Excretory system
Encyclopedia
The excretory system is a passive biological system that removes excess, unnecessary or dangerous materials from an organism, so as to help maintain homeostasis
within the organism and prevent damage to the body. It is responsible for the elimination of the waste products
of metabolism
as well as other liquid and gaseous wastes. As most healthy functioning organs
produce metabolic and other wastes, the entire organism
depends on the function of the system; however, only the organs specifically for the excretion process are considered a part of the excretory system. The excretory system gets rid of waste called urine
or "pee".
As it involves several functions that are only superficially related, it is not usually used in more formal classifications of anatomy or function.
Within each kidney there are an estimated one million microscopic nephrons, where blood filtration takes place. Each nephron contains a cluster of capillaries called a glomerulus
. A cup-shaped sac called a bowmans capsule surrounds each glomerulus. The blood that flows through the glomerulus is under great pressure. This causes water, glucose and urea to enter the bowmans capsule. White blood cells, red blood cells and proteins remain in the blood. As the blood continues in the excretory system, it passes through the renal tubule. During this time, reabsorption occurs: glucose and chemicals such as potassium, sodium, hydrogen, magnesium and calcium are reabsorbed into the blood. Almost all the water removed during filtration returns to the blood during the reabsorption phase. The kidneys control the amount of liquid in our bodies. Now only wastes are in the nephron. These wastes are called urine and include urea, water and inorganic salts. The cleansed blood goes into veins that carry the blood from the kidneys and back to the heart.
not excretion, specifically for temperature control and pheromone release. Therefore, its role as a part of the excretory system is minimal at best. Specifically, the skin secretes a fluid waste called sweat, or perspiration.
s of organisms constantly secrete gaseous wastes from the bloodstream as a regular part of respiration
.
Kidneys perform several homeostatic functions:
. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum. Undigested food may also be expelled this way; this process is called egestion.
, the ureters are muscular ducts that propel urine
from the kidneys to the urinary bladder
. In the adult, the ureters are usually 25–30 cm (10–12 in) long. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This "pelviureteric junction" is a common site for the impaction of kidney stones (the other being the uteterovesical valve). The ureters run posteriorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves. In the female, the ureters pass through the mesometrium
on the way to the urinary bladder
.
is the organ that collects urine
excreted by the kidneys prior to disposal by urination
. It is a hollow muscular, and distensible (or elastic) organ, and sits on the pelvic floor
. Urine enters the bladder via the ureters and exits via the urethra
.
Embryologically, the bladder is derived from the urogenital sinus
and, it is initially continuous with the allantois
. In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate
, and separated from the rectum
by the rectovesical excavation
. In females, the bladder sits inferior to the uterus and anterior to the vagina. It is separated from the uterus
by the vesicouterine excavation
. In infants and young children, the urinary bladder is in the abdomen
even when empty.
, the urethra (from Greek
οὐρήθρα - ourethra) is a tube which connects the urinary bladder
to the outside of the body. The urethra has an excretory function in both sexes to pass urine
to the outside, and also a reproductive function in the male, as a passage for semen
during sexual activity.
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
within the organism and prevent damage to the body. It is responsible for the elimination of the waste products
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...
of metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
as well as other liquid and gaseous wastes. As most healthy functioning organs
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
produce metabolic and other wastes, the entire organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
depends on the function of the system; however, only the organs specifically for the excretion process are considered a part of the excretory system. The excretory system gets rid of waste called urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
or "pee".
As it involves several functions that are only superficially related, it is not usually used in more formal classifications of anatomy or function.
Excretory functions
Removes metabolic and liquid toxic wastes as well as excess water from the organism.Within each kidney there are an estimated one million microscopic nephrons, where blood filtration takes place. Each nephron contains a cluster of capillaries called a glomerulus
Glomerulus
A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...
