Urination
Created | Updated Apr 1, 2005
As a species we produce about 13 million litres of urine every day. Certainly a lot of pee but only equivalent to about 6% of the flow over Niagara. Urine is the waste product resulting from the filtration process that takes place in the kidneys. In order to survive our bodies need to keep fluid and electrolyte balances and vital acid-base levels regulated within very narrow limits. The kidneys carry out this task using filtration and re-absorption processes.
The urine forming parts of the kidney are highly efficient filtration units called nephrons packed tightly into the outer, solid part of the kidney. At the top of the nephron is a cup shaped structure known as Bowman’s capsule and projecting into the capsule is the glomerulus, a tufted network of capillaries. Blood in the capillaries is at a higher pressure than the tissue fluids around them and the filtrate is forced through the walls. There are over one million nephrons in each kidney. The filtering area of a single kidney is as large as the whole body surface and if all the glomerular capillaries were laid end to end they would stretch over 35 miles. Each bean shaped filtration system, i.e. kidney, weighs typically 150gm and filters approximately 1,000 litres of blood per day. From this 200 litres of dilute filtrate is produced and, after re-absorption, 1.7 litres of urine remain. If one kidney is removed the remaining kidney will enlarge to do the extra work.
The capsule forms the top of the proximal tubule which descends, through a hairpin bend called Henle’s loop, into several, small cup like chambers called calyces. The dilute filtrate flows slowly through the tubules where water and dissolved materials are reabsorbed . The remaining urine passes via larger calyces into a funnel shaped cavity called the kidney pelvis, the lower end of which forms the ureter, a tube that drains into the bladder and not to be confused with the urethra, the tube that conveys urine from the bladder to the outside world.
So this golden fluid the product of the prodigious, silent labours of the kidneys gathers in the urinary bladder, a hollow muscular organ, the shape and size of which varies according to the amount of urine it contains. When empty it is tetrahedral and when full ovoid. As the bladder fills with urine there is a rise in internal pressure. The bladder walls then relax to accommodate the contents. As urine continues to gather this tensing and relaxing process continues with the pressure rising in steps with the final pressure always rising.
The desire to micturate or urinate begins when the content reaches 400 ml but can be overridden until the content reaches about 800ml. It is a complex activity partly unconscious controlled by lower spinal cord centres and partly conscious controlled by the higher brain centres. Messages from the brain stimulate the detrusor or emptying muscles of the bladder and relaxing both utrethral sphincters. Evacuation is assisted by the abdominal wall contracting, the diaphragm descending, the breath is held and the perineal floor relaxes. Once begun urination is carried through to completion by higher and lower centres acting in concert and messages sent from the urethra distended by the flow of urine.
Unconscious or involuntary urination is known as incontinence and has several categories such as stress, including fight or flight response, overflow and urge incontinence caused by various biological factors. Until potty trained young babies are incontinent and at the other end of life’s journey, as we loose control over our higher faculties we may again become incontinent. Young men can suffer from an inability to urinate particularly if they imagine they are being observed known as shy or bashful bladder syndrome and older men sometimes struggle due to the enlargement of the prostrate gland.
Generally men urinate standing up and women sitting down a custom that leads to much friction between the sexes regarding the resulting position of the toilet seat and men’s inability to accurately aim a stream of urine. However in men’s defence the evidence of a miss hit only becomes evident when the light straw coloured healthy urine dries to a deeper yellow stain on the rim.
The urine forming parts of the kidney are highly efficient filtration units called nephrons packed tightly into the outer, solid part of the kidney. At the top of the nephron is a cup shaped structure known as Bowman’s capsule and projecting into the capsule is the glomerulus, a tufted network of capillaries. Blood in the capillaries is at a higher pressure than the tissue fluids around them and the filtrate is forced through the walls. There are over one million nephrons in each kidney. The filtering area of a single kidney is as large as the whole body surface and if all the glomerular capillaries were laid end to end they would stretch over 35 miles. Each bean shaped filtration system, i.e. kidney, weighs typically 150gm and filters approximately 1,000 litres of blood per day. From this 200 litres of dilute filtrate is produced and, after re-absorption, 1.7 litres of urine remain. If one kidney is removed the remaining kidney will enlarge to do the extra work.
The capsule forms the top of the proximal tubule which descends, through a hairpin bend called Henle’s loop, into several, small cup like chambers called calyces. The dilute filtrate flows slowly through the tubules where water and dissolved materials are reabsorbed . The remaining urine passes via larger calyces into a funnel shaped cavity called the kidney pelvis, the lower end of which forms the ureter, a tube that drains into the bladder and not to be confused with the urethra, the tube that conveys urine from the bladder to the outside world.
So this golden fluid the product of the prodigious, silent labours of the kidneys gathers in the urinary bladder, a hollow muscular organ, the shape and size of which varies according to the amount of urine it contains. When empty it is tetrahedral and when full ovoid. As the bladder fills with urine there is a rise in internal pressure. The bladder walls then relax to accommodate the contents. As urine continues to gather this tensing and relaxing process continues with the pressure rising in steps with the final pressure always rising.
The desire to micturate or urinate begins when the content reaches 400 ml but can be overridden until the content reaches about 800ml. It is a complex activity partly unconscious controlled by lower spinal cord centres and partly conscious controlled by the higher brain centres. Messages from the brain stimulate the detrusor or emptying muscles of the bladder and relaxing both utrethral sphincters. Evacuation is assisted by the abdominal wall contracting, the diaphragm descending, the breath is held and the perineal floor relaxes. Once begun urination is carried through to completion by higher and lower centres acting in concert and messages sent from the urethra distended by the flow of urine.
Unconscious or involuntary urination is known as incontinence and has several categories such as stress, including fight or flight response, overflow and urge incontinence caused by various biological factors. Until potty trained young babies are incontinent and at the other end of life’s journey, as we loose control over our higher faculties we may again become incontinent. Young men can suffer from an inability to urinate particularly if they imagine they are being observed known as shy or bashful bladder syndrome and older men sometimes struggle due to the enlargement of the prostrate gland.
Generally men urinate standing up and women sitting down a custom that leads to much friction between the sexes regarding the resulting position of the toilet seat and men’s inability to accurately aim a stream of urine. However in men’s defence the evidence of a miss hit only becomes evident when the light straw coloured healthy urine dries to a deeper yellow stain on the rim.
