Urinary system

The processes of urine production:

(1) filtration, (2) reabsorption, (3) secretion. This figure also shows the pathway for the components of urine (yellow), as well as blood flow associated with a nephron (red). Bold blue arrows show the direction materials are moving during each process.

The urinary system:

(a) anterior view, (b) view of a cadaver showing the kidneys, ureters, and urinary bladder (the parietal peritoneum has been removed).

Nephron

(a) as it occurs in a kidney, (b) juxtaglomerular apparatus.

The urinary bladder and urethra:

(a) female, (b) male. Although it appears in this figure that the ureters enter the urinary bladder at the top, they actually travel behind the bladder and enter at the bladder's base. The openings where they enter the bladder are labeled ureteral openings in this figure.

Organs of the excretory system:

(a) skin, (b) lungs, (c) liver, (d) kidney.

Nephr/o

Kidney

The process through which the urinary bladder is emptied is called urination or another scientific term is

Micturition

Azot/o

Nitrogen

Pyel/o

Renal pelvis

Hydronephrosis

The buildup of urine in the kidney. This results when the flow of urine from the kidney is blocked due to factors such as kidney stones, blood clots, tumors, or birth defects that cause a narrowing of the ureter. This backup of urine causes the kidney to swell, and the increased pressure inside the kidney causes damage. Diagnostic tests used to diagnose this condition are CT, intravenous pyelography, ultrasound, blood tests, and urine tests. If diagnosed early, hydronephrosis is treatable with no resulting kidney damage. Treatment involves removing the obstruction to restore the normal flow of urine from the kidney.

Action of aldosterone.

The pathway shown in red represents the negative feedback mechanism to restore homeostasis.

Ureter/o

ureter

Microscopic anatomy of a nephron:

(a) location of a nephron in a kidney, (b) anatomy of a nephron stretched out so that the parts can be more easily seen, (c) the renal corpuscle. Functional unit of the kidney is microscopic. Each kidney contains over 1 million nephrons. These structures produce urine. The anatomy of a nephron appears fairly complicated when seen in total. There are two principal parts to a nephron—the renal corpuscle and the renal tubule.

Gross anatomy of the kidney:

(a) major anatomical features of the kidneys, (b) coronal section of a cadaver kidney. Each kidney has three layers: the thin, outer fibrous renal capsule; a layer deep to the capsule called the renal cortex; and the inner renal medulla. In the diagram, the renal medulla appears to be composed of triangles called pyramids. The renal pyramids are actually three-dimensional cones, each leading to a funnel-like structure called a minor calyx. Two or more minor calyces may merge to form a major calyx, which empties into the renal pelvis. The calyces look and act like funnels that collect urine and deliver it to the renal pelvis. Many blood vessels that are present in this frontal section of the kidney. The kidney has and needs a very rich blood supply to function properly.

Retroperitoneal position of the kidney:

(a) transverse section through the torso at the level of the kidney, (b) sagittal cut through the torso at the right kidney.

Intravenous pyelography

An X-ray of the kidneys and the urinary tract that involves using contrast dye injected intravenously.

Water Conservation

Approximately 1.5 L of urine is excreted per day. This is the same amount as three-fourths of a 2 L bottle. This amount represents only 1% of the water filtered out of the blood at the glomeruli because the rest was reabsorbed. If there were no reabsorption, 150 L of urine (75 2 L bottles) would be produced per day, assuming you would have the time to take in that much water.

Excretion

The excretory system removes the body's metabolic wastes.

Cystoscopy

A procedure in which a lighted cystoscope is used to visualize the urinary bladder, lower urinary tract, and prostate.

Biopsy

A procedure in which tissue is collected and examined for the presence of abnormal cells.

Blood test

A procedure that involves obtaining a sample of blood and analyzing its contents. In regard to excretory disorders, blood tests can reveal the blood's composition, which may indicate a problem with kidney function.

Urinalysis

A test that involves a physical, chemical, and microscopic examination of urine. Results that do not fall within normal limits may indicate a condition or disease.

