the bladder, and the urethra. They work in constant unison to perform many roles in the body.

The first organ of the bunch is the kidney, and have two of these bean-shaped structures in your body. As the most complex in structure, the kidneys are the main actors in filtering blood and making urine. Knowing its structures will help you understand the production of urine later on.

Here are some functions of the kidneys:

1. The kidney regulates the salt, potassium, and acid content of the body (National Kidney Foundation, n.d.).
2. They also release the hormone called renin. Renin is an important hormone in regulating blood pressure (Mescher, 2021).
3. It controls the production of red blood cells. It does this by secreting erythropoietin, another hormone (National Kidney Foundation, n.d.).

• It produces the active form of Vitamin D called calcitriol. This helps promote the health of the bones in the body (National Kidney Foundation, n.d.).
• It removes drugs from the body along with other bioactive substances (Mescher, 2021).
• It balances the amount of fluids in the body (National Kidney Foundation, n.d.).

Moving along, the next organ in the urinary system is the ureter. The ureter is tubular in structure. It channels urine from the kidney to the bladder (Mescher, 2021).

Each kidney has a ureter attached to it. That means that you have two ureters in your body. Each ureter connects to the bladder, which is the next organ in the system.

The urinary bladder is where the urine from the kidney goes and awaits excretion. It does something amazing – it changes from triangular to oval as it collects urine. A latter section of this article will reveal how the bladder is able to do this.

Finally, at the bottom part of the bladder is the urethra. It is a single tube that is longer in males than in females. The urethra is where the urine flows from the bladder to the outside of the body.

What organ of the urinary system produces urine?

Our blood carries waste products that the body needs to remove in the form of urine.

Inside the urinary system, an organ acts like a filtering factory. The primary organ of the urinary system that produces urine is the kidney.

The kidney allows blood to enter and exit while filtering its contents. It does this through a series of tubes of varying sizes. The blood comes from the renal artery and exits through the renal vein.

The following is the process of making urine within the kidney according to Mescher (2021):

1. Blood enters the kidney through the renal artery

• The renal artery branches into smaller arteries – segmental, interlobar, arcuate, and interlobular. Blood travels through these vessels until they reach the nephron.

• In the nephron, blood pressure forces fluids and waste into the glomerulus.

• The remaining blood exits the glomerulus. The filtrate flows through tubules in the nephron.
• The tubules reabsorb water and needed substances back into the bloodstream.

• Cleansed blood and reabsorbed materials flow through a system of veins. They pass through the interlobular veins to the arcuate, and interlobar, renal veins.

• Waste products left behind in the tubules become urine. It passes through the collecting ducts through the renal pyramids. Urine then passes through the calyces and to the ureter.

What types of tissue can you find in the urinary system?

Despite having only four organs, the urinary system contains all four major tissues. The presence of these ensures the proper functionality of the system as a whole. The urinary system contains different types of muscular, epithelial, and connective tissues.

Muscle Tissue

Smooth muscles compose the muscularis layers of the ureters, bladder, and urethra. At the middle part of the urethra, you can find a sphincter composed of striated muscle. This is the part that you can control when you want to either urinate or hold your pee.

Connective Tissue

Loose areolar connective tissue surrounds the blood vessels and nerves. Dense connective tissue composes the capsule and the lamina propria of the bladder.

Adipose tissue is also present in the perirenal fat that cushions the kidney (Pirie, 2023).

Nervous Tissue

Many nerves provide signals from the nervous system to the urinary system. Some are the thoracolumbar splanchnic nerve, the vagus nerve, and the intermesenteric plexus. In the ureter, one can also find many nerve plexuses that innervate it.

The urinary system contains a wide variety of epithelial tissue. You can read about this in the following section.

What type of epithelium lines the urinary system?

The urinary system contains various epithelial tissues. It has simple cuboidal, columnar, transitional, and simple squamous epithelia.

• It has simple cuboidal epithelia, which are present in the kidney tubules. You can also find it in the collecting ducts.
• Principal cells of the collecting system can range from cuboidal to columnar epithelia.
• The urinary system also has transitional epithelia present in the bladder and ureters.
• Simple squamous epithelia line the thin descending limb of the loop of Henle.

• Pseudostratified epithelia line the membranous urethra.

• Among males, stratified columnar epithelia line the spongy urethra.

• Nonkeratinized squamous epithelium line the end of the urethra among females.

The variety of these epithelia found in different parts of the urinary system is crucial. The structure of these epithelia reflects their function in the system.

What is the histology of the kidney?

Given its many parts, you could expect that there are many types of tissues present in the kidney. These tissues serve different roles that all contribute to the function of kidneys. The kidney contains connective, epithelial, muscular, and nervous tissues.

From the outside view of the kidney, you can find two structures. The first is the capsule surrounding the kidney made of fibrous connective tissue. The other is the hilum, where the nerves, blood and lymph vessels, and ureters enter (Mescher, 2021).

Inside the kidney, you can find even more complex structures that all take part in making urine. The one end of the ureter that enters the kidney extends as the renal pelvis. The pelvis divides into major calyces that branch into minor calyces (Mescher, 2021).

The kidney also has the renal cortex and the renal medulla. The renal cortex is the outer part of the inner structure and has many corpuscles and tubules. The medulla contains renal pyramids whose bases meet at the cortex (Mescher, 2021).