. A cup-shaped sac called a bowmans capsule surrounds each glomerulus. The blood that flows through the glomerulus is under great pressure. This causes water, glucose and urea to enter the bowmans capsule. White blood cells, red blood cells and proteins remain in the blood. As the blood continues in the excretory system, it passes through the renal tubule. During this time, reabsorption occurs: glucose and chemicals such as potassium, sodium, hydrogen, magnesium and calcium are reabsorbed into the blood. Almost all the water removed during filtration returns to the blood during the reabsorption phase. The kidneys control the amount of liquid in our bodies. Now only wastes are in the nephron. These wastes are called urine and include urea, water and inorganic salts. The cleansed blood goes into veins that carry the blood from the kidneys and back to the heart.
Skin
Excretion by definition is passive and deals with metabolic wastes as filtered by the kidneys. Though the sweat may contain a trace amount of metabolic wastes, sweating is an active process of secretionSecretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
not excretion, specifically for temperature control and pheromone release. Therefore, its role as a part of the excretory system is minimal at best. Specifically, the skin secretes a fluid waste called sweat, or perspiration.
Lungs
The lungs and gillGill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s of organisms constantly secrete gaseous wastes from the bloodstream as a regular part of respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
.
Kidneys
In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful irritant that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra.Kidneys perform several homeostatic functions:
- Maintain volume of extracellular fluid
- Maintain ionic balance in extracellular fluid
- Maintain pH and osmotic concentration of the extracellular fluid.
- Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.
Defecation
Organisms eliminate solid, semisolid or liquid waste material (feces) from the digestive tract via the anus during the process of defecationDefecation
Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum. Undigested food may also be expelled this way; this process is called egestion.
Ureter
In human anatomyAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, the ureters are muscular ducts that propel urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
from the kidneys to the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
. In the adult, the ureters are usually 25–30 cm (10–12 in) long. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This "pelviureteric junction" is a common site for the impaction of kidney stones (the other being the uteterovesical valve). The ureters run posteriorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves. In the female, the ureters pass through the mesometrium
Mesometrium
The mesometrium is the mesentery of the uterus. It constitutes the majority of the broad ligament of the uterus, excluding only the portions adjacent to the uterine tube and ovary.It is adjacent to the mesosalpinx....
on the way to the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
.
Urinary bladder
The urinary bladderUrinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
is the organ that collects urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
excreted by the kidneys prior to disposal by urination
Urination
Urination, also known as micturition, voiding, peeing, weeing, pissing, and more rarely, emiction, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control...
. It is a hollow muscular, and distensible (or elastic) organ, and sits on the pelvic floor
Pelvic floor
The pelvic floor or pelvic diaphragm is composed of muscle fibers of the levator ani, the coccygeus, and associated connective tissue which span the area underneath the pelvis. The pelvic diaphragm is a muscular partition formed by the levatores ani and coccygei, with which may be included the...
. Urine enters the bladder via the ureters and exits via the urethra
Urethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
.
Embryologically, the bladder is derived from the urogenital sinus
Urogenital sinus
The definitive urogenital sinus is a part of the human body only present in the development of the urinary and reproductive organs...
and, it is initially continuous with the allantois
Allantois
Allantois is a part of a developing animal conceptus . It helps the embryo exchange gases and handle liquid waste....
. In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....
, and separated from the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
by the rectovesical excavation
Rectovesical excavation
Between the rectum and the bladder the peritoneal cavity forms, in the male, a pouch, the rectovesical excavation , the bottom of which is slightly below the level of the upper ends of the vesiculae seminales—i. e., about 7.5 cm. from the orifice of the anus...
. In females, the bladder sits inferior to the uterus and anterior to the vagina. It is separated from the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
by the vesicouterine excavation
Vesicouterine excavation
In human female anatomy, the vesicouterine excavation is a second, but shallower, pouch formed from the peritoneum over the uterus and bladder, continued over the intestinal surface and fundus of the uterus onto its vesical surface, which it covers as far as the junction of the body and cervix...
. In infants and young children, the urinary bladder is in the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
even when empty.
Urethra
In anatomyAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, the urethra (from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
οὐρήθρα - ourethra) is a tube which connects the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
to the outside of the body. The urethra has an excretory function in both sexes to pass urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
to the outside, and also a reproductive function in the male, as a passage for semen
Semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...
during sexual activity.