Renal Tubule

A tubule is simply a hollow tube. The renal tubule can be divided into three sections on the basis of anatomy and location—the proximal convoluted tubule (PCT), the nephron loop (loop of Henle), and the distal convoluted tubule (DCT). The walls of the tubule are simple epithelia that allow for the exchange of materials. What flows through the tubules eventually becomes urine. The proximal convoluted tubule is directly connected to the glomerular capsule. It twists and turns (convolutes) before descending to form the nephron loop in the renal pyramid. The renal tubule then ascends out of the renal pyramid to form the distal convoluted tubule in the renal cortex. Several distal convoluted tubules connect to a shared collecting duct that empties at the very end of the renal pyramid into a minor calyx. The renal corpuscle, the proximal convoluted tubule, and the distal convoluted tubule are located in the renal cortex, while the nephron loop and the collecting duct are located in the renal pyramid of the renal medulla. The epithelial cells of the distal convoluted tubule are very close together, forming a structure called the macula densa. You can also see specialized smooth muscle cells—juxtaglomerular cells surrounding the afferent arteriole. Together, the juxtaglomerular cells and the macula densa make up a structure called the juxtaglomerular apparatus.

Atrial Natriuretic Hormone ANH results in increased urine production in four ways.

ANH dilates the afferent arterioles while constricting the efferent arterioles in the kidney. This causes increased pressure in the glomeruli, so the glomerular filtration rate is increased. More water, glucose, amino acids, and mineral salts move across the filtration membrane into the glomerular capsule. ANH inhibits the production of renin by the juxtaglomerular apparatus. Inhibiting renin means that angiotensin I, angiotensin II, and aldosterone will not be produced. This inhibits water and sodium reabsorption. ANH inhibits the secretion of ADH from the posterior pituitary. This also limits water conservation. ANH inhibits sodium reabsorption in the nephron directly. Since water follows sodium, if less sodium is reabsorbed, it follows that less water is reabsorbed.

Effects of Aging on the Excretory System

Aging of the excretory system primarily affects the urinary system. Some of the effects involve the kidneys' production of urine, but the voiding of urine can be affected as well. The effects on urine production are as follows: Typically, the size of the kidneys and the number of functioning nephrons decrease by one-third by the age of 80. This is partly due to the narrowing and hardening of the arteries supplying the kidneys and glomeruli. With the reduced number of functioning nephrons, the glomerular filtration rate decreases along with the reserve capacity. Even so, the waste removal by the kidneys is normally sufficient in the elderly. However, other diseases may put pressure on the urinary system and cause it to fail more quickly in elderly individuals. Drugs are cleared less efficiently with age, so drugs remain in circulation longer. Drug dosages may need to be adjusted in elderly people to compensate for the poor clearance. Responsiveness to ADH also decreases in older persons, making water balance a problem. The sense of thirst may also be diminished, which means they may become dehydrated. The effects of aging on the passing or voiding of urine affect both men and women: By the time they are 50 years old, 50% of men experience benign prostatic hyperplasia (BPH). This will increase to 80% of men over 80.1 In this condition, the prostate enlarges toward its center, compressing the urethra. This makes emptying the bladder more difficult. Elderly women are prone to incontinence (urine leakage), especially if vaginal childbirths have weakened the pelvic floor muscles and external urethral sphincter. Disorders of the excretory system may or may not have anything to do with aging

Nitrogenous wastes

Ammonia is produced from the breakdown of amino acids. It is extremely toxic, but it is quickly converted by the liver to urea, a less toxic waste. Urea is the most common nitrogenous waste produced in the body, accounting for 50% of that waste. It is ultimately formed from the breakdown of proteins. Uric acid is formed from the breakdown of nucleic acids. Creatinine is formed from the breakdown of creatine phosphate, a stable energy storage molecule

Ultrasound

An imaging technique in which sound waves create visual images of internal structures. In the excretory system, ultrasound may be used to examine the urinary tract.

Computed tomography (CT)

An imaging technique used to visualize internal structures. The scan produces images in "slices" of areas throughout the body. In regard to excretory system disorders, CT can be used to determine changes in the organs located in the lower abdomen and pelvic regions.

Bladder Cancer

Bladder cancer is usually caused by transitional cell carcinoma. Risk factors for bladder cancer include smoking and exposure to certain drugs and chemicals. Symptoms include hematuria, anemia, dysuria, and possible pelvic pain. Diagnosis of bladder cancer involves the viewing of the urinary bladder using an endoscope (cystoscopy) and biopsy to determine the presence of cancerous cells. For cancers that are noninvasive, removal of the cancerous tissue followed by chemotherapy may be the preferred treatment method. In more severe cases, a cystectomy (removal of the urinary bladder) may be necessary.