The end of each pyramid opposite the cortex called the renal papilla connects to a minor calyx. Between the renal pyramids are extensions of the cortex – renal columns (Mescher, 2021).

The functional units of the kidney are the nephrons. Each nephron has a renal corpuscle and a system of tubes composed of three parts. These are the proximal and distal convoluted tubules and the loop of Henle (Mescher, 2021).

Crumbie (2023) says both the sympathetic and parasympathetic nervous systems innervate the kidneys. The thoracolumbar splanchnic nerve provides vasomotor supply. The kidney also has fibers of the vagus nerve and the intermesenteric plexus.

What is the histology of the ureter and bladder?

The ureter and the bladder have similar histologic structures. The ureter consists of three layers: the mucosa, muscularis, and adventitia. Meanwhile, the bladder has four layers: the mucosa, submucosa, muscularis, and adventitia.

The mucosa of both the ureter and bladder consists of the urothelium. The urothelium is a transitional epithelium, and these have a special ability. They can become thick or squamous cells depending on the conditions of these organs. According to Mescher (2021), the urothelium has three layers:

1. Basal cells forming a single layer on top of the basement membrane
2. A middle region of at least one layer of cuboidal or low columnar cells

• An apical layer of umbrella cells, which are bulbous or elliptical. They specialize in protecting underlying cells from the hypertonicity of urine. They are most developed in the urinary bladder.

An adventitial layer covers both the ureter and the bladder. An exception to this would be the upper part of the bladder. Instead of adventitia, a covering of serous peritoneum lines this area (Mescher, 2021).

In the ureter, you can find a thick muscularis. These function to move the urine from the kidney to the bladder through peristalsis. The muscularis also makes folds in the mucosa when the lumen of the ureter is empty.

Meanwhile, the urothelium of the urinary bladder varies in thickness (Ferng, 2023). The thickness depends on the amount of urine stored in the bladder. Its thickness when the bladder is full is half the thickness when the bladder is empty.

A lamina propria and an underlying submucosa surround the urothelium. The submucosa consists of dense irregular connective tissue. Within these layers, you can find many blood vessels.

Underneath the submucosa is a dense layer of muscle followed by an adventitia. The muscularis consists of the detrusor muscle, an interwoven layer of smooth muscles. They contract to empty urine from the bladder (Mescher, 2021).

What is the histology of the loop of Henle?

The loop of Henle consists of a thin descending limb and a thick ascending limb. Together, they form a U-shaped structure. The loop of Henle contains simple squamous as well as simple cuboidal epithelium.

The histology of the thin limb consists of simple squamous epithelia. It also has few mitochondria and organelles (Mescher, 2021). It functions in the passive reabsorption of sodium and chloride ions.

Meanwhile, the thick ascending limb consists of simple cuboidal epithelium. It contains no microvilli and has many mitochondria (Mescher, 2021). Its function is the active reabsorption of various electrolytes.

The thin limb sits in the medulla while the thick limb sits in the medulla and the medullary rays (Mescher, 2021).

What is the histology of the glomerulus?

The glomerulus is a bed of capillaries. It connects to an afferent arteriole at one end and an efferent arteriole at the other. These capillaries consist of fenestrated endothelial cells.

The glomerulus has a covering called the glomerular capsule or Bowman’s capsule. The outside of this is a layer of squamous epithelium supported by a basal lamina (Mescher, 2021). It is continuous with the proximal tubule, where the epithelia become simple cuboidal.

The capillaries of the glomerulus work together with podocytes. They work together with these stellate epithelia to proceed with renal filtration. Primary processes from the podocytes curve around these capillaries.

The primary processes also have many pedicels covering the capillary surfaces. These pedicels are in contact with the basal lamina. Between these pedicels are slit diaphragms – tight junctions containing nephrins (Mescher, 2021).

A thick glomerular basement membrane lies between the capillaries and the podocytes. This membrane is important in separating blood from the capsular space. This restricts large proteins and some organic ions (Mescher, 2021).

You can also find mesangial cells, which support the capillaries of the glomerulus. They also adjust the contractions based on the blood pressure. Mesangial cells also work in immune defense and glomerular repair (Mescher, 2021).

Conclusion

The urinary system plays a vital role in maintaining our health. It filters waste and extra water from the blood, balancing fluids and electrolytes. By removing toxins, it regulates blood pressure and prevents buildup.

It is thus important to take care of your urinary system. Drink plenty of water and eat the right food. Keep the urinary system happy as it works non-stop to keep you healthy.

References:

Crumbie, L. (2023). Neurovascular supply of the kidney. Kenhub. https://www.kenhub.com/en/library/anatomy/neurovascular-supply-of-the-kidney

Ferng, A. (2023). Urinary bladder. Kenhub. https://www.kenhub.com/en/library/anatomy/urinary-bladder

Mescher, A. (2021). Junqueira’s basic histology: Text & atlas. McGraw-Hill Education.

National Kidney Foundation. (n.d.). How your kidneys work. National Kidney Foundation. https://www.kidney.org/kidneydisease/howkidneyswrk

Pirie, E. (2023). Kidney histology. Kenhub. https://www.kenhub.com/en/library/anatomy/kidney-histology