Last Updated: March 23, 2021
Why does the bladder eliminate yellow liquid, or urine, out of the body? How does the excretory system work? The truth is that most people don’t know or understand much about the excretory system. The urinary excretory system is quite remarkable.
The excretory system, also known as the renal system, is responsible for transporting any unnecessary, dangerous, or excess material from the body—this process allows the body to sustain homeostasis, balance within the body, thereby preventing any destruction from happening. Most organs in the body, when functioning normally, will produce waste and because of this, the whole body relies on the urinary excretory system to excrete it.
The two bean-shaped kidneys are the essential organs of a functioning urinary system. Protected by the ribcage, the kidneys reside at the posterior, or rear, wall of each side of the abdominal cavity above the waistline. The majority of people have two kidneys; however, a person can live a healthy life with one functioning kidney.
The kidneys have three major regions:
Each kidney contains over 1 million nephrons, which function to regulate water and soluble substances through filtering the blood and excreting the rest as urine. The components of a single nephron consists of the:
Different components of a nephron reside in different sections of the kidney:
The following components of the urinary excretory system are accessory structures to assist in the elimination of the urine from the body.
These two narrow, muscular tubes transport urine to the bladder from the kidneys. The muscles in the ureter walls tighten and relax to force urine downward, away from the kidneys. When urine backs up or is at a stand still in the kidneys, a kidney infection will likely develop. Approximately every 10 to 15 seconds small amounts of urine is expelled into the bladder from the ureters.
The bladder is a reservoir for urine—this triangle-shaped, hollow organ is in the lower abdomen. Ligaments, attached to other organs and the pelvic bone, hold the bladder in place. The bladder’s wall relaxes and expands to hold urine and contacts and flattens to release the urine down the urethra. The average adult bladder can hold up to two cups, or 500 mL, of urine for two to five hours.
The final passageway for the outflow of urine is the urethra—it is a thin-walled tube that passes urine from the urinary bladder to outside of the body. The brain signals the bladder muscles to contract, which squeeze the urine out of the bladder. Simultaneously, the sphincter muscles of the bladder relax to let urine exit through the urethra. When all the signals happen in this exact order, normal urination occurs.
The renal veins are veins that empty the kidney by carrying out the filtered blood. The inferior vena cava, the large vein that transports deoxygenated blood to the heart, connects to the renal veins to carry blood away from the kidney.
The renal arteries branch off the abdominal aorta and supply each of the kidneys with oxygenated blood. The renal arteries carry a substantial portion of the total blood flow to the kidneys—when a person is at rest, a quarter of the total cardiac output, on average, passes through the renal arteries to be filtered by the kidneys.
Urine, one of the body’s waste products, is mainly comprised of water and urea. Urea is a nitrogenous waste that the body must constantly remove. Formation of urine occurs in three stages:
Each nephron has its own blood supply from the renal artery. A network (or tuft) of capillaries known as the glomerulus, is located at the beginning of each nephron in the kidney—it serves as the first stage in making urine by separating the liquid part of the blood, the plasma.
Surrounding the glomerulus is a cup-like sac known as the Bowman’s capsule. The blood plasma is filtered through the capillaries of the glomerulus and the filtered liquid from the blood, known as the filtrate, collects into the Bowman’s capsule.
The speed at which the kidneys filter the blood is called the glomerular filtration rate—the amount of filtrate produced every minute. The driving force for the filtering process is the blood pressure as it enters the glomerulus. The blood pressure is counteracted by an inward pressure from the fluid and proteins left within the small vessels.
Creatinine is produced in the muscle when creatine is used, or metabolized, to generate energy; therefore, it is a waste product of creatine metabolism. Creatinine is exclusively filtered through the kidneys and cannot be reabsorbed. Its rate of excretion from the blood stream is directly related to how efficient the kidneys are filtering. By measuring the amount of creatinine in a blood sample, and combining this with age, height and weight, gender, and ethnicity, a health-care practitioner can determine how well the kidneys are working.
The filtrate produced by the glomerulus initially contains small molecules like urea, amino acids, glucose, bicarbonate, water, and creatinine as well as ions such as sodium, potassium, chloride, calcium, and phosphate. The filtrate should not contain large cells, such as protein, or albumin, and red blood cells.
The Bowman’s capsule then empties the filtrate, into the proximal tubule of the nephron. This region of the kidney is responsible for the reabsorption of glucose, amino acids, various ions, and water. During the filtrate’s voyage, many substances will be added and subtracted.
The filtrate then passes through the loop of Henle where it loses water and ions such as sodium, chloride, calcium, and potassium. What is not reabsorbed within the loop of Henle eventually passes into the distal convoluted tubule (DCT). The DCT selectively reabsorbs or secretes based on several factors, one being hormonal control.
In the secretion stage, waste, such as various ions and hydrogen ions; nitrogen-containing waste products of protein metabolism; and urea and creatinine, are all combined into the remaining filtrate to become urine.
The urine passes out of the nephron tubule into a collecting duct. The collecting duct can reabsorb water to concentrate urine if necessary—this occurs in the presence of antidiuretic hormone (ADH) (see table 2). The urine then passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.
Table 1. Defining terms and key functions of the urinary excretory system.
|Maintenance of homeostasis||Excretion of wastes|
Note. These terms are an overview and are not all inclusive.
Table 2. Hormones produced by the kidney and the hormones that control the kidney function.
|Production of hormones||Hormones that control kidney function|
|Calcitriol||Antidiuretic hormone (ADH)|
|Atrial natriuretic peptide (ANP)|
Note. The hormones associated with the urinary secretory system play a crucial role in homeostasis.
Urinary tract infections (UTIs) are amongst the most frequent infections and are easy to diagnose with a urine sample. Antibiotic therapy is usually helpful in treating an infection. UTIs occur when bacteria populate and infect the urinary tract. Several parts of the urinary tract can be affected by bacteria, but the most common type of UTI is a bladder infection, known as cystitis. Typical symptoms include burning or pain during urination, frequent urination, urgency, feeling the need to urinate despite having an empty bladder, pressure or pain the lower abdomen, fever, cloudy or bloody urine.
Kidney stones, pebble-like solid pieces, can occur in one or both of the kidneys when high levels of certain minerals are in the urine. Kidney stones are pebble-like pieces. The stones vary in size and shape. The Size doesn’t necessarily determine the pain. A small kidney stone may pass through the urinary tract on its own, causing little or no pain for one person and cause a high level of pain for another. Severe pain or bleeding from is often associated with a larger stone that is stuck, causing a block in the flow of urine. Kidney stones rarely cause permanent damage if treated promptly by a health-care professional.
Kidney cancer is on the top 10 list of the most common cancers diagnosed in the United States. It is estimated that in 2017, about 63,990 people will be diagnosed. About 14,000 people will die from this disease. Of the people who are diagnosed in the beginning stages of cancer, 75–80% will survive. Kidney tumors may be silent, not showing any signs or pain. Sometimes, a tumor in the kidney will cause pain in the side, abdomen, or back that does not go away; blood in the urine; a lump in the abdomen; weight loss; loss of appetite; and anemia and fatigue.
Table 3. Other disorders and clinical findings of the urinary secretory system.
|Disorders and findings||Disorders and findings cont.|
|Pyelonephritis||Acute Kidney Injury (AKI)|
|Benign prostatic hyperplasia||Neurogenic bladder|
|Diabetes insipidus||Chronic kidney disease|
|Renal artery disease||Uremia|
Note. These disorders and findings are an overview and are not all inclusive.