Blood Flow to a Nephron

Blood enters the kidney through the renal artery. It travels through smaller and smaller arteries leading to the afferent arteriole, which feeds the glomerulus. From the glomerulus, blood flows out the efferent arteriole to the peritubular capillaries, which then feed into venules, to larger and larger veins, and, finally, to the renal vein that exits the kidney.

Kidney Stones

Calcium or uric acid can precipitate out of urine and form solid stones in the renal pelvis. Small stones often pass without notice, but larger stones can block the renal pelvis or ureters. The blocked flow of urine increases the pressure within the kidney and can result in damage to nephrons. The kidneys continue to produce urine whether the flow is blocked or not. This increases pressure on the stone. The continued pressure on the sharp-edged stone may cause it to move toward the bladder and may cause intense pain as it passes along the ureter. Possible treatments include medication to dissolve the stone, a procedure using sound waves to break up the stone (lithotripsy), and surgery to remove the stone.

Filtration

Filtration occurs between the glomerulus and the glomerular capsule of the renal corpuscle. The thin capillary walls of the glomerulus and the cells that cover them act as a filter that allows materials to cross, depending on size—small molecules may pass through, while larger molecules cannot. Filtration is a passive process (does not require energy). An example of filtration in your home is your coffee maker: Gravity forces water and the essence of coffee through the filter because they are small, while coffee grounds remain behind because they are big. However, gravity does not drive filtration in the kidney. Instead, high pressure (blood pressure) forces materials out of the glomerular capillaries to the space inside the glomerular capsule. Adjusting the diameter of the afferent and efferent arterioles regulates this high pressure and ultimately the glomerular filtration rate (GFR). If the diameter of the afferent arteriole is greater than that of the efferent arteriole, more blood can enter than can leave the glomerulus. This causes high pressure in the glomerulus, forcing materials out of the glomerular capillaries. The higher the pressure, the greater the glomerular filtration rate and the greater the amount of materials filtered. The direction of movement in filtration is from the capillaries to the tubules. The materials moved are water, some nitrogenous wastes, amino acids, glucose, and mineral salts, such as sodium and calcium. These materials comprise the beginnings of urine. Blood cells and proteins do not filter out because they are too big.

Glomerul/o

Glomerulus

Blood glucose levels are higher than normal in uncontrolled diabetes mellitus.

More glucose is filtered out of the blood in the glomerulus. Reabsorption is time-limited: It can occur only while the filtrate is flowing through the renal tubule. If there is more glucose in the filtrate than can be reabsorbed in that amount of time, not all of the glucose will be reabsorbed. So some of the excess glucose will be found in the urine excreted from the body. Although this does bring down the abnormally high blood glucose levels, the levels will rise again with the next meal. This is not a homeostasis mechanism; it is a sign that the body is not using glucose properly.

Consider the two statements that follow, which you may intuitively agree make sense. They sum up the kidneys' role in the homeostasis of fluids and electrolytes in the body. Once you grasp the goal, you can proceed to investigate how the goal is reached.

If the blood's concentration of solutes is higher than normal, the kidneys will put out small volumes of concentrated (many solutes) urine. In this way, the kidneys conserve water in the blood and eliminate excess solutes from the blood. If the blood's concentration of solutes is lower than normal, the kidneys will put out large volumes of dilute (few solutes) urine. In this case, water is removed from the blood, and the solutes in the blood are conserved.

Diuretics

In addition to hormonal and nervous mechanisms of control, diuretics, such as alcohol, caffeine, and diuretic drugs, can also affect urine production. A diuretic is anything that increases urine volume. The following list contains more specific information about the various types of diuretics and their impact on the body: Alcohol inhibits the secretion of ADH. If large quantities of alcohol are consumed, this effect may be so great that the large quantity of dilute urine that is excreted may actually exceed the amount of fluids consumed. This can lead to dehydration and excessive thirst. Caffeine increases the blood flow to the kidney, and this increases the glomerular filtration rate. It also decreases the amount of sodium reabsorbed, so the net effect is large volumes of urine containing sodium. Diuretic drugs are often prescribed for hypertensive patients to reduce their blood pressure. Many of these drugs work by inhibiting the active transport of sodium. These drugs result in large volumes of urine containing sodium. Although reducing blood volume through increased urine production does decrease blood pressure, care must be given in administering these medications so that the body's electrolytes remain balanced and homeostasis is maintained.

