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Urinary System 2

Urinary System 2
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Urinary System 2

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Urinary System 2:
The urinary system performs the functions of producing and excreting urine from the body.  
The constancy of body fluid volumes and the level of many important chemicals depend on normal urinary system function.  
The urinary system performs vital excretory functions and eliminates the dissolved organic waste products generated by the body's cells.  


Regulating blood volume and blood pressure by adjusting the volume of water lost in the urine and releasing the hormones erythropoietin and renin.
Regulating blood concentrations of sodium, potassium, chloride and other ions by controlling the quantities lost in the urine.
Stabilizing blood pH by controlling the loss of ions in the urine
Conserving valuable nutrients by selectively preventing losses while eliminating waste products.

The kidneys clear or clean the blood of many waste products that are continually being produced as a result of metabolism of food in the cells.  
If the kidneys fail to function properly, toxic wastes start to accumulate in the body.  Toxic wastes accumulating in the cells cause them to suffocate and literally poison themselves.

The urinary system consists of 2 kidneys (that form the urine), 2 ureters, 1 bladder and 1 urethra.   .The 2 kidneys produce urine, which is a liquid containing water, ions, and small soluble compounds.  Urine leaving the kidneys travels along the paired ureters to the urinary bladder for temporary storage.  When urination or micturation occurs, contraction of the muscular bladder forces the urine through the urethra and out of the body.


Location:  The kidneys lie just above the waistline on the posterior aspect of your body.  Usually the right kidney is a little lower than the left.  They are located between the 12th thoracic vertebrae and the 3rd lumbar vertebrae, one on each side of the vertebral column.  The kidneys, partially protected by the lower ribs, lie in shallow depressions against the posterior abdominal wall and the parietal peritoneum or the membrane that covers the abdominal organs.  This is why kidneys are called retroperitoneal meaning in back of or behind the peritoneum.  

The kidney is the primary organ of the urinary system.  Each kidney and it's blood vessels is enclosed within a mass of fat tissue called the adipose capsule.  In turn, each kidney and the adipose capsule is enclosed in a tough, fibrous tissue called the renal fascia or fibrous capsule.  The adipose capsule is a protective measure to keep the kidneys from being damaged.


The kidney is bean shaped and has an indentation on the medial aspect called the hilum or the hilus.  The hilus is the point of entry for the renal artery and the exit for the renal vein and ureter.  

Cutting the kidney in half lengthwise reveals the internal structure. The upper end of each ureter flares into a funnel shaped structure known as the renal pelvis.  See page 459-A
The hilum leads to a large cavity, called the renal sinus within the kidney.  
The kidney is divided into 2 layers: an outer, granular layer called the cortex and an inner, striated layer called the medulla.  
The medulla is red and consists of striated cones called renal pyramids. Within these pyramids are straight tubular structures and blood vessels.   The base of each renal pyramid faces the cortex, while it's apex or renal papilla empties into cuplike cavities called calyces.  There is a major and a minor calyx.  The major calyx being the larger of the two, which then empties into the renal pelvis.

The cortex, reddish brown is composed of millions of microscopic functional units of the kidney called nephrons.  The nephron is the basic structural and functional unit of the kidney.
The nephron is within the renal pyramid and is a very complex and important structure.  This cortical tissue is interspersed or placed between the renal pyramids, separating and supporting them.  These supports are called renal columns.  

Urine production begins in the renal pyramids and overlying areas of renal cortex.  Ducts within each renal papilla discharge urine into a minor calyx.


The nephron has two parts the renal corpuscle and the renal tubule.    Interestingly enough the number of nephrons does not increase after birth.  Growth of the kidney is due to enlargement of the individual nephrons.  When nephrons are damaged they are not replaced or regenerated.  

A nephron begins with the afferent arteriole.  This carries blood from the renal artery.  The afferent arteriole enters a double walled hollow capsule called the Bowman's capsule.  Within this capsule the arteriole divides or breaks off into very fine or small divisions that form a knotty ball called the glomerulus, which contains over 50 capillaries.  The combination of the Bowman's capsule and the glomerulus is called the renal corpuscle.  

The Bowman's capsule sends off a highly convoluted or twisted tubular branch, which is called the proximal convoluted tubule.  This tubule descends into the medulla to form the loop of Henle.  Eventually, this tubule opens into a larger, straight vessel known as the collecting tubule.  Several distal convoluting tubules join to form this single straight collection tubule.  The collecting tubule then empties into the ureter.  

After the afferent arteriole branches out to form the glomerulus, it leaves the Bowman's capsule as the efferent arteriole.  The efferent arteriole branches to form capillaries surrounding the renal tubules.  All of these capillaries eventually join together to form a small branch of the renal vein, which carries blood away from the kidney.


The formation of urine involves three major processes.
glomerular filtration-  blood from the renal artery enters the smaller afferent arteriole which then turns into even smaller capillaries of the glomerulus.  As the blood vessels travel this course the vessels get narrower and narrower.  This results in an increase in blood pressure.  In most of the capillaries throughout the body, blood pressure is about 25mm/Hg; in the glomerulus, it is between 60-90mm/Hg.

