# Human Physiology/The Urinary System

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## Introduction

The **Urinary System** is a group of organs in the body concerned with
filtering out excess fluid and other substances from the bloodstream.
The substances are filtered out from the body in the form of **urine**.
Urine is a liquid produced by the kidneys, collected in the bladder and
excreted through the urethra. Urine is used to extract excess minerals
or vitamins as well as blood corpuscles from the body. The Urinary
organs include the kidneys, ureters, bladder, and urethra. The Urinary
system works with the other systems of the body to help maintain
homeostasis. The kidneys are the main organs of homeostasis because they
maintain the acid base balance and the water salt balance of the blood.

## Functions of the Urinary System

One of the major functions of the Urinary system is the process of
excretion. Excretion is the process of eliminating, from an organism,
waste products of metabolism and other materials that are of no use. The
urinary system maintains an appropriate fluid volume by regulating the
amount of water that is excreted in the urine. Other aspects of its
function include regulating the concentrations of various electrolytes
in the body fluids and maintaining normal pH of the blood. Several body
organs carry out excretion, but the kidneys are the most important
excretory organ. The primary function of the kidneys is to maintain a
stable internal environment (homeostasis) for optimal cell and tissue
metabolism. They do this by separating urea, mineral salts, toxins, and
other waste products from the blood. They also do the job of conserving
water, salts, and electrolytes. At least one kidney must function
properly for life to be maintained. **Six important roles of the kidneys
are:**

**Regulation of plasma ionic composition.** Ions such as sodium,
potassium, calcium, magnesium, chloride, bicarbonate, and phosphates are
regulated by the amount that the kidney excretes.

**Regulation of plasma osmolarity.** The kidneys regulate osmolarity
because they have direct control over how many ions and how much water a
person excretes.

**Regulation of plasma volume.** Your kidneys are so important they even
have an effect on your blood pressure. The kidneys control plasma volume
by controlling how much water a person excretes. The plasma volume has a
direct effect on the total blood volume, which has a direct effect on
your blood pressure. Salt(NaCl)will cause osmosis to happen; the
diffusion of water into the blood.

**Regulation of plasma hydrogen ion concentration (pH).** The kidneys
partner up with the lungs and they together control the pH. The kidneys
have a major role because they control the amount of bicarbonate
excreted or held onto. The kidneys help maintain the blood Ph mainly by
excreting hydrogen ions and reabsorbing bicarbonate ions as needed.

**Removal of metabolic waste products and foreign substances from the
plasma.** One of the most important things the kidneys excrete is
nitrogenous waste. As the liver breaks down amino acids it also releases
ammonia. The liver then quickly combines that ammonia with carbon
dioxide, creating **urea** which is the primary nitrogenous end product
of metabolism in humans. The liver turns the ammonia into urea because
it is much less toxic. We can also excrete some ammonia, creatinine and
uric acid. The **creatinine** comes from the metabolic breakdown of
creatine phospate (a high-energy phosphate in muscles). **Uric acid**
comes from the break down of nucleotides. Uric acid is insoluble and too
much uric acid in the blood will build up and form crystals that can
collect in the joints and cause gout.

**Secretion of Hormones** The endocrine system has assistance from the
kidney\'s when releasing hormones. Renin is released by the kidneys.
Renin leads to the secretion of aldosterone which is released from the
adrenal cortex. Aldosterone promotes the kidneys to reabsorb the sodium
(Na+) ions. The kidneys also secrete erythropoietin when the blood
doesn\'t have the capacity to carry oxygen. Erythropoietin stimulates
red blood cell production. The Vitamin D from the skin is also activated
with help from the kidneys. Calcium (Ca+) absorption from the digestive
tract is promoted by vitamin D.

