∞ generated and posted on 2020.03.15 ∞

Glomerular filtrate is processed within nephrons into urine as it passes through a series of tubes.

From the nephron, urine flows into collecting ducts, from which water can still be removed, and on into the renal pelvis, then ureters and urinary bladder, with the transition from the latter to the urethra controlled by urethral sphincters.

This page contains the following terms: Convoluted tubules, Loop of Henle, Collecting ducts, Urine, Concentrated urine, Dilute urine, Ureters, Urinary bladder, Urethra, Internal urethral sphincter, External urethral sphincter, Diuretic, Micturition, Continence Cystitis, Kidney stone, Renal failure

The above video provides a very toungue-in-cheek (dare I say dry?) overview of urine and the urinary system. Almost not worth watching, but…

Convoluted tubules

Carrier of glomerular filtrate through much of the nephron especially within the renal cortex.
The convoluted tubules are differentiated into the proximal convoluted tubule, which spans immediately post the Bowman's capsule until the loop of Henle, and the distal convoluted tubule which spans post the loop of Henle until the collecting duct.

The term "convoluted" refers to their meandering nature, which packs more tubule into the distances spanned, and of course "tubule", which means that they are small tubes. The proximal convoluted tubule additionally is lined with microvilli, which has the effect of increasing the absorptive area further.

In association with the convoluted tubules, substantial amounts of tubular reabsorption and tubular secretion occur.

Links to terms of possible interest: Ascending limb of loop of Henle, Bowman's capsule, Collecting duct, Convoluted tubule, Descending limb of loop of Henle, Distal convoluted tubule, Glomerular capsule, Glomerular filtrate, Glomerulus, Kidney, Loop of Henle, Nephron, Podocyte, Proximal convoluted tubules, Renal corpuscle, Renal cortex, Renal medulla, Renal pelvis, Tubular reabsorption, Tubular secretion, Ureter

The above video provides an overview of renal tubules.

Tubular reabsorption, that is, from glomerular filtrate to the blood, via the proximal convoluted tubule.

Tubular reabsorption of sodium ion via the proximal convoluted tubule.

Tubular reabsorption of bicarbonate ion via the proximal convoluted tubule in combination with tubular secretion of hydrogen ion, that is, into the glomerular filtrate.

Tubular reabsorption of potassium ion via the proximal convoluted tubule.

Tubular reabsorption of bicarbonate ion via the proximal convoluted tubule in combination with tubular secretion of hydrogen ion, that is, into the glomerular filtrate.

Tubular reabsorption of water and urea via the proximal convoluted tubule plus discussion of how urea is formed.

Tubular reabsorption, that is, from glomerular filtrate to the blood, via the distal convoluted tubule.

Tubular reabsorption of sodium ion via the distal convoluted tubule.

Tubular reabsorption of water via the distal convoluted tubule.

Tubular secretion, that is, to glomerular filtrate (ultimately) from the blood, via the distal convoluted tubule as well as the collecting duct, of potassium ion.

Loop of Henle

Nephron component responsible for generating the osmotic gradient of the renal medulla.
A key means by which water is removed from glomerular filtrate is during its passage, within collecting ducts, from the renal cortex to the renal pelvis. This passage is through the renal medulla, which can osmotically draw water out of the forming urine. It is the loop of Henle that is responsible for generating the necessary osmotic gradient about these collecting ducts.

The loop of Henle varies in length, helping to define the two different types of nephron, the cortical nephrons versus the juxtamedullary nephrons. The loop of Henle is longer in the juxtamedullary nephrons, and indeed it is the latter that is typically depicted when a nephron is illustrated. It particularly is this longer loop of Henle as found in the juxtamedullary nephrons that is responsible for generating the osmotic gradient of the renal medulla.

Links to terms of possible interest: Collecting ducts, Cortical nephrons, Glomerular filtrate, Juxtamedullary nephrons, Loop of Henle, Minor calyx, Nephron, Osmotic gradient, Renal cortex, Renal medulla, Renal pelvis

The above video provides a very simplified discussion of how the loop of Henle functions though not the importance of the collecting duct in contributing to osmoregulation.

