Humans get rid of wastes from the body through the urinary system. The urinary system is functional in turning toxic substances into the urine, storing and carrying urine, and safely eliminating it from the body.
In addition to waste removal, which is the main function of the urinary system, it is also involved in other vital functions such as maintaining proper chemical and water balance and ensuring the body is hydrated enough by controlling the electrolyte levels. Further, the system regularly monitors and regulates the acidity of the body fluids.
Urinary system – Parts
The main parts of the urinary system are –
- Kidneys
- Ureters
- Urinary bladder
- Urethra
- Urinary sphincter muscles
Kidneys
Kidneys are paired bean-shaped structures forming the core of the urinary system. They are situated on either side of the spine towards the back, just below the rib cage. The right kidney is somewhat lower compared to the left to accommodate the liver. Each kidney in an average adult weighs about 5 ounces and is about 5 inches in length, 3 inches in width and 1 inch in thickness.
Each kidney is protected and encased by three layers of tissue –
- Renal capsule – The innermost layer formed by the smooth fibrous membrane
- Adipose capsule – a layer of fatty tissue
- Renal fascia – the outermost layer comprising connective tissue holding the kidney to the abdominal wall
The outer section of the kidney is known as the cortex. The medulla is in the centre of the kidney and comprises 10-15 cone-shaped collecting ducts referred to as renal pyramids. These structures pour the urine into the minor calyces – the cup-shaped receptacles. Urine from here flows into the major calyces – the larger openings. It takes place via the renal pelvis (funnel-shaped) and to the ureter and the bladder.
The majority of the kidney function occurs in cup-shaped capsules referred to as nephrons. There are about one million nephrons in each kidney. The blood gets filtered here. Water and required nutrients are reabsorbed in the system, while the toxic wastes are eliminated. Further, nephrons regulate the pH level of blood. Nephrons comprise the glomerulus – a network of small capillaries or blood vessels that are contained in a thin sac referred to as a glomerular capsule (Bowman’s capsule).
Through the renal artery, blood enters each kidney into the hilus – a curved notch near the centre on the side of each kidney. Then, it branches into the capillaries of the glomerulus. The blood that exits from the glomerulus passes through a membrane (three-layered) initiating the process of filtration.
When the glomerular filtrate arrives at the glomerular capsule, it moves through the PCT (proximal convoluted tubule) – a small coiled tube. Here, water and other substances are reabsorbed into the bloodstream through small capillaries. The remaining fluid contains substances that are not reabsorbed by the blood – water, excess salts and urea (waste generated by the disintegration of proteins) – are passed through the loop of Henle to the DCT (distal convoluted tubule).
Numerous distal tubules pour wastes called urine into one collecting duct, which in turn pours urine into the papillary ducts. As it approaches the papillary duct, the solution is filtered continually hence when it reaches the ureters, about 99% of the actual glomerular filtrate has been reabsorbed into the blood.
The purified blood moves to the heart through the renal vein, and the urine moves into the calyces of the renal pelvis.
Functions of Kidney
Mainly, the kidney is involved in three functions –
- Homeostasis
- Hormone secretion
- Removing wastes
The kidneys are involved in keeping the body free from any impurities. It maintains a healthy chemical and water balance and monitors the electrolyte composition. Further, the kidneys are involved in regulating blood pressure and in the secretion of vital hormones.
Wastes products in the bloodstream
The digestive enzymes disintegrate nutrients into small substances as the food moves through the stomach and intestines. This disintegration releases different toxic wastes into the bloodstream, they are –
- Urea – protein metabolism yields amino acids which are disintegrated in the liver forming ammonia. Since it is toxic to process by the body, the liver transforms it into less toxic urea to remove
- Ketone bodies – produced by the disintegration of excess fatty acids in the liver
- Uric acid – formed by the disintegration of purines in the tissues
- Creatinine – muscle metabolism yields it as a by-product
If these wastes are allowed to accumulate in the blood, it would gradually poison the cells and blood. Kidneys filter the dissolved wastes from the blood, forming urine that is eliminated from the body over time.
