Urinary system: Structure and function

Definitions & Key takeaways

The urinary system is composed of several organs and structures that work together to produce, store, and eliminate urine from the body. The urinary system has several important functions in the body, including excretion of waste products, regulation of fluid balance, regulation of electrolyte balance and maintenance of blood pressure.

The main organs of the urinary system are the kidneys, ureters, bladder, and urethra. The kidneys are two bean-shaped organs located in the back of the abdominal cavity, one on each side of the spine. They are responsible for filtering waste products and excess water from the blood to produce urine. The urine flows from the kidneys through the ureters, which are two long, narrow tubes that connect the kidneys to the bladder.

The bladder is a muscular sac located in the pelvis that stores urine until it is ready to be eliminated from the body. When the bladder is full, it signals the brain, and the person experiences the sensation of needing to urinate. The urine is then eliminated from the body through the urethra, which is a tube that extends from the bladder to the external opening.

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Intro0:00–0:36

The main function of the urinary system is to control the volume and composition of blood by removing excess fluid and waste products from the body.
The structures of the urinary system consist of two kidneys, two ureters, the urinary bladder, and the urethra. The kidneys filter the blood to produce urine.
Once produced, urine will flow through the ureters into the bladder for storage until it’s ready to be excreted through the urethra.
Let’s explore each part of the urinary system in more detail, starting with the kidneys. They are bean-shaped organs located toward the back of the upper abdominal cavity, one on each side of the spinal column.

Structure of the urinary system 0:36–4:53

They’re protected by the bottom of the rib cage and a layer of fat. Inside each kidney there are about 1 million nephrons.
The nephron is the basic functioning unit of the kidney. Each nephron consists of a glomerulus and a series of complicated twisted tubules.
The glomerulus is a network of capillaries located within a structure called Bowman’s capsule.Now, in order to filter blood, the kidneys need a good blood supply, so they are supplied by two large arteries: the left and right renal arteries that branch directly off of the aorta, the largest artery in the body.
Once inside the kidney, the left and right renal arteries branch into smaller and smaller arteries and eventually give rise to the capillary within the glomerulus.
These capillaries have tiny openings in them that let small molecules filter out of the blood. Through this process, waste products are removed from the blood, while nutrients remain within the blood.
At the end of filtration, we’ll basically have two distinct liquids: one is the filtered blood, which remains within the capillaries, and the other one is the filtrate in the Bowman’s capsule, which forms the basis of urine.
The filtrate leaves the Bowman’s capsule and flows through the tubules that make up the rest of the nephron. These tubules will reabsorb some of the substances that the body wants to keep, like water, nutrients, and minerals from the filtrate.
By the time it reaches the end of the tubules, only excess fluid and waste products remain, and this is called urine. The kidneys are able to produce about 160 to 180 liters of filtrate per day, but only 1 to 1.5 liters are excreted from the body as urine.
The urine produced by each kidney is emptied into a collecting area called the renal pelvis. From the renal pelvis, urine flows through the ureters.
The ureters are muscular tubes of approximately 10 to 13 inches, or about 25 to 33 cm, and they carry the urine from the kidneys to the bladder.
They have smooth muscles in their walls, which contract rhythmically to move urine from the kidney into the bladder. These are called peristaltic movements, similar to those you can see in the digestive tract as food is moved through the intestines.
At the point where the ureters enter the bladder, there’s a small triangular fold that forms to keep urine from flowing back into the ureters.
Next up is the bladder, which is basically a reservoir for holding urine. Kidneys never take a break, so urine is constantly produced and transported through the ureters into the bladder and stored there until urination occurs.
Normally, when empty, the bladder is very small but becomes pretty large as it fills with urine. Just like the ureters, the bladder is lined with a mucous membrane to protect it from infections.
The bladder also contains three layers of smooth muscle. Urination occurs when the walls of the bladder contract.
The bladder is continued by a tube called the urethra.At the point where the bladder and the urethra join, called the bladder outlet, is the internal sphincter, which is an involuntary muscle that keeps the bladder closed while it fills with urine.
Now, the urethra is a tube that carries urine from the bladder to the outside of the body. The urethra begins just below the internal sphincter and ends at the external urinary opening, which is called the urinary meatus or urethral orifice.
Below the internal sphincter, there’s the external urethral sphincter, which is a ring of voluntary muscle that relaxes voluntarily to allow urine to pass during urination.
Male and female urethras are very different in size and function. In females, the urethra is about 2 inches long and is used only for the passage of urine.
In males, the urethra is about 6 to 8 inches and serves as a passageway for both urine and semen.Okay, now we’re going to talk about the functions of the urinary system, which is to maintain the body's homeostasis by controlling the composition and volume of blood.

Functions of the urinary system 4:53–6:21

The kidneys can filter out waste, and they also regulate the level of minerals and pH. Too little fluid in the body leads to dehydration, while too much fluid in the body leads to fluid overload and swelling of certain parts of the body, like the limbs.
Minerals like potassium, calcium, and sodium are important to our body's function, and the kidneys either retain or excrete them through urine.
Sodium is especially important for maintaining fluid balance. If levels of sodium are high, more water will be retained.
On the other hand, loss of sodium through vomiting or diarrhea can lead to dehydration. Potassium and calcium are important to maintain proper skeletal muscle function as well as cardiac muscle function.
For example, if there’s too much potassium in the body, this can affect the heart and lead to serious issues, even death.
So, when this happens, the kidneys will try to excrete the excess potassium to keep the organs functioning normally. Finally, human blood has a pH that’s slightly above 7, but in certain conditions, it can become either too acidic or too alkaline.
In both cases, it can lead to serious injury of the cells. The kidneys can detect abnormal pH in the blood and release the substances necessary to restore the pH balance.Moving on, aging can affect multiple systems, including the urinary system.

Effects of aging on the urinary system6:21–7:50

First, after a person reaches 40 years of age, the number of functioning nephrons in the kidneys start to decrease. As a result, the process of filtration isn’t as effective, so the ability to remove waste products is decreased.
Second, loss of muscle tone as a result of aging can reduce bladder capacity, which can lead to increased frequency of urination.
Third, in biological males, the enlargement of the prostate gland is common. It can be benign, which is due to normal aging, or it can be malignant, which can be the result of prostate cancer.
As the prostate gland enlarges, it narrows the urethra, which makes it difficult for urine to get out. So after urination, not all urine from the bladder is excreted.
Because the bladder doesn’t empty completely, this means that it will quickly refill and, at this point, urine simply overflows.
As a result, the person might dribble urine in between visits to the bathroom. Finally, in elderly individuals, there’s an increased risk for urinary tract infections.
Incomplete emptying of the bladder can lead to infection because the urine sitting in the bladder over long periods of time is a perfect environment for bacterial growth.
In order to help the kidneys filter blood properly and to prevent urinary tract infection by flushing harmful bacteria, drinking plenty of fluid is necessary, especially in most elderly people.
All right, as a quick recap... The urinary system consists of two kidneys, two ureters, the bladder, and the urethra.

Recap7:50–8:30

The kidneys can filter out waste, and they also regulate the level of minerals and pH in the body. The nephron is the kidney’s basic functioning unit.
Each nephron has a glomerulus and a series of tubules. The glomerulus is a network of capillaries located within Bowman’s capsule.
In the glomerulus, filtration occurs, which will result in the formation of the filtrate that becomes urine after traveling through the tubules.
Urine then leaves the kidney through the ureters and is stored in the bladder until it’s excreted through the urethra.