**CC: Chapter Check: Name the role of the kidneys and how they work?**

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## Organs in the Urinary System

### Kidneys And Their Structure

!Kidney Diagram: 1. Renal pyramid 2. Interlobar artery 3. Renal artery
4. Renal vein 5. Renal hylum 6. Renal pelvis 7. Ureter 8. Minor calyx 9.
Renal capsule 10. Inferior renal capsule 11. Superior renal capsule 12.
Interlobar vein 13. Nephron 14. Minor calyx 15. Major calyx 16. Renal
papilla 17. Renal
column{width="250"}

The **kidneys** are a pair of bean shaped, brown organs about the size
of your fist.It measures 10-12 cm long. They are covered by the renal
capsule, which is a tough capsule of fibrous connective tissue. Adhering
to the surface of each kidney is two layers of fat to help cushion them.
There is a concaved side of the kidney that has a depression where a
renal artery enters, and a renal vein and a ureter exit the kidney. The
kidneys are located at the rear wall of the abdominal cavity just above
the waistline, and are protected by the ribcage. They are considered
retroperitoneal, which means they lie behind the peritoneum. There are
three major regions of the kidney, **renal cortex**, **renal medulla**
and the **renal pelvis**. The outer, granulated layer is the renal
cortex. The cortex stretches down in between a radially striated inner
layer. The inner radially striated layer is the renal medulla. This
contains pyramid shaped tissue called the renal pyramids, separated by
renal columns. The ureters are continuous with the renal pelvis and is
the very center of the kidney.

#### Renal Vein

The **renal veins** are veins that drain the kidney. They connect the
kidney to the inferior vena cava. Because the inferior vena cava is on
the right half of the body, the left renal vein is generally the longer
of the two. Unlike the right renal vein, the left renal vein often
receives the left gonadal vein (left testicular vein in males, left
ovarian vein in females). It frequently receives the left suprarenal
vein as well.

#### Renal Artery

The **renal arteries** normally arise off the abdominal aorta and supply
the kidneys with blood. The arterial supply of the kidneys are variable
and there may be one or more renal arteries supplying each kidney. Due
to the position of the aorta, the inferior vena cava and the kidneys in
the body, the right renal artery is normally longer than the left renal
artery. The right renal artery normally crosses posteriorly to the
inferior vena cava. The renal arteries carry a large portion of the
total blood flow to the kidneys. Up to a third of the total cardiac
output can pass through the renal arteries to be filtered by the
kidneys.

### Ureters

The **ureters** are two tubes that drain urine from the kidneys to the
bladder. Each ureter is a muscular tube about 10 inches (25 cm) long.
Muscles in the walls of the ureters send the urine in small spurts into
the bladder, (a collapsible sac found on the forward part of the cavity
of the bony pelvis that allows temporary storage of urine). After the
urine enters the bladder from the ureters, small folds in the bladder
mucosa act like valves preventing backward flow of the urine. The outlet
of the bladder is controlled by a sphincter muscle. A full bladder
stimulates sensory nerves in the bladder wall that relax the sphincter
and allow release of the urine. However, relaxation of the sphincter is
also in part a learned response under voluntary control. The released
urine enters the urethra.

### Urinary Bladder

The **urinary bladder** is a hollow, muscular and distendible or elastic
organ that sits on the pelvic floor (superior to the prostate in males).
On its anterior border lies the pubic symphysis and, on its posterior
border, the vagina (in females) and rectum (in males). The urinary
bladder can hold approximately 17 to 18 ounces (500 to 530 ml) of urine,
however the desire to micturate is usually experienced when it contains
about 150 to 200 ml. When the bladder fills with urine (about half
full), stretch receptors send nerve impulses to the spinal cord, which
then sends a reflex nerve impulse back to the sphincter (muscular valve)
at the neck of the bladder, causing it to relax and allow the flow of
urine into the urethra. The Internal urethral sphincter is involuntary.
The ureters enter the bladder diagonally from its dorsolateral floor in
an area called the trigone. The trigone is a triangular shaped area on
the postero-inferior wall of the bladder. The urethra exits at the
lowest point of the triangle of the trigone. The urine in the bladder
also helps regulate body temperature. A bladder when operating normally
empties completely upon a complete discharge, otherwise it is a sign
that its elasticity is compromised, when it becomes completely void of
fluid, it may cause a chilling sensation due to the rapid change of body
temperature.