The above video is the first-in-this-series discussion of the loop of Henle.

The above video is the second-in-this-series discussion of the loop of Henle; note that the loop of Henle is a pretty complicated system to fully appreciate.

The above video is a bit broader than considering just the loop of Henle in osmoregulation but does provide a fairly detailed yet not overwhelming overview how the loop of Henle impacts osmoregulation, starting at 3:59.

Collecting ducts

Immediately post-nephron tubes found within kidneys through which still-forming urine passes.
The collecting ducts continue to refine glomerular filtrate as they pass from the renal cortex through the renal medulla and into the renal pelvis. Specifically, water as well as urea can continue to be removed, with the urea contributing to the osmotic gradient that is found within the renal pyramids.

The collecting ducts also go by the names ducts of Bellini and papillary ducts. Furthermore, they pass fully formed urine into what are known as minor calyces (minor calyx is the singular), which fuse together (converge) thus channeling further urine flow into major calyces and then into the renal pelvis, which is followed by a ureter, etc.

Links to terms of possible interest: Active transport, ADH, Antidiuretic hormone, Aquaporins, Collecting ducts, Concentrated urine, Countercurrent multiplier, Descending limb of nephron loop, Ducts of Bellini, Glomerular filtrate, H2O reabsorption, Inner medulla kidney, Interstitial fluid, Kidneys, Major calyces, Minor calyx, Nephron, Osmolarity, Outer medulla kidney, Papillary ducts, Passive transport, Posterior pituitary, Renal cortex, Renal medulla, Renal pelvis, Renal pyramids, Urea, Ureter, Urine

The above video considers the role of collecting ducts in osmoregulation starting at 6:22. In terms of the functioning of the collecting ducts in water reclamation, this later portion of this video is well worth watching. At about 5:00 the means by which the osmotic gradient that drives this water reclamation from the collecting ducts is described.


Product of kidney function that passes into the renal pelvis and then into ureters.
Urine is produced over the course of flow of glomerular filtrate through the nephron and then collecting duct. Once this filtrate is found within the renal pelvis, however, it is no longer glomerular filtrate but instead is now bona fide urine. This urine can be concentrated or dilute – or, of course, somewhere in between – and contains mostly water, typically on the order of about 95%.

Post the renal pelvis the urine then passes though ureters into the urinary bladder, then through the urethra and out of the body. You produce one to two liters of urine per day, or more if you consume excess fluids or less if you consume insufficient fluids.

In addition to water, urine consists primarily of salts (electrolytes) and urea.

The presence of glucose in the urine ("sugar") is an abnormality indicating an overwhelming of the nephronal glucose reabsorption processes, owing to excess glucose in the blood (i.e., as seen with diabetes mellitus).

Kidney disease, on the other hand, can be detected by the presence of protein in the blood, which normally should not enter glomerular filtrate to begin with.

Links to terms of possible interest: Collecting ducts, Dehydrated, Glomerular filtrate, Kidney, Nephron, Nephronal glucose reabsorption, Renal pelvis, Ureters, Urethra, Urinary bladder, Urine, Renal pelvis

The above video provides yet another overview of nephron functioning, leading to urine formation, with a brief discussion of urine itself towards the end (1:29).

Concentrated urine

Product of kidney functioning possessing relatively low levels of water.
In addition to its excretion and regulatory functions (regulation particularly of blood chemistry), the kidneys in land vertebrates are also charged with sufficiently retaining water that body dehydration is at least forestalled. This process is accomplished, in a general sense, via the formation of concentrated urine.

The "concentrated" of concentrated urine refers to the removal of substantial amounts of water from urine during its formation. In this way, the urine becomes concentrated with whatever is left behind. The result is that water that otherwise had found its way into glomerular filtrate is spared from being excreted.

This sparing of water from excretion can be viewed as "concentrated blood" giving rise to concentrated urine. The process of concentration, however, is not nearly as simple as that statement would suggest, involving something known as a countercurrent mechanism (or countercurrent multiplier), which in fact is rather complicated.

