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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. In infants, elderly individuals and those with neurological injury, urination may occur as an involuntary reflex. In other animals, in addition to expelling waste material, urination can mark territory or express submissiveness. Physiologically, micturition involves coordination between the central, autonomic, and somatic nervous systems. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex. In males urine is ejected through the penis, and in females through the urethral opening.



Every one of us depends on the process of urination for the removal of certain waste products in the body. The production of urine is vital to the health of the body. Most of us have probably never thought of urine as valuable, but we could not survive if we did not produce it and eliminate it. Urine is composed of water, certain electrolytes, and various waste products that are filtered out of the blood system. Remember, as the blood flows through the body, wastes resulting from the metabolism of foodstuffs in the body cells are deposited into the bloodstream, and this waste must be disposed of in some way. A major part of this "cleaning" of the blood takes place in the kidneys and, in particular, in the nephrons, where the blood is filtered to produce the urine. Both kidneys in the body carry out this essential blood cleansing function. Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration. This is called the filtration fraction. The rest of the blood (about 80%) does not go through the filtering portion of the kidney, but flows through the rest of the body to service the various nutritional, respiratory, and other needs that are always present.
For the production of urine, the kidneys do not simply pick waste products out of the bloodstream and send them along for final disposal. The kidneys' 2 million or more nephrons (about a million in each kidney) form urine by three precisely regulated processes: filtration, reabsorption, and secretion.


Urine formation begins with the process of filtration, which goes on continually in the renal corpuscles. As blood courses through the glomeruli, much of its fluid, containing both useful chemicals and dissolved waste materials, soaks out of the blood through the membranes (by osmosis and diffusion) where it is filtered and then flows into the Bowman's capsule. This process is called glomerular filtration. The water, waste products, salt, glucose, and other chemicals that have been filtered out of the blood are known collectively as glomerular filtrate. The glomerular filtrate consists primarily of water, excess salts (primarily Na+ and K+), glucose, and a waste product of the body called urea. Urea is formed in the body to eliminate the very toxic ammonia products that are formed in the liver from amino acids. Since humans cannot excrete ammonia, it is converted to the less dangerous urea and then filtered out of the blood. Urea is the most abundant of the waste products that must be excreted by the kidneys. The total rate of glomerular filtration (glomerular filtration rate or GFR) for the whole body (i.e., for all of the nephrons in both kidneys) is normally about 125 ml per minute. That is, about 125 ml of water and dissolved substances are filtered out of the blood per minute. The following calculations may help you visualize how enormous this volume is. The GFR per hour is:

125 ml/min X 60min/hr= 7500 ml/hr.
The GFR per day is:
7500 ml/hr X 24 hr/day = 180,000 ml/day or 180 liters/day.

Now, see if you can calculate how many gallons of water we are talking about. Here are some conversion factors for you to consider: 1 quart = 960 ml, 1 liter = 1000 ml, 4 quarts. = 1 gallon. Remember to cancel units and you will have no problem.
Now, what we have just calculated is the amount of water that is removed from the blood each day - about 180 liters per day. (Actually it also includes other chemicals, but the vast majority of this glomerular filtrate is water.) Imagine the size of a 2-liter bottle of soda pop. About 90 of those bottles equals 180 liters! Obviously no one ever excretes anywhere near 180 liters of urine per day! Why? Because almost all of the estimated 43 gallons of water (which is about the same as 180 liters - did you get the right answer?) that leaves the blood by glomerular filtration, the first process in urine formation, returns to the blood by the second process - reabsorption.


Reabsorption, by definition, is the movement of substances out of the renal tubules back into the blood capillaries located around the tubules (called the peritubular copillaries). Substances reabsorbed are water, glucose and other nutrients, and sodium (Na+) and other ions. Reabsorption begins in the proximal convoluted tubules and continues in the loop of Henle, distal convoluted tubules, and collecting tubules. Let's discuss for a moment the three main substances that are reabsorbed back into the bloodstream.
Large amounts of water - more than 178 liters per day - are reabsorbed back into the bloodstream from the proximal tubules because the physical forces acting on the water in these tubules actually push most of the water back into the blood capillaries. In other words, about 99% of the 180 liters of water that leave the blood each day by glomerular filtration returns to the blood from the proximal tubule through the process of passive reabsorption.

The nutrient glucose (blood sugar) is entirely reabsorbed back into the blood from the proximal tubules. In fact, it is actively transported out of the tubules and into the peritubular capillary blood. None of this valuable nutrient is wasted by being lost in the urine. However, even when the kidneys are operating at peak efficiency, the nephrons can reabsorb only so much sugar and water. Their limitations are dramatically illustrated in cases of diabetes mellitus, a disease which causes the amount of sugar in the blood to rise far above normal. As already mentioned, in ordinary cases all the glucose that seeps out through the glomeruli into the tubules is reabsorbed into the blood. But if too much is present, the tubules reach the limit of their ability to pass the sugar back into the bloodstream, and the tubules retain some of it. It is then carried along in the urine, often providing a doctor with her first clue that a patient has diabetes mellitus. The value of urine as a diagnostic aid has been known to the world of medicine since as far back as the time of Hippocrates. Since then, examination of the urine has become a regular procedure for physicians as well as scientists.