### Urethra

!250 px\|thumb\|left\|Female urethra (labeled at bottom
right.)")
!200 px\|thumb\|right\|Male Sphincter urethrae muscle - The male
urethra laid open on its anterior (upper) surface. (Region visible, but
muscle not
labeled.) surface. (Region visible, but muscle not labeled.)")

The **urethra** is a muscular tube that connects the bladder with the
outside of the body. The function of the urethra is to remove urine from
the body. It measures about 1.5 inches (3.8 cm) in a woman but up to 8
inches (20 cm) in a man. Because the urethra is so much shorter in a
woman it makes it much easier for a woman to get harmful bacteria in her
bladder this is commonly called a bladder infection or a UTI. The most
common bacteria of a UTI is E-coli from the large intestines that have
been excreted in fecal matter. **Female urethra**

In the human female, the urethra is about 1-2 inches long and opens in
the vulva between the clitoris and the vaginal opening.

Men have a longer urethra than women. This means that women tend to be
more susceptible to infections of the bladder (cystitis) and the urinary
tract.

**Male urethra**

In the human male, the urethra is about 8 inches long and opens at the
end of the head of the penis.

The length of a male\'s urethra, and the fact it contains a number of
bends, makes catheterisation more difficult.

The **urethral sphincter** is a collective name for the muscles used to
control the flow of urine from the urinary bladder. These muscles
surround the urethra, so that when they contract, the urethra is closed.

-   There are two distinct areas of muscle: the internal sphincter, at
    the bladder neck and
-   the external, or distal, sphincter.

Human males have much stronger sphincter muscles than females, meaning
that they can retain a large amount of urine for twice as long, as much
as 800mL, i.e. \"hold it\".

### Nephrons

A nephron is the basic structural and functional unit of the kidney. The
name nephron comes from the Greek word (nephros) meaning kidney. Its
chief function is to regulate water and soluble substances by filtering
the blood, reabsorbing what is needed and excreting the rest as urine.
Nephrons eliminate wastes from the body, regulate blood volume and
pressure, control levels of electrolytes and metabolites, and regulate
blood pH. Its functions are vital to life and are regulated by the
endocrine system by hormones such as antidiuretic hormone, aldosterone,
and parathyroid hormone.

Each nephron has its own supply of blood from two capillary regions from
the renal artery. Each nephron is composed of an initial filtering
component (the renal corpuscle) and a tubule specialized for
reabsorption and secretion (the renal tubule). The renal corpuscle
filters out large solutes from the blood, delivering water and small
solutes to the renal tubule for modification.

#### Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from
an afferent arteriole of the renal circulation. The glomerular blood
pressure provides the driving force for fluid and solutes to be filtered
out of the blood and into the space made by Bowman\'s capsule. The
remainder of the blood not filtered into the glomerulus passes into the
narrower efferent arteriole. It then moves into the vasa recta, which
are collecting capillaries intertwined with the convoluted tubules
through the interstitial space, where the reabsorbed substances will
also enter. This then combines with efferent venules from other nephrons
into the renal vein, and rejoins with the main bloodstream.

**Afferent/Efferent Arterioles**

The afferent arteriole supplies blood to the glomerulus. A group of
specialized cells known as **juxtaglomerular cells** are located around
the afferent arteriole where it enters the renal corpuscle. The efferent
arteriole drains the glomerulus. Between the two arterioles lies
specialized cells called the **macula densa.** The juxtaglomerular cells
and the macula densa collectively form the **juxtaglomerular
apparatus.** It is in the juxtaglomerular apparatus cells that the
enzyme **renin** is formed and stored. Renin is released in response to
decreased blood pressure in the afferent arterioles, decreased sodium
chloride in the distal convoluted tubule and sympathetic nerve
stimulation of receptors (beta-adrenic) on the juxtaglomerular cells.
Renin is needed to form Angiotensin I and Angiotensin II which stimulate
the secretion of aldosterone by the adrenal cortex.