This high pressure forces a plasma-like fluid to filter from the blood in the glomerulus into the Bowman's capsule.  This fluid is called the filtrate.  It consists of water, glucose amino acids, some salts, and urea.  The filtrate does not contain plasma proteins because they are too large to pass through the pores of the capillary membrane so they remain in the blood.  Waste products are dissolved in blood plasma so they pass into the renal filtrate.  Useful materials such as nutrients and minerals are also dissolved in plasma and are also present in renal filtrate.  Therefore, renal filtrate is very much like blood plasma; except that there is far less protein and no blood cells are present.  The Bowman's capsule filters out 125 ml of fluid from the blood in a single minute. YIKES!!!!!!!!!

Therefore in one hour about 7500ml of filtrate leave the blood; this amounts to 180 liters in a 24-hour period.  Depending on how you think of it that also amounts to 90 2-liter bottles of soda.

As you have guessed we cannot dispose of that much water each day.  In fact, we only lose about a liter and a half per day.  This is because the filtrate continues along the tubules and 99% of the water is reabsorbed back into the blood stream.

tubular reabsorption-  This process includes the reabsorption of useful substances from the filtrate within the renal tubules into the capillaries around the tubules.  These substances are water, glucose, amino acids, vitamins, bicarbonate ions, and the chloride salts of calcium, magnesium, sodium and potassium.  Reabsorption starts in the proximal tubules and continues through the Henle's loop, to the distal tubule and then to the collecting tubules.  The proximal tubules reabsorb about 80% of the water filtered out of the blood in the glomerulus.  From what we have learned it becomes apparent that most of the renal filtrate does not become urine.  Only 1%-5% of the filtrate will enter the renal pelvis as urine.  The cells of the renal tubule use ATP to transport most of the useful materials from the filtrate to the blood.  If, however, the blood glucose level is above normal, the amount of glucose in the filtrate will also be above normal and will exceed the level of possible reabsorption, therefore some glucose will be present in the urine.  There are many hormones that are secreted by the endocrine system that affect reabsorption.
ATP or adenosine triphosphate is used to transport most of the useful materials from the filtrate to the blood.  PTH or parathyroid hormone assists with the increased reabsorption of calcium.  PTH is secreted when the blood calcium levels decrease.  Aldosterone increases the reabsorption of sodium and the excretion of potassium.
tubular secretion-  is the opposite of reabsorption.  This mechanism also changes the composition of urine.   Some substances are actively secreted into the tubules.  Substances secreted into the urine include ammonia, hydrogen ions, potassium and some drugs.

Summary of urine formation

The kidneys form urine from blood plasma.  Blood flow through the kidneys is a major factor in determining urinary output.
Glomerular filtration is the first step in urine formation.  High blood pressure in the glomeruli forces plasma, dissolved materials, and small proteins into the Bowman's capsule, the fluid is now called renal filtrate.
Tubular reabsorption is selective in terms of usefulness.  Reabsorption takes place from the filtrate in the renal tubules to the blood in the peritubular capillaries.
Tubular secretion takes place from the blood in the peritubular capillaries to the filtrate in the renal tubules and can ensue that wastes are actively put into the filtrate to be excreted.
Hormones influence the reabsorption of water and help to maintain normal blood volume and blood pressure.  Waste products remain in the renal filtrate and are excreted in urine.


Urine passes from the kidneys out of the collecting tubules into the renal pelvis, then down the ureters and into the urinary bladder.  There are two ureters, one from each kidney carrying urine from the kidneys to the bladder.  They are long and narrow tubes.  Mucous membranes line both the renal pelvis and the ureters.  Beneath the mucous membrane lining there are smooth muscle fibers.  When these muscles contract, peristalsis is initiated, pushing urine down the ureter into the urinary bladder.  As the bladder fills. it expands and compresses the lower ends of the ureters to prevent backflow of urine.


The urinary bladder is a hollow, muscular sac made of elastic fibers and involuntary muscle, which lies below the peritoneum and behind the pubic bones.  In women, the bladder is inferior to the uterus and in men; the bladder is superior to the prostate gland.  
The bladder is a reservoir for accumulating urine, and it contracts to eliminate urine.  The size and shape of the bladder varies according to the amount of urine it contains and with pressure from surrounding organs.  The inner lining of the bladder is a mucous membrane.  When the bladder is empty it has numerous folds or rugae in it.  These folds allow for expansion as the bladder fills to it's potential.  It stores the urine until about one pint has been accumulated.  The bladder then becomes uncomfortable and must be emptied.  Emptying the bladder or voiding, takes place by muscular contractions of the bladder, which are involuntary, although they can be controlled by some extent through the nervous system.  
Contraction of the bladder muscles forces the urine through a narrow canal, the urethra, which extends to the outside opening or the urinary meatus.

Urinary tract infections or UTI's occur more frequently in women than in men because of the differences in the urethra.  In females, the urethral opening is in close proximity to the anal opening, which gives intestinal bacteria easier access to the urethra.  The female urethra is short, which allows any infection to spread to the urinary bladder.  An infection of the urethra is called urethritis and one of the urinary bladder is called cystitis.  The urethra is much longer in men than it is in females.  The urethra in the male measures about 7-8 inches long and in the female the urethra measures about 1.5 inches long.  

An involuntary internal urethral sphincter and a voluntary external urethral sphincter control the flow of urine through the urethra.   


Blood enters the afferent arteriole...passes through the the Bowman's it becomes filtrate (blood minus the RBC's and plasma proteins)...continues through the proximal convoluted the loop of the distal convoluted the collecting tubule (at this time about 99% of the filtrate has been reabsorped)....approximately 1ml of urine is formed per minute...the 1ml of urine goes to the renal the the the the urinary meatus.



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