Aldosterone

Is a mineralocorticoid produced in the adrenal cortex that targets the kidneys. It maintains homeostasis by regulating the amount of active transport in the nephron. If aldosterone levels are up, more sodium ions are actively transported from the tubule to the peritubular capillaries and more potassium ions are secreted. Water follows the sodium by osmosis, so more water moves to the peritubular capillaries too. The result of aldosterone secretion is reduced urine output, with both water and sodium conserved in the blood, and increased potassium in urine. In the case of ADH, the hypothalamus monitors the blood and sends the signal for ADH release. How does the adrenal cortex know when to secrete aldosterone? The juxtaglomerular apparatus, regulates aldosterone secretion. The juxtaglomerular apparatus monitors blood traveling through the afferent arteriole. It also secretes renin under any of the conditions listed here: Blood pressure falls (hypotension). The level of sodium in the blood is too low (hyponatremia). The level of potassium in the blood is too high (hyperkalemia). Renin is a chemical that helps maintain homeostasis by converting angiotensinogen, a protein from the liver, to angiotensin I. An enzyme called angiotensin-converting enzyme (ACE), produced in the lungs and kidneys, then converts angiotensin I to angiotensin II. Angiotensin II targets the adrenal cortex, telling it to secrete aldosterone. Aldosterone's effect on urine production prevents a further drop in blood pressure from the loss of water to urine, maintains the level of sodium in the blood, and lowers the level of potassium in the blood.

Ren/o

Kidney

Reabsorption

Reabsorption begins between the proximal convoluted tubule and the peritubular capillaries and continues along the renal tubule. In this process, 100% of the glucose, 100% of the amino acids, and variable amounts of the mineral salts that were filtered out of the blood are actively transported (requiring energy) from the tubules to the capillaries. In addition, 99% of the water that was filtered into the glomerular capsule is reabsorbed by osmosis into the bloodstream.

Functions of the Excretory System

Removal of metabolic wastes. The skin removes small amounts of urea with sweat, lungs remove CO2 from blood, liver removes bilirubin from the breakdown of hemoglobin and puts it in bile, spleen puts bilirubin in plasma for the kidneys to remove, and kidneys remove nitrogenous wastes. Maintenance of the body's fluid and electrolyte balance. Eating salty popcorn, but not drinking fluids. The net result is increased sodium in blood. kidneys will conserve water to maintain blood volume while allowing the excess sodium to exit with urine. net urine output will be a small volume of concentrated urine. Maintenance of the body's acid-base balance. kidneys secrete any excess H+ in the blood, and the lungs remove CO2 from the blood with every breath to maintain blood pH at homeostatic levels. The respiratory rate will fluctuate with the blood pH, increasing if the blood pH falls and decreasing if the pH rises. Regulation of blood pressure. All the salty popcorn is likely to increase thirst. The hypothalamus monitors the sodium level of the blood. Because it is high from all the salty popcorn and lack of additional fluids, hypothalamus increases the thirst. Once begin to drink fluids again, blood volume will rise. If the increased fluid intake increases blood volume and therefore blood pressure too much, the kidneys will produce an increased volume of urine to reduce blood volume and reduce blood pressure.

The two forms of acidosis based on cause are described in the following list:

Respiratory acidosis happens if the respiratory system cannot eliminate sufficient CO2. For example, a patient with emphysema may not have sufficient ventilation of the alveoli in his lungs due to the breakdown of his alveolar walls. So his respiratory system cannot eliminate enough CO2 to keep up with the CO2 produced from cellular respiration in his tissues and homeostasis cannot be maintained. Metabolic acidosis happens if there is decreased kidney elimination of hydrogen ions or increased production of acidic substances through metabolism. For example, anaerobic respiration results in lactic acid buildup in the muscles, which makes its way into the bloodstream. Metabolic acidosis can also occur in diabetics who have poor control of their blood sugars. You have already learned that diabetes mellitus is a wasting disease in which the body may need to break down fats and proteins for energy because it cannot use the sugar in the blood. This type of metabolism produces acidic ketones, and their presence in the blood lowers the pH. Whatever the cause, whenever the body goes into acidosis, both the respiratory and the excretory systems will try to fix the imbalance. The respiratory system increases the respiratory rate (hyperventilation), and the excretory system increases secretion of the excess hydrogen ions to bring the pH of the blood back to homeostasis.

two forms of alkalosis based on cause:

Respiratory alkalosis occurs during hyperventilation. In this case, too much CO2 is being blown off and homeostasis cannot be maintained. Metabolic alkalosis is relatively rare, but it can occur if there is prolonged vomiting, which results in the repeated loss of stomach acids. Whatever the cause, whenever the body goes into alkalosis, the respiratory system will try to fix it by reducing the respiratory rate. This keeps more CO2 in the blood. The kidneys can raise the pH of the blood by secreting excess H+ during secretion, but they cannot add more H+ to the blood to lower the pH. The kidneys can manage only whatever amount of H+ is in the blood in the first place. They cannot add to it

Neural control of micturition

Steps 1 to 3 involve a reflex, and steps 4 to 6 involve higher brain control.