#### Glomerular Capsule or Bowman\'s Capsule

**Bowman\'s capsule** (also called the **glomerular capsule**) surrounds
the glomerulus and is composed of visceral (simple squamous epithelial
cells) (inner) and parietal (simple squamous epithelial cells) (outer)
layers. The visceral layer lies just beneath the thickened glomerular
basement membrane and is made of podocytes which send foot processes
over the length of the glomerulus. Foot processes interdigitate with one
another forming filtration slits that, in contrast to those in the
glomeruluar endothelium, are spanned by diaphragms. The size of the
filtration slits restricts the passage of large molecules (e.g.,
albumin) and cells (e.g., red blood cells and platelets). In addition,
foot processes have a negatively-charged coat (glycocalyx) that limits
the filtration of negatively-charged molecules, such as albumin. This
action is called electrostatic repulsion.

The parietal layer of Bowman\'s capsule is lined by a single layer of
squamous epithelium. Between the visceral and parietal layers is
Bowman\'s space, into which the filtrate enters after passing through
the podocytes\' filtration slits. It is here that smooth muscle cells
and macrophages lie between the capillaries and provide support for
them. Unlike the visceral layer, the parietal layer does not function in
filtration. Rather, the filtration barrier is formed by three
components: the diaphragms of the filtration slits, the thick glomerular
basement membrane, and the glycocalyx secreted by podocytes. 99% of
glomerular filtrate will ultimately be reabsorbed.

The process of filtration of the blood in the Bowman\'s capsule is
ultrafiltration (or glomerular filtration), and the normal rate of
filtration is 125 ml/min, equivalent to ten times the blood volume
daily. Measuring the glomerular filtration rate (GFR) is a diagnostic
test of kidney function. A decreased GFR may be a sign of renal failure.
Conditions that can affect GFR include: arterial pressure, afferent
arteriole constriction, efferent arteriole constriction, plasma protein
concentration and colloid osmotic pressure.

Any proteins that are roughly 30 kilodaltons or under can pass freely
through the membrane. Although, there is some extra hindrance for
negatively charged molecules due to the negative charge of the basement
membrane and the podocytes. Any small molecules such as water, glucose,
salt (NaCl), amino acids, and urea pass freely into Bowman\'s space, but
cells, platelets and large proteins do not. As a result, the filtrate
leaving the Bowman\'s capsule is very similar to blood plasma in
composition as it passes into the proximal convoluted tubule. Together,
the glomerulus and Bowman\'s capsule are called the renal corpuscle.

#### Proximal Convoluted Tubule (PCT)

The proximal tubule can be anatomically divided into two segments: the
proximal convoluted tubule and the proximal straight tubule. The
proximal convoluted tubule can be divided further into S1 and S2
segments based on the histological appearance of it\'s cells. Following
this naming convention, the proximal straight tubule is commonly called
the S3 segment. The proximal convoluted tubule has one layer of cuboidal
cells in the lumen. This is the only place in the nephron that contains
cuboidal cells. These cells are covered with millions of microvilli. The
microvilli serve to increase surface area for reabsorption.

Fluid in the filtrate entering the proximal convoluted tubule is
reabsorbed into the peritubular capillaries, including approximately
two-thirds of the filtered salt and water and all filtered organic
solutes (primarily glucose and amino acids). This is driven by sodium
transport from the lumen into the blood by the Na+/K+ ATPase in the
basolateral membrane of the epithelial cells. Much of the mass movement
of water and solutes occurs in between the cells through the tight
junctions, which in this case are not selective.