Secretion of hormones
Kidneys either activate or secrete three vital hormones –
- Calcitriol – it facilitates bone growth by increasing calcium and phosphorus levels in the blood
- Erythropoietin – triggers the production RBCs in the bone marrow
- Aldosterone – regulation of blood pressure and sodium balance by increasing water reabsorption, filtration of blood in kidneys, reducing the amount of sodium which is lost
Ureters
Ureters extend from each kidney – hollow, fine tubes reaching into the bladder. Each ureter extends about 12 inches, its widest part is about 0.5 inches. Ureters penetrate the walls of the bladder from either side, which forms a U shape. The Ureters at the terminal of the U connect to the triangular-shaped section on the bladder base known as the trigone.
As the kidneys turn the wastes into urine, muscles which line the ureter walls aid in pushing the urine to the bladder to store. Through the ureteral orifices (openings), the urine enters the bladder. The mucous membrane in the ureters stores urine in the bladder and prevents it from moving back up towards the kidneys where it can cause an infection.
Bladder
The muscular hollow bladder keeps urine till eliminated. It is positioned in the abdomen behind the pubic bone. At the base of the trigone, there is an opening to the urethra in the neck of the bladder, via which urine moves out of the body.
The interior of the bladder comprises three layers –
- Serosa – outer coat
- Detrusor muscle – collectively used term for 3 layers of smooth muscle
- Mucosa – inner layer
When the bladder is full, a signal is sent to the spinal cord to urinate. The bladder muscles relax during filling and contract during urinating.
Prostate
This doughnut-shaped gland surrounding the juncture where it connects to the bladder is seen only in males. Prostate in young men is of the shape and size of a walnut; it increases with age and can enlarge about the size of an orange.
The main role of the prostate is the addition of fluid volume and nutrients to the sperm. Numerous small glands in the prostate produce a liquid which gets mixed with the sperm in the urethra at the time of orgasm. The semen (combined fluids), gets released via the penis at the time of ejaculation. Furthermore, the prostate protects the bladder from any infection. Its muscular fibres squeeze, helping to control the urine flow into the urethra.
Urethra
The fine, muscular tube of the urethra connects the bladder to the outside, rendering a route for urine to exit the body. The urethra’s wall comprises a mucous membrane and a layer of smooth muscle tissue. There is a difference in the function, path and length of the urethra in males and females.
In males, the urethra is ~8 inches long, connecting from the bladder to the head of the penis. It carries urine and serves as a duct to conduct semen that is released at the time of ejaculation.
In females, the urethra is ~1.5 inches long, hence women are comparatively more prone to urinary tract infections. The short length makes it easier for urine to move back up in the bladder and source an infection. The urethra links to the apex of the vagina of a woman, just underneath the clitoris. It is mainly involved in eliminating urine outside the body.
Urinary Sphincter
Sphincters are two groups of muscles, which regulate the urine flow out of the bladder. The urethra and bladder meet at a ring of smooth muscles – internal urethral sphincters, which are involuntary muscles stopping urine from back-flow up the urethra to the bladder. The external urethral sphincters present at the end of the urethra are voluntary muscles which release and strain to start and stop the flow of urine.
Both sphincter muscles, when open, allow urine to move out of the body. When they are closed, it prevents the urine from escaping. These muscles are framed to function with the bladder. When the bladder relaxes, allowing urine to enter, the sphincter muscles stay closed to avert any leakage. When the bladder contracts at the time of urination, the sphincter muscles relax, which allows urine to flow out of the body. In the event that they fail to function in sync, chances are that urine can leak from the bladder, resulting in incontinence.
4 Astonishing Facts about the Human Urinary System
|
You read about the different parts of the human urinary system with a diagram in detail. For related information on other topics, visit BYJU’S.
More here:
Frequently Asked Questions on Human Urinary System
What are the 3 main steps involved in urine formation?
Each day, ~180 litres of blood plasma pass through the nephrons of the kidneys. Most of which is cleansed and returned to the bloodstream. The remaining is eliminated as urine. The formation of urine is through these 3 processes –
- Filtration
- Reabsorption
- Secretion
What is the pH of urine?
Averaging around 6.0, the normal pH of urine ranges from 4.5 to 8.0.
What is incontinence?
Incontinence is losing voluntary control over urination.
Babies wet naturally, as their spinal cords and brains have not matured enough to control the sphincter muscles. But in adults, incontinence is typically due to a disease or an injury to the nerves which control the bladder, any damage or infection to the external sphincter. The inability to urinate is retention.
Comments