Flow of Urine Components through a Nephron

The glomerular capsule catches whatever is removed from the blood in the glomerulus. This material collected by the glomerular capsule is called filtrate. . Materials flow in this direction: Glomerular capsule → PCT → nephron loop → DCT → collecting duct → minor calyx The efferent arteriole leads to a complex capillary bed surrounding the renal tubule—the peritubular capillaries. The two capillary beds—the glomerulus and the peritubular capillaries—form a portal route so that materials can be exchanged twice between the nephron and the blood in the capillaries before the blood exits the kidney.

Anatomy of the Kidney

The kidneys are dark red, bean-shaped organs about the size of a tightly clenched fist. Like the pancreas, the kidneys are retroperitoneal (posterior to the parietal peritoneum). The kidneys extend from T11 to L3 (vertebrae) on each side of the vertebral column and are somewhat protected superiorly by the ribs. The right kidney is slightly lower than the left due to the position of the liver. Fibrous renal capsule, surrounded by adipose tissue (perirenal fat capsule), also protects the kidney. This fatty pad absorbs the mechanical shock to the kidney that may occur with a fall. Renal fascia (a connective tissue covering) anchors the kidney to the posterior muscle wall of the body's abdomen.

Ureters

The minor and major calyces of the kidney deliver urine to the renal pelvis. The ureters are an extension of the renal pelvis. Like the kidneys, these muscular tubes are also retroperitoneal. The ureters carry urine from the renal pelvis to the urinary bladder. Each ureter travels posterior to the urinary bladder and enters the bladder at its base. A small flap at the ureter's opening to the bladder prevents backflow of urine to the ureter when the urinary bladder contracts.

Kidney Cancer

The most common type of kidney cancer is renal cell carcinoma. The malignant tumors involved in renal cell carcinoma are adenocarcinomas (tumors that originate in glandular epithelial tissue). Renal cell carcinoma usually affects people between the ages of 50 and 70 years, occurring more often in men than in women. Other risk factors for renal cell carcinoma include smoking, obesity, use of certain drugs, exposure to certain chemicals, and a history of polycystic kidney disease. Renal cell carcinoma can go undetected for some time because symptoms often do not appear until the cancer has progressed, making it harder to treat. Symptoms include blood in the urine (hematuria), pain between the lower back and the upper abdomen (flank), fever, and the presence of a mass. Renal cell carcinoma is usually diagnosed by CT scan or MRI, and its treatment depends on the stage of the cancer. Surgery to remove the kidney (nephrectomy) may be performed.

Secretion

The nephron removes the rest of the wastes that remain in the blood. In this process, materials move from the peritubular capillaries to the tubules. These materials include the rest of the nitrogenous wastes (those that could not be filtered because of their size), excess hydrogen ions, excess potassium, and the by-products of drug metabolism. Removing excess hydrogen ions is crucial if the blood's pH is to remain in the homeostatic range of 7.35 to 7.45. Acidosis results if the blood's pH falls below this range, and alkalosis results if blood pH rises above this range. Both conditions are potentially lethal. Acidosis may first be seen as disorientation that may lead to coma. Alkalosis may start with hyperexcitability of the PNS along with spontaneous stimulation of muscle contractions and continue to spasms, convulsions, and possible death.

Micturition

The passing or voiding of urine. A micturition reflex controls the voiding of urine in infants, but once toilet training has been accomplished, impulses from higher centers in the brain can, and likely will, influence the reflex. Reflex explained 1-The reflex arc for micturition begins with stretch receptors in the urinary bladder's walls (afferent neurons). 2-As the bladder fills, these receptors send signals to the sacral region of the spinal cord (integration center). 3-Parasympathetic neurons go from the spinal cord to the detrusor muscle (effector), causing it to contract, and to the internal urethral sphincter (effector), causing it to relax. Urine is then voided. 4-At the same time as step 3, the afferent signals are also sent to the pons and ultimately to the cerebrum. 5-If it is timely to pass urine, the pons sends a signal to the sacral region of the spinal cord. 6-Motor neurons send signals to the external urethral sphincter, causing it to relax so that urine can be passed. However, if it is not timely to pass urine, the pons will respond to the afferent signals from the stretch receptors by sending inhibitory signals to the external urethral sphincter. These signals will prevent the external sphincter from relaxing in order to retain urine in the bladder.