The solutes are absorbed isotonically, in that the osmotic potential of
the fluid leaving the proximal tubule is the same as that of the initial
glomerular filtrate. However, glucose, amino acids, inorganic phosphate,
and some other solutes are reabsorbed via secondary active transport
through cotransport channels driven by the sodium gradient out of the
nephron.

#### Loop of the Nephron or Loop of Henle

!The Nephron Loop or Loop of
Henle.{width="450"}

The loop of Henle (sometimes known as the nephron loop) is a U-shaped
tube that consists of a descending limb and ascending limb. It begins in
the cortex, receiving filtrate from the proximal convoluted tubule,
extends into the medulla, and then returns to the cortex to empty into
the distal convoluted tubule. Its primary role is to concentrate the
salt in the interstitium, the tissue surrounding the loop.

Descending limb:Its descending limb is permeable to water but completely impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more hypertonic than shorter limbs.

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Ascending limb:Unlike the descending limb, the ascending limb of Henle\'s loop is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex.

#### Distal Convoluted Tubule (DCT)

The distal convoluted tubule is similar to the proximal convoluted
tubule in structure and function. Cells lining the tubule have numerous
mitochondria, enabling active transport to take place by the energy
supplied by ATP. Much of the ion transport taking place in the distal
convoluted tubule is regulated by the endocrine system. In the presence
of parathyroid hormone, the distal convoluted tubule reabsorbs more
calcium and excretes more phosphate. When aldosterone is present, more
sodium is reabsorbed and more potassium excreted. Atrial natriuretic
peptide causes the distal convoluted tubule to excrete more sodium. In
addition, the tubule also secretes hydrogen and ammonium to regulate pH.
After traveling the length of the distal convoluted tubule, only 3% of
water remains, and the remaining salt content is negligible. 97.9% of
the water in the glomerular filtrate enters the convoluted tubules and
collecting ducts by osmosis.

#### Collecting ducts

Each distal convoluted tubule delivers its filtrate to a system of
collecting ducts, the first segment of which is the connecting tubule.
The collecting duct system begins in the renal cortex and extends deep
into the medulla. As the urine travels down the collecting duct system,
it passes by the medullary interstitium which has a high sodium
concentration as a result of the loop of Henle\'s countercurrent
multiplier system. Though the collecting duct is normally impermeable to
water, it becomes permeable in the presence of antidiuretic hormone
(ADH). As much as three-fourths of the water from urine can be
reabsorbed as it leaves the collecting duct by osmosis. Thus the levels
of ADH determine whether urine will be concentrated or dilute.
Dehydration results in an increase in ADH, while water sufficiency
results in low ADH allowing for diluted urine. Lower portions of the
collecting duct are also permeable to urea, allowing some of it to enter
the medulla of the kidney, thus maintaining its high ion concentration
(which is very important for the nephron).

Urine leaves the medullary collecting ducts through the renal papilla,
emptying into the renal calyces, the renal pelvis, and finally into the
bladder via the ureter. Because it has a different embryonic origin than
the rest of the nephron (the collecting duct is from endoderm whereas
the nephron is from mesoderm), the collecting duct is usually not
considered a part of the nephron proper.

**Renal Hormones**

1\. Vitamin D- Becomes metabolically active in the kidney. Patients with
renal disease have symptoms of disturbed calcium and phosphate balance.

2\. Erythropoietin- Released by the kidneys in response to decreased
tissue oxygen levels (hypoxia).

3\. Natriuretic Hormone- Released from cardiocyte granules located in
the right atria of the heart in response to increased atrial stretch. It
inhibits ADH secretions which can contribute to the loss of sodium and
water.

## Formation of Urine

Urine is formed in three steps: Filtration, Reabsorption, and Secretion.

