Urinary System – Anatomy, Structure, Functions

The urinary system’s function is to filter blood and create urine as a waste by-product. The organs of the urinary system include the kidneys, renal pelvis, ureters, bladder and urethra. The body takes nutrients from food and converts them to energy.

The urinary system is the kidneys of our body’s sewage treatment plant. They filter toxins out of the body, as well as other substances that we no longer need. These waste products leave your body in the urine produced in your kidneys. This is how water and substances like urea, uric acid, salts, and amino acids are removed from the blood. Every day, all of the blood in your body (between five and six liters) passes through the kidneys about 300 times. So your kidneys filter about 1,700 liters of blood per day in total. This leads to the daily production of about 170 liters of primary urine (glomerular filtrate) – which later becomes urine.

Inside the kidney, there is a renal medulla, which has small tubules and larger collecting tubes running through it. As the primary urine flows through this system of tubes, the kidney cells re-absorb about 99 percent of the fluid in it, as well as many substances that can still be used, and at the same time release other substances. About 1.7 liters of urine are produced like this each day. The urine passes from the kidneys through the ureter into the urinary bladder, where it is stored.

Overview of the Urinary System

The urinary system works with the lungs, skin, and intestines to maintain the balance of chemicals and water in the body. Adults eliminate about 27 to 68 fluid ounces (800 to 2,000 milliliters) per day based on typical daily fluid intake of 68 ounces (2 liters), National Institutes of Health (NIH). Other factors in urinary system function include fluid lost through perspiring and breathing. In addition, certain types of medications, such as diuretics that are sometimes used to treat high blood pressure, can also affect the amount of urine a person produces and eliminates. Some beverages, such as coffee and alcohol, can also cause increased urination in some people.

The primary organs of the urinary system are the kidneys, which are bean-shaped organs that are located just below the rib cage in the middle of the back. The kidneys remove urea — a waste product formed by the breakdown of proteins — from the blood through small filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

Muscles in the ureter walls continually tighten and relax to force urine away from the kidneys, according to the NIH. A backup of urine can cause a kidney infection. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.

The bladder is a hollow, balloon-shaped organ that is located in the pelvis. It is held in place by ligaments attached to other organs and the pelvic bones, according to the Kidney & Urology Foundation of America. The bladder stores urine until the brain signals the bladder that the person is ready to empty it. A normal, healthy bladder can hold up to 16 ounces (almost half a liter) of urine comfortably for two to five hours.

To prevent leakage, circular muscles called sphincters close tightly around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body. The only difference between the female and male urinary system is the length of the urethra, according to Merck Manuals. In females, the urethra is about 1.5 to 2 inches long (3.8 to 5.1 cm) and sits between the clitoris and the vagina. In males, it is about 8 inches (20 cm) long, runs the length of the penis and opens at the end of the penis. The male urethra is used to eliminate urine as well as semen during ejaculation.

The urinary system maintains blood homeostasis by filtering out excess fluid and other substances from the bloodstream and secreting waste.

Structure

The urinary system refers to the structures that produce and transport urine to the point of excretion. In the human urinary system, there are two kidneys that are located between the dorsal body wall and parietal peritoneum on both the left and right sides.

The formation of urine begins within the functional unit of the kidney, the nephrons. Urine then flows through the nephrons, through a system of converging tubules called collecting ducts. These collecting ducts then join together to form the minor calyces, followed by the major calyces that ultimately join the renal pelvis. From here, urine continues its flow from the renal pelvis into the ureter, transporting urine into the urinary bladder. The anatomy of the human urinary system differs between males and females at the level of the urinary bladder. In males, the urethra begins at the internal urethral orifice in the trigone of the bladder, continues through the external urethral orifice, and then becomes the prostatic, membranous, bulbar, and penile urethra. Urine exits through the external urethral meatus. The female urethra is much shorter, beginning at the bladder neck and terminating in the vaginal vestibule.

The Renal System

The renal system, which is also called the urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH.

The renal system organs include the kidneys, ureters, bladder, and urethra. Metabolic wastes and excess ions are filtered out of the blood, along with water, and leave the body in the form of urine.

Renal System Functions

The renal system has many functions. Many of these functions are interrelated with the physiological mechanisms in the cardiovascular and respiratory systems.

• Removal of metabolic waste products from the body (mainly urea and uric acid).
• Regulation of electrolyte balance (e.g., sodium, potassium, and calcium).
• Osmoregulation controls the blood volume and body water contents.
• Blood pressure homeostasis: The renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.
• Regulation of acid-base homeostasis and blood pH, a function shared with the respiratory system.

Many of these functions are related to one another as well. For example, water follows ions via an osmotic gradient, so mechanisms that alter sodium levels or sodium retention in the renal system will alter water retention levels as well.

Organs of the Renal System

Kidneys and Nephrons

Kidneys are the most complex and critical part of the urinary system. The primary function of the kidneys is to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. The kidneys have an extensive blood supply from the renal arteries that leave the kidneys via the renal vein.

Nephrons are the main functional component inside the parenchyma of the kidneys, which filter blood to remove urea, a waste product formed by the oxidation of proteins, as well as ions like potassium and sodium. The nephrons are made up of capsule capillaries (the glomerulus) and a small renal tube.

The renal tube of the nephron consists of a network of tubules and loops that are selectively permeable to water and ions. Many hormones involved in homeostasis will alter the permeability of these tubules to change the amount of water that is retained by the body.

Kidney and urinary system parts and their functions

• Two kidneys. This pair of purplish-brown organs is located below the ribs toward the middle of the back. Their function is to:
  • Remove waste products and drugs from the body
  • Balance the body’s fluids
  • Release hormones to regulate blood pressure
  • Control production of red blood cells

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

• Two ureters. These narrow tubes carry urine from the kidneys to the bladder. Muscles in the ureter walls continually tighten and relax forcing urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.
• Bladder. This triangle-shaped, hollow organ is located in the lower abdomen. It is held in place by ligaments that are attached to other organs and the pelvic bones. The bladder’s walls relax and expand to store urine, and contract and flatten to empty urine through the urethra. The typical healthy adult bladder can store up to two cups of urine for two to five hours.

Upon examination, specific “landmarks” are used to describe the location of any irregularities in the bladder. These are:

• Trigone: a triangle-shaped region near the junction of the urethra and the bladder
• Right and left lateral walls: walls on either side of the trigone
• Posterior wall: back wall
• Dome: roof of the bladder
• Two sphincter muscles. These circular muscles help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder.
• Nerves in the bladder. The nerves alert a person when it is time to urinate or empty the bladder.
• Urethra. This tube allows urine to pass outside the body. The brain signals the bladder muscles to tighten, which squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.

Human Osmoregulation

The kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from the glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), renin, aldosterone, and angiotensin I and II.

A basic example is that a decrease in water concentration of blood is detected by osmoreceptors in the hypothalamus, which stimulates ADH release from the pituitary gland to increase the permeability of the wall of the collecting ducts and tubules in the nephrons. Therefore, a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.

The extent of blood volume and blood pressure regulation facilitated by the kidneys is a complex process. Besides ADH secretion, the renin-angiotensin feedback system is critically important to maintain blood volume and blood pressure homeostasis.

Function

The main functions of the urinary system and its components are to:

• Regulate blood volume and composition (e.g. sodium, potassium and calcium)
• Regulate blood pressure.
• Regulate pH homeostasis of the blood.
• Contributes to the production of red blood cells by the kidney.
• Help synthesize calcitriol (the active form of Vitamin D).
• Stores waste products (mainly urea and uric acid) before it and other products are removed from the body.

References

Show More