Contrast concentrated urine with dilute urine.

Links to terms of possible interest: Blood chemistry, Collecting duct, Concentrated urine, Countercurrent multiplier, Dehydration, Dilute urine, Excretion, Glomerular filtrate, Kidney, Late distal tubule, Osmolarity, Peritubular fluid, Renal pelvis, Urine

The video explains how urine is concentrated in terms of both the generation of an osmolarity gradient by the loop of Henle, that is, high salt solution concentrations, and the subsequent extraction of water from the collecting ducts into that osmolarity gradient.

Dilute urine

Product of kidney functioning possessing relatively high levels of water.
The formation of dilute urine is the major means by which your body avoids having its tissues bathed in excessively dilute fluids, a.k.a., hypotonic fluid. The production of dilute urine is fairly straightforward as it involves mostly reduced water reabsorption within the collecting duct so that this water – as originally found in the glomerular filtrate – remains within the urine over the course of its formation.

Contrast with concentrated urine.

Links to terms of possible interest: Antidiuretic hormone, Collecting duct, Concentrated urine, Dilute urine, Diuretics, Glomerular filtrate, Kidney, Urine, Vasopressin, Water reabsorption

The video explains how urine is concentrated in terms of both the generation of an osmolarity gradient (high salt/solution concentrations) by the loop of Henle and subsequent extraction of water from the collecting ducts into that osmolarity gradient; dilute urine occurs when the hormone ADH (antidiuretic hormone, a.k.a., vasopressin) is relatively absent from the blood.


Tubes connecting kidneys to bladder.
The ureters carry fully formed urine from the renal pelvis to the urinary bladder. They are more than just tubes, however, as associated smooth muscle additionally displays peristalsis, which serves as a urine-moving aid (since you, and indeed most vertebrate land animals, are not always standing up with their head high in the air!).

You have two ureters, one for each kidney.

Between the ureters and the bladder there exist one-way valves known as ureterovesical valves, which serve to prevent the backflow of urine from bladder to kidney, as too does the peristalsis in the ureters. One way to view the purpose of these valves is that though a bladder infection can be uncomfortable, a kidney infection can kill you, and these valves prevent bacteria that may have reached the bladder from finding their way up to the kidney.

Links to terms of possible interest: Bladder, Histology, Kidneys, Lamina propria, Lumen, Peristalsis, Renal pelvis, Smooth muscle, Ureterovesical valves, Ureters, Urinary bladder, Urine, Urothelium

Urinary bladder

Collecting organ of the water and wastes excreted by the kidney.
The urinary bladder – or simply bladder for short and as it is usually referred to – is found between our pubic bone and our rectum. In women the uterus is found in approximately the same location, between the bladder and rectum, resulting in less volume within the abdominal cavity available to the bladder in females versus males (and resulting typically in a smaller bladder in females than in males, and even less volume available to the bladder during pregnancy).

The bladder is more than just a sack that holds urine but instead possesses smooth muscle as well, called Detrusor muscle. This muscle contributes substantially to the voiding of the bladder, which involves somewhat more than just the pull of urine out of your body via gravity. The epithelium that lines the urinary bladder furthermore is capable of reversibly stretching. Your bladder thus can stretch to hold fairly large amounts of urine, and then forcibly expel that urine once the opportunity arises.

Links to terms of possible interest: Abdominal cavity, Bladder, Detrusor muscle, External urethral sphincter, Internal urethral sphincter, Kidney, Lamina propria, Pubic bone, Rectum, Submucosa, Transitional epithelium, Ureter, Ureteral openings, Urinary bladder, Urine

The first half of the above video considers the normal functioning of a urinary bladder while the second half addresses the issue of stress-induced urinary incontinence.


Tube connecting the bladder to outside of the body.
While there are two ureters, one for each kidney, we have only a single urethra, and its length differs substantially between males and females. That is, it is somewhat longer in males than it is in females.