### Filtration

Blood enters the afferent arteriole and flows into the glomerulus. Blood
in the glomerulus has both filterable blood components and
non-filterable blood components. Filterable blood components move toward
the inside of the glomerulus while non-filterable blood components
bypass the filtration process by exiting through the efferent arteriole.
Filterable Blood components will then take a plasma like form called
glomerular filtrate. A few of the filterable blood components are water,
nitrogenous waste, nutrients and salts (ions). Nonfilterable blood
components include formed elements such as blood cells and platelets
along with plasma proteins. The glomerular filtrate is not the same
consistency as urine, as much of it is reabsorbed into the blood as the
filtrate passes through the tubules of the nephron.

### Reabsorption

Within the peritubular capillary network, molecules and ions are
reabsorbed back into the blood. Sodium Chloride reabsorbed into the
system increases the osmolarity of blood in comparison to the glomerular
filtrate. This reabsorption process allows water (H2O) to pass from the
glomerular filtrate back into the circulatory system.

Glucose and various amino acids also are reabsorbed into the circulatory
system. These nutrients have carrier molecules that claim the glomerular
molecule and release it back into the circulatory system. If all of the
carrier molecules are used up, excess glucose or amino acids are set
free into the urine. A complication of diabetes is the inability of the
body to reabsorb glucose. If too much glucose appears in the glomerular
filtrate it increases the osmolarity of the filtrate, causing water to
be released into the urine rather than reabsorbed by the circulatory
system. Frequent urination and unexplained thirst are warning signs of
diabetes, due to water not being reabsorbed.

Glomerular filtrate has now been separated into two forms: Reabsorbed
Filtrate and Non-reabsorbed Filtrate. Non-reabsorbed filtrate is now
known as tubular fluid as it passes through the collecting duct to be
processed into urine.

### Secretion

Some substances are removed from blood through the peritubular capillary
network into the distal convoluted tubule or collecting duct. These
substances are Hydrogen ions, creatinine, and drugs. Urine is a
collection of substances that have not been reabsorbed during glomerular
filtration or tubular reabsorbtion.

## Maintaining Water-Salt Balance

It is the job of the kidneys to maintain the water-salt balance of the
blood. They also maintain blood volume as well as blood pressure. Simple
examples of ways that this balance can be changed include ingestion of
water, dehydration, blood loss and salt ingestion.

### Reabsorption of water

Direct control of water excretion in the kidneys is exercised by the
anti-diuretic hormone (ADH), released by the posterior lobe of the
pituitary gland. ADH causes the insertion of water channels into the
membranes of cells lining the collecting ducts, allowing water
reabsorption to occur. Without ADH, little water is reabsorbed in the
collecting ducts and dilute urine is excreted. There are several factors
that influence the secretion of ADH. The first of these happen when the
blood plasma gets too concentrated. When this occurs, special receptors
in the hypothalamus release ADH. When blood pressure falls, stretch
receptors in the aorta and carotid arteries stimulate ADH secretion to
increase volume of the blood.

### Reabsorption of Salt

The Kidneys also regulate the salt balance in the blood by controlling
the excretion and the reabsorption of various ions. As noted above, ADH
plays a role in increasing water reabsorption in the kidneys, thus
helping to dilute bodily fluids. The kidneys also have a regulated
mechanism for reabsorbing sodium in the distal nephron. This mechanism
is controlled by aldosterone, a steroid hormone produced by the adrenal
cortex. Aldosterone promotes the excretion of potassium ions and the
reabsorption of sodium ions. The release of Aldosterone is initiated by
the kidneys. The juxtaglomerular apparatus is a renal structure
consisting of the macula densa, mesangial cells, and juxtaglomerular
cells. Juxtaglomerular cells (JG cells, also known as granular cells)
are the site of renin secretion. Renin is an enzyme that converts
angiotensinogen (a large plasma protein produced by the liver) into
Angiotensin I and eventually into Angiotensin II which stimulates the
adrenal cortex to produce aldosterone. The reabsorption of sodium ions
is followed by the reapsorption of water. This causes blood pressure as
well as blood volume to increase.

**Atrial natriuretic hormone (ANH**) is released by the atria of the
heart when cardiac cells are stretched due to increased blood volume.
ANH inhibits the secretion of renin by the juxtaglomerular apparatus and
the secretion of the aldosterone by the adrenal cortex. This promotes
the excretion of sodium. When sodium is excreted so is water. This
causes blood pressure and volume to decrease.

#### Hypernatremia

An increase in plasma sodium levels above normal is **hypernatremia**.
Sodium is the primary solute in the extracellular fluid. Sodium levels
have a major role in osmolarity regulation. For excitable cells the
electrochemical gradient for sodium across the plasma membrane is
critical for life. Water retention and an increased blood pressure
usually are signs of hypernatremia. If the plasma sodium levels are
below normal it is called **hyponatremia**. Signs of this are low plasma
volume and hypotension.

#### Diuretics

A diuretic (colloquially called a water pill) is any drug that elevates
the rate of bodily urine excretion (diuresis). Diuretics also decrease
the extracellular fluid (ECF) volume, and are primarily used to produce
a negative extracellular fluid balance. Caffeine, cranberry juice and
alcohol are all weak diuretics. In medicine, diuretics are used to treat
heart failure, liver cirrhosis, hypertension and certain kidney
diseases. Diuretics alleviate the symptoms of these diseases by causing
sodium and water loss through the urine. As urine is produced by the
kidney, sodium and water -- which cause edema related to the disease --
move into the blood to replace the volume lost as urine, thereby
reducing the pathological edema. Some diuretics, such as acetazolamide,
help to make the urine more alkaline and are helpful in increasing
excretion of substances such as aspirin in cases of overdose or
poisoning. The antihypertensive actions of some diuretics (thiazides and
loop diuretics in particular) are independent of their diuretic effect.
That is, the reduction in blood pressure is not due to decreased blood
volume resulting from increased urine production, but occurs through
other mechanisms and at lower doses than that required to produce
diuresis. Indapamide was specifically designed with this is mind, and
has a larger therapeutic window for hypertension (without pronounced
diuresis) than most other diuretics. Chemically, diuretics are a diverse
group of compounds that either stimulate or inhibit various hormones
that naturally occur in the body to regulate urine production by the
kidneys. Alcohol produces diuresis through modulation of the vasopressin
system.

## Diseases of the Kidney

**Diabetic nephropathy** (nephropatia diabetica), also known as
Kimmelstiel-Wilson syndrome and intercapillary glomerulonephritis, is a
progressive kidney disease caused by angiopathy of capillaries in the
kidney glomeruli. It is characterized by nodular glomerulosclerosis. It
is due to longstanding diabetes mellitus, and is a prime cause for
dialysis in many Western countries. !An image of a **kidney
stone**. In medicine,
**hematuria** (or \"haematuria\") is the presence of blood in the urine.
It is a sign of a large number of diseases of the kidneys and the
urinary tract, ranging from trivial to lethal.

**Kidney stones**, also known as nephrolithiases, urolithiases or renal
calculi, are solid accretions (crystals) of dissolved minerals in urine
found inside the kidneys or ureters. They vary in size from as small as
a grain of sand to as large as a golf ball. Kidney stones typically
leave the body in the urine stream; if they grow relatively large before
passing (on the order of millimeters), obstruction of a ureter and
distention with urine can cause severe pain most commonly felt in the
flank, lower abdomen and groin. Kidney stones are unrelated to
gallstones.

**Case Study** I was 34 weeks pregnant when I noticed blood in my urine.
I immediately went to my OBGYN where I was told that I had a bladder
infection and given an antibiotic. The next morning I experienced the
most intense pain. I was rushed to the ER where I was told that I had
kidney stones. The doctors explained that there was nothing they could
do as long as I was pregnant. The next 3 weeks of my life were filled
with intense pain and multiple painkillers. After I delivered my baby,
CAT scans were done and I was informed that I had 6 kidney stones. It
took three more weeks for me to pass all of the stones the largest
measuring 5 mm. The stones were tested and I was informed that my body
had been building up calcium due to my pregnancy and this was the cause
of the kidney stones. I continued to have kidney pain for 6 months after
passing the stones. I now live my life on a low calcium diet and the
hope that my body will not develop more kidney stones.