Action of antidiuretic hormone.

The pathway shown in red represents the negative feedback mechanism to restore homeostasis.

Action of atrial natriuretic hormone.

The pathways shown in red represent negative feedback mechanisms to restore homeostasis.

Renal Corpuscle

The renal corpuscle is like an elaborate filter in a cup. It is composed of a glomerulus and a glomerular capsule (Bowman's capsule). An afferent arteriole delivers blood to a capillary bed called the glomerulus (the filter) inside the glomerular capsule (the walls of the cup). Cells of the capsule extend over each of the capillaries in the glomerulus, forming a filtration membrane. Whatever is filtered out of the blood through this membrane is caught in the glomerular capsule space and delivered to the next part of the nephron—the renal tubule. Meanwhile, the blood in the glomerular capillaries continues on its journey out of the renal corpuscle through the efferent arteriole.

organs and the excretions they are responsible for carrying out are explained in the following list:

The skin removes some salts, lactic acid, and urea with sweat. The lungs remove carbon dioxide with every humidified, expired breath. The liver removes bilirubin by putting it in bile. The kidneys remove nitrogenous wastes (wastes containing nitrogen), excess minerals, bilirubin, and excess hydrogen ions by producing urine. water is often used to eliminate metabolic wastes. Sweat, humidified air, bile, and urine all contain water.

Nervous System Mechanisms of Control

The sympathetic nervous system exerts its control of urine production during heavy exercise or acute conditions like a traumatic drop in blood pressure that may occur with sudden blood loss. In these cases, sympathetic neurons cause constriction in the kidney's afferent arterioles. Because this reduces the amount of blood entering the glomeruli, the glomerular filtration rate is also decreased. In order to maintain homeostasis, blood is diverted from the kidney and sent to the brain, heart, and skeletal muscles instead.

Urethra

The urethra is a tube that delivers urine from the urinary bladder to the outside. It begins at the internal urinary sphincter at the urinary bladder's base. The urethra passes through the pelvic floor, where it is encircled by a skeletal muscle called the external urethral sphincter. Since it is skeletal muscle, the external urinary sphincter is under voluntary control. The biggest difference in male and female urinary anatomy is the length of the urethra. A female urethra is approximately 3 to 4 cm and opens to the outside (external urethral orifice) between the clitoris and the vaginal opening. The male urethra is approximately 11 to 18 cm long and can be divided into three distinct areas: The prostatic urethra is surrounded by the prostate; the membranous urethra penetrates the pelvic floor; the longest section, the penile urethra, passes through the length of the penis to the external urethral orifice. Male reproductive structures, such as the prostate gland and bulbourethral glands, also secrete fluids into the male urethra.

Urinary Bladder

The urinary bladder is covered by the parietal peritoneum superiorly, and it sits posterior to the pubic symphysis. The urinary bladder functions to store urine until its release. So the bladder's anatomy has the ability to stretch. The mucosa lining the bladder is transitional epithelial tissue, and the lining has many folds (rugae) that are less conspicuous when the bladder is stretched. Urine fills the bladder from the bottom. As a result, the urinary bladder stretches and expands upward as it fills. The rugae flatten, and the transitional epithelium gets thinner. The maximum amount the bladder can hold is 700 to 800 mL. A feeling of fullness is typically felt at 500 mL. Three openings at the base of the bladder—two ureters and a urethral opening—define a triangular area called the trigone. This area is often the site of infection in the urinary bladder. Three layers of smooth muscle make up the detrusor muscle of the bladder's walls. This muscle is very important in the physiology of passing urine. At the base of the bladder, the detrusor muscle thickens to form the internal urethral sphincter. This muscle compresses the tube leading from the bladder (the urethra), so urine remains in the bladder.

Atrial Natriuretic Hormone

This is the third hormone that helps maintain homeostasis by regulating urine production. Unlike the first two hormones—ADH and aldosterone—this hormone is likely to be new to you. Cells in the right atrium of the heart produce atrial natriuretic hormone (ANH) when the blood pressure in the right atrium is too high.