Differences in length matter because they help to determine the ability of pathogenic microorganisms to move upward, essentially against the flow of urine (when flowing), into the urinary bladder, where they potentially can cause a urinary tract infection. As a consequence of the shorter length of the urethra, as well as its closer proximity to the anal opening, females tend to be more susceptible to urinary tract infections than are men.

Links to terms of possible interest: Anal opening, Bladder, Distal urethra, Female, Male, Penis, Prostate gland, Proximal urethra, Rectum, Testes, Ureters, Urethra, Urinary bladder, Urinary tract infection, Urine, Uterus

The above video is by a plastic surgeon so discusses not just female urogenital anatomy but how that can be modified surgically; nonetheless, it provides a good introduction to female urogenital anatomy including in terms of the location of the urethra and its exterior opening.

Internal urethral sphincter

Effector of involuntary control over voiding of the bladder.
The internal urethral sphincter relaxes in response to excess pressure within the urinary bladder. The muscle itself represents a continuation of the muscles otherwise associated with the bladder, called detrusor muscle.

The internal urethral sphincter represents the major impediment to the release of urine. That is, when the internal urethral sphincter relaxes you really really have to pee, assuming that you haven't started already!

Links to terms of possible interest: Bladder, Body wall, Detrusor muscles, External urethral sphincter, Female, Female urinary tract, Internal urethral sphincter, Male, Male urinary tract, Prostate gland, Smooth muscle, Urinary bladder, Ureter, Ureteral opening, Urethra, Urine, Voiding of the bladder

The above video discusses the urethral sphincters with emphasis on nervous system control of their functioning within the context of the micturition reflex.

External urethral sphincter

Effector of voluntary control over voiding of the bladder.
The external urethral sphincter gives you some choice about when and where to pee, though the extent to which you truly have a choice is dependent on how full your bladder is in combination with whether or not your internal urethral sphincter has opened.


Drug that increases the amount of water present in urine.
Diuretic drugs, that is, give rise to the production of more dilute urine, which can be used to combat water retention within the rest of the body. The action of diuretics has the effect of lowering blood pressure.

Links to terms of possible interest: Aldosterone, Ascending limb of loop of Henle, Collecting duct, Descending limb of loop of Henle, Dilute urine, Distal convoluted tubule, Diuretic, Diuretic drugs, Glomerulus, Hormone, Kidney, Loop of Henle, Nephron, Outer medulla kidney, Proximal convoluted tubule, Renal cortex, Tubular reabsorption, Tubular secretion, Urine, Water retention

The above video takes yet another look at kidney functioning, though in some detail in terms of tubular secretion and tubular reabsorption and also within the context of diuretics, which interfere with these functions particularly in terms of a failure of reabsorption of sodium ions, which in turn interferes with water removal from urine.

The above video considers the functioning of a diversity of diuretics.


Clinical term for urination, i.e., the voiding of one's bladder.
The process of urination involves more than just the opening of various sphincters but also the contraction of various muscles. These include particularly the detrusor muscles of the bladder itself.

Links to terms of possible interest: Afferent pathway, Bladder, Brain, Cerebral cortex, Detrusor muscles, Efferent pathway, External urinary sphincter, Internal urinary sphincter, Interneuron, Intramural ganglion, Micturition, Micturition reflex, Muscle contraction, Parasympathetic preganglionic motor fiber, Pelvic nerve, Postganglionic neuron, Proximal, Sensory fiber, Skeletal muscle, Sphincter, Spinal cord, Stretch receptor, Thalamus, Urethra, Urination, Urine, Voiding of one's bladder

The above video provides a nice overview to the micturition reflex, that is, why and how in terms of our nervous system and urinary system anatomy we pee.


Voluntary control over urination.
The converse, urinary incontinence, represents a lack of control over urination/micturition. That is, with urinary incontinence, upon opening of the involuntarily controlled internal urinary sphincter, there is no further control over the occurrence of urination.