Sodium ions (Na+) and other ions are only partially reabsorbed from the renal tubules back into the blood. For the most part, however, sodium ions are actively transported back into blood from the tubular fluid. The amount of sodium reabsorbed varies from time to time; it depends largely on how much salt we take in from the foods that we eat. (As stated earlier, sodium is a major component of table salt, known chemically as sodium chloride.) As a person increases the amount of salt taken into the body, that person's kidneys decrease the amount of sodium reabsorption back into the blood. That is, more sodium is retained in the tubules. Therefore, the amount of salt excreted in the urine increases. The process works the other way as well. The less the salt intake, the greater the amount of sodium reabsorbed back into the blood, and the amount of salt excreted in the urine decreases.


Now, let's describe the third important process in the formation of urine. Secretion is the process by which substances move into the distal and collecting tubules from blood in the capillaries around these tubules. In this respect, secretion is reabsorption in reverse. Whereas reabsorption moves substances out of the tubules and into the blood, secretion moves substances out of the blood and into the tubules where they mix with the water and other wastes and are converted into urine. These substances are secreted through either an active transport mechanism or as a result of diffusion across the membrane. Substances secreted are hydrogen ions (H+), potassium ions (K+), ammonia (NH3), and certain drugs. Kidney tubule secretion plays a crucial role in maintaining the body's acid-base balance, another example of an important body function that the kidney participates in.


Arginine vasopressin (AVP), also known as vasopressin, argipressin or antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals. Vasopressin is responsible for increasing water absorption in the collecting ducts of the kidney nephron.Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels in the kidney nephron collecting duct plasma membrane.Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's permeability. It also increases peripheral vascular resistance, which in turn increases arterial blood pressure. It plays a key role in homeostasis, by the regulation of water, glucose, and salts in the blood. It is derived from a preprohormone precursor that is synthesized in the hypothalamus and stored in vesicles at the posterior pituitary. Most of it is stored in the posterior pituitary to be released into the bloodstream; however, some AVP is also released directly into the brain, where it plays an important role in social behavior and bonding.


Atrial natriuretic peptide (ANP), atrial natriuretic factor (ANF), atrial natriuretic hormone (ANH), or atriopeptin, is a powerful vasodilator, and a protein (polypeptide) hormone secreted by heart muscle cells.It is involved in the homeostatic control of body water, sodium, potassium and fat (adipose tissue). It is released by muscle cells in the upper chambers (atria) of the heart (atrial myocytes) in response to high blood pressure. ANP acts to reduce the water, sodium and adipose loads on the circulatory system, thereby reducing blood pressure.

ANP binds to a specific set of receptors - ANP receptors. Receptor-agonist binding causes a reduction in blood volume and therefore a reduction in cardiac output and systemic blood pressure. Lipolysis is increased and renal sodium reabsorption is decreased. The overall effect of ANP on the body is to counter increases in blood pressure and volume caused by the renin-angiotensin system.

● as the bladder fills with urine, pressure increases and micturition reflex is activated by the stretch of the bladder
● action potentials are conducted from the bladder to the spinal cord through the
pelvic nerves
● parasymphathetic nerves carries action potentials that causes the bladder to
● decreased action potentials carried by the somatic motor nerves causes the
external sphincter to relax
● 2 ways on how the micturtion reflex is controlled by higher brain centers:
● in the ascending tract, when the bladder is stretched, there is an
increased frequency of action potentials up in the spinal cord to the pons and
cerebrum. This increases the conscious desire to urinate
● descending tracts, it carries action potentials from the cerebrum to the sacral region of the spinal cord to stimulate the micturition reflex and when one voluntarily chooses to urinate
● micturition reflex is inhibited by the higher brain centers by sending action
poptentials through the spinal cord to decrease the intensity of the autonomic
reflex that stimulates nerve fibers that keeps the external urinary sphincter
● the ability to voluntarily inhibit micturition develops at the age of 2-3 years

adult male – 60% consists of water
adult female – 50% consists of water
*smaller percentage of the body weight of the adult female consists of water
because females generally have a greater percentage of body fats than males
2 major compartments:
● intracellular fluid compartment
- fluid inside all the cells of the body
- enclosed by trillions of small compartments
- composition and movement of fluid across the cell membrane are equal
- 2/3 of all the water in the body
● extracellular fluid compartment - all the fluid outside the cell
- 1/3 of total body water
- interstitial fluid, plasma and fluid in the lymphatic vessels
- separated into subcompartments
- includes the aqueous and vitreous humor of
the eye, cerebrospinal fluid, synovial fluid, serous fluid and fluid secreted by glands

● intracellular fluid contains relatively high concentration of ions: K+,
MG2+, PO43-, SO42-
● extracellular fluid has a lower concentration of Na, Ca, Cl- and HCO –
● concentration of protein in the intracellular fluid is greater than that
in the extracellular fluid
● the extracellular fluid also has a fairly consistent composition from
one area of the body to another
● cell membranes that separate the body fluid compartments are
selectively permeable
● much less permeable to ions dissolve
● water movement is regulated by hydrostatic pressure differences and
osmotic differences
● the intracellular fluid can help maintain the extracellular fluid volume
if it is depleted
● helps to maintain blood volume
● can help prolong the time a person can survive conditions such as
dehydration or cardiovascular shock
● if concentration of ions decreases in the extracellular fluid, water
moves from the extracellular fluid into the cells causing the cells to