**Pyelonephritis** When an infection of the renal pelvis and calices,
called pyelitis, spreads to involve the rest of the kidney as well, the
result is pyelonephritis. It usually results from the spread of fecal
bacterium Escherichia coli from the anal region superiorly through the
urinary tract. In severe cases, the kidney swells and scars, abscesses
form, and the renal pelvis fills with pus. Left untreated, the infected
kidney may be severely damaged, but administration of antibiotics
usually achieve a total cure.

**glomerulonephritis** Inflammation of the glomerular can be caused by
immunologic abnormalities, drugs or toxins, vascular disorders, and
systemic diseases. Glomerulonephritis can be acute, chronic or
progressive. Two major changes in the urine are distinctive of
glomerulonephritis: hematuria and proteinuria with albumin as the major
protein. There is also a decrease in urine as there is a decrease in GFR
(glomerular filtration rate). Renal failure is associated with oliguria
(less than 400 ml of urine output per day).

**Renal Failure** Uremia is a syndrome of renal failure and includes
elevated blood urea and creatinine levels. Acute renal failure can be
reversed if diagnosed early. Acute renal failure can be caused by severe
hypotension or severe glomerular disease. Diagnostic tests include BUN
and plasma creatinine level tests. It is considered to be chronic renal
failure if the decline of renal function to less than 25%.

## Diabetes Insipidus

This is caused by the deficiency of or decrease of ADH. The person with
(DI) has the inability to concentrate their urine in water restriction,
in turn they will void up 3 to 20 liters/day. There are two forms of
(DI), neurogenic, and nephrogenic. In nephrogenic (DI) the kidneys do
not respond to ADH. Usually the nephrogenic (DI) is characterized by the
impairment of the urine concentrating capability of the kidney along
with concentration of water. The cause may be a genetic trait,
electrolyte disorder, or side effect of drugs such as lithium. In the
neurogenic (DI), it is usually caused by head injury near the
hypophysisal tract.

## Urinary tract infections (UTI\'s)

The second most common type of bacterial infections seen by health care
providers is UTI\'s. Out of all the bacterias that colonize and cause
urinary tract infections the big gun is *Escherichia coli*. In the
hospital indwelling catheters and straight catheterizing predispose the
opportunity for urinary tract infections. In females there are three
stages in life that predispose urinary tract infections, that is
menarche, manipulation between intercourse, and menopause. However, a
small percentage of men and children will get urinary tract infections.
In men it is usually due to the prostate gland growth which usually
occurs in older age men. In children it can occur 3% to 5% in girls and
1% in boys, uncircumcised boys it is more common than circumcised ones
to have a urinary tract infection, in girls it may be the result of
onset of toilet training, some predispositions for getting urinary tract
infection include family history and urinary tract anomalies. In
neonates urinary tract infections is most common when bacteremia is
present.

## Dialysis and Kidney Transplant

!Plugged into
dialysis{width="200"}

Generally, humans can live normally with just one kidney. Only when the
amount of functioning kidney tissue is greatly diminished will renal
failure develop. If renal function is impaired, various forms of
medications are used, while others are contraindicated. Provided that
treatment is begun early, it may be possible to reverse chronic kidney
failure due to diabetes or high blood pressure. If creatinine clearance
(a measure of renal function) has fallen very low (\"end-stage renal
failure\"), or if the renal dysfunction leads to severe symptoms,
dialysis is commenced. Dialysis is a medical procedure, performed in
various different forms, where the blood is filtered outside of the
body.

Kidney transplantation is the only cure for end stage renal failure;
dialysis, is a supportive treatment; a form of \"buying time\" to bridge
the inevitable wait for a suitable organ.

The first successful kidney transplant was announced on