Urinary incontinence is the normal human state prior to early-life maturation, which results in the development of continence. Continence in turn can be lost due to injury or disease, but also as a consequence, temporary or less so, of pregnancy.

Links to terms of possible interest: Continence, Incontinence, Internal urinary sphincter, Involuntary control, Micturition, Urethral blockage, Urinary incontinence, Urination, Voluntary control


Inflammation of the urinary bladder such as caused by urinary tract infections.
Another, more common name for cystitis, as caused by infection, is bladder infection. Urinary tract infections (UTIs) that involve the bladder also can be described as an "acute cystitis", and more generally UTIs include infections not just of bladder but also of the urethra.

Links to terms of possible interest: Acute cystitis, Bladder, Bladder infections, Cystitis, Female, Female urinary tract, Inflammation, Kidney, Lower urinary tract infections, Male, Male urinary tract, Scrotum, Testicle, Ureter, Urethra, Urethral opening, Urinary bladder, Urinary system, Urinary tract infections

The above video specifically considers lower urinary tract infections and particularly in terms of women, who tend to experience especially bladder infections more frequently than men.

Kidney stone

Common name for renal calculus.
Renal referring to the kidneys and calculus meaning stone or concretion, kidney stones consist of various solid materials and are found in various locations in or downstream from the kidney. They generally form in men though can form in women as well.

They will dislodge and pass out of the body with urine unless they reach sufficient size that they become lodged particularly in the ureter. If this occurs, then urine passage can be blocked and pain along with other symptoms can occur.

Links to terms of possible interest: Calyceal stone, Kidney, Kidney stone, Nephrons, Renal, Renal calculus, Renal pelvic stone, Renal pelvis, Symptoms, Upper ureteral stone, Ureter, Urine

The above video describes the formation of kidney stones, why they can a problem, and also how to treat kidney stones.

Renal failure

Lack of sufficient filtration of the blood by the kidneys.
Renal failure is also described as kidney failure and/or kidney insufficiency. This results from either injury or instead disease. Renal failure can be differentiated into acute renal failure (shorter duration and/or more-rapid onset) versus chronic renal failure (longer duration and often less-rapid onset), with the latter typically not reversible.

Renal failure can involve either production of too little urine (as associated usually with production of too little glomerular filtrate) or instead the production of urine that possesses materials that otherwise should not belong in urine, such as proteins, which are an indication that unusually large materials are successfully entering into glomerular filtrate.

If too much material is lost in urine, then this can deplete the blood and therefore the body of needed substances (e.g., calcium). If too little material is removed into urine, including water, then hemodialysis can be necessary. Renal failure, versus underlying disease, can be treated also via kidney transplantation.

Links to terms of possible interest: Acute tubular necrosis, Bilateral ureteral obstruction, Bladder outlet obstruction, Blood pressure, Congestive heart failure, Extracellular fluid, Glomerular filtrate, Hemodialysis, Hypertension, Kidney failure, Kidney transplant, Kidneys, Renal failure, Symptoms, Urine

The above video discusses especially the symptoms as well as treatment of kidney failure.


Means of artificial filtration of blood outside of the body as required given reduced kidney functioning.
Typically referred to simply as dialysis, hemodialysis is a fairly routine procedure that involves the routing of a flow of blood from a patient into an ultrafiltration devise. Note that dialysis also can be accomplished by alternative means to hemodialysis.

In addition to removing materials normally removed by kidney function, such as urea, it can be necessary during hemodialysis to assure the removal of water as well (as too does the kidney) if kidney functioning is sufficiently reduced that normal urination no longer adequately occurs.

Links to terms of possible interest: Arterial blood, Artery, Blood port, Blood proteins, Blood pressure, Blood pump, Cellophane tubes, Circulation, Clot, Dialysate, Dialysis, Dialyzer, Filtration of blood, Formed elements, Hemodialysis, Kidney, Semipermeable membrane, Ultrafiltration, Vascular, Venous blood, Waste products

The above video is a nice, relatively low level introduction to hemodialysis.

The above video provides a slightly more detailed look at different dialysis procedures.