Patients with high blood pressure (hypertension) may be prescribed ACE inhibitors

This medication interferes with the enzyme that converts angiotensin I to angiotensin II. Aldosterone is produced in the adrenal cortex only when angiotensin II fits into receptors. So ACE inhibitors inhibit aldosterone secretion. Without aldosterone, urine output is increased and blood volume (and therefore blood pressure) is reduced.

The region on the floor of the urinary bladder bounded by the two ureteral orifices and the urethral orifice is called the

Trigone

Urethr/o

Urethra

Cyst/o

Urinary bladder

What is the hollow muscular organ that lies just posterior to the symphysis pubis that holds urine?

Urinary bladder

Ur/o

Urinary tract, urine

Regulation of Urine Volume and Concentration

You can significantly adjust the intake of water and electrolytes by what you consume. However, the kidney's urine production is the only way to significantly adjust the amount of water and electrolytes in the blood through what leaves the body. The kidney cannot increase the amount of water or electrolytes in the blood, but it can prevent their loss by adjusting the amount of water and electrolytes that may exit during urine production. The principal electrolyte in this process is sodium. Where sodium goes, water usually follows. The volume of urine is determined by the amount of water in it, while the concentration of urine is determined by the relative amount of solutes it contains. Sodium is a very important solute in the regulation of urine volume. Another important electrolyte in urine production is potassium.

kidney's external and internal anatomy

You can see a notch on the medial surface of the kidneys. This notch in a kidney is called the renal hilum. As with the hilum in the lung, all structures entering or leaving the kidney do so at the hilum. The renal artery enters the kidney, while the renal vein and the renal pelvis (leading to the ureter) exit the kidney. If you were to grab hold and remove the renal pelvis, renal artery, and renal vein from the kidney, you would be left with the space they occupied. This space is the renal sinus. Adipose tissue fills whatever space is available in the renal sinus.

Renal Failure

You really need only part of one kidney to carry out the necessary functions for a normal life. Yet the body has two kidneys, and this provides tremendous reserve capacity. The ability to clear nitrogenous waste from the blood can be measured by assessing the blood urea nitrogen (BUN). This blood test expresses the amount of one of the nitrogenous wastes—urea—in the blood. Slightly higher levels indicate renal insufficiency (azotemia). Seriously elevated levels indicate uremia, characterized by the vomiting, diarrhea, and arrhythmias associated with increased nitrogenous waste in the blood. Complete kidney failure usually results in convulsions, coma, and death within a few days. Treatment for kidney failure is kidney transplantation or dialysis, in which a machine filters the excess fluid, salt, and nitrogenous wastes in the blood.

Water in the Body

Your body is approximately 50% to 75% water. Men tend to have slightly more water than women because women deposit more fat, which does not contain much water. Body water is located in two major fluid compartments—intracellular and extracellular. Sixty-five percent of body water is in the cytoplasm of your cells (intracellular). The other 35% of water is outside your cells (extracellular), as tissue fluid, blood plasma, lymph, CSF, synovial fluid, fluids of the eye (humors), bile, and serous fluid. Water moves between the two fluid compartments by osmosis, traveling easily across membranes to equalize the concentration of solutes on both sides and maintain homeostasis. Osmosis occurs very quickly to minimize the formation of concentration gradients of solutes in order to maintain homeostasis. Most of the solutes in the fluids are electrolytes, such as sodium in the extracellular fluids and potassium in the intracellular fluids. Fluid and electrolyte balance are therefore tied together. Water enters the body through the fluids you drink. This is the major source of water for the body, but not the only one. You may remember that water is also formed in the cells through cellular respiration (C6H12O6 + 6O2 → 6CO2 + 6H2O + energy). This additional source is considered to be metabolic water because it is derived from a chemical process that occurs in the cells. The body's daily intake and output of water should be equal to maintain homeostasis. Although most of the water you take in is from drinks, food and metabolic water do make significant contributions to water balance. At the same time, urine output is the major way the body rids itself of water, while sweat, water evaporated from the skin, expired air, and feces also make significant contributions to the amount of water leaving the body. Since urine output is so vital to maintaining fluid and electrolyte balance in the body, we focus next on how urine production and its volume are regulated.

Urethritis

an infection of the urethra, which can be caused by bacteria, viruses, or fungi. Usually, the bacteria that cause this infection reside in or around the anus and make their way to the urethra, where infection can result. Urethritis is more common in women because the opening of the urethra is closer to the anus than it is in men. Other organisms, like those that cause sexually transmitted infections, can also cause urethritis in both men and women. Symptoms of urethritis include painful and frequent urination. A discharge may also be present if the cause of the infection is a sexually transmitted organism. If urethritis is not treated, the infection can travel up to other parts of the excretory system and continue to cause infections such as cystitis.

Cystitis

an inflammation of the urinary bladder, usually caused by a bacterial infection. It is far more common in women than men because the pathway for the bacteria to the urinary bladder (the urethra) is far shorter in women. The symptoms include the frequent passing of small amounts of urine accompanied by a burning sensation. The prevalence of cystitis is increased when women become sexually active because of the introduction of more bacteria to the genital area. The infection can travel up the ureters to the renal pelvis (pyelitis) and even on to the renal cortex (pyelonephritis). In general, cystitis, pyelitis, and pyelonephritis are classified as urinary tract infections and are treated with antibiotics.

Polycystic kidney disease (PKD)

an inherited disorder that causes multiple cysts to form on the kidneys. The cysts interrupt the normal function of the kidney, causing high blood pressure and kidney infections. PKD can be diagnosed using CT, MRI, intravenous pyelography, and blood tests. PKD is an inherited kidney disease passed down to family members as an autosomal dominant trait; this means that if one parent carries the gene for PKD, the offspring have a 50% chance of developing the disease. Genetic testing can be done to determine whether a person has the PKD gene and is at risk for developing the disease.

There are three main hormones that regulate urine production in the kidneys.

antidiuretic hormone (ADH), aldosterone, and atrial natriuretic hormone (ANH) Both ADH and aldosterone result in less urine produced, but they do so in very different ways. On the other hand, ANH increases urine production.

Kidneys

are the primary organs of this system. The excretory system is sometimes referred to as the urinary system when the focus is on only the kidneys and their urine production. The nitrogenous wastes removed by the kidneys can be lethal to the body if they are allowed to accumulate in the blood in excessive amounts.

Urine production involves three processes:

filtration, reabsorption, and secretion.

Urinary tract infections (UTIs)

infections that affect any structure—including the urethra, urinary bladder, ureters, and kidneys—along the urinary tract. You will explore a few of these infections in the following paragraphs.

Glomerulonephritis

inflammation of the filtration membrane in the glomerulus of the nephron. There are two forms of this disorder—acute and chronic. Acute glomerulonephritis usually occurs 1 to 3 weeks after a severe bacterial infection in the body. Antibodies are produced to fight the infection, and in doing so, they attach to antigens. The antibody-antigen complexes batter the walls of the glomeruli in the kidneys due to the increased pressure. As a result of the irritation, the filtration membrane in the renal corpuscle becomes inflamed and more permeable, allowing plasma proteins and leukocytes in the filtrate. Water follows the plasma proteins, resulting in higher-than-normal volumes of urine containing protein and blood cells. Acute glomerulonephritis is usually time-limited. Once the antibody-antigen complexes are cleared from the blood by macrophages, the inflammation is resolved. Chronic glomerulonephritis is just that—chronic. In this form of glomerulonephritis, the constant irritation to the filtration membrane causes it to thicken and be replaced by connective tissue. This may decrease the amount of filtration to the point of renal failure.

Antidiuretic Hormone

is produced by the hypothalamus, but it is stored and then released from the posterior pituitary when commanded by the hypothalamus. The hypothalamus monitors blood sodium concentration and blood pressure. If blood sodium concentration increases or blood pressure falls, the hypothalamus sends nerve signals to the posterior pituitary telling it to release ADH. ADH targets distal convoluted tubules and collecting ducts in the nephron. The effect of ADH on these structures is that it makes them more permeable, so more water is reabsorbed. This decreases water loss to urine and therefore helps maintain blood volume, blood pressure, and homeostasis. It is important to note that ADH has an effect on only water reabsorption, not sodium reabsorption. Under the influence of ADH, the kidneys conserve water, but not sodium. Sodium is allowed to exit with urine, reducing the sodium concentration in the blood.

Overview of Kidney Function

kidneys are important in the body's homeostasis of calcium through their role in vitamin D synthesis. They are also important to the cardiovascular system in several ways. For example, the kidneys produce erythropoietin to stimulate red blood cell production when blood oxygen levels are low. The kidneys also help regulate blood volume, blood pressure, and the blood's concentration of solutes by adjusting the amount of water they use to produce urine

Metabolic wastes

wastes produced by the cells—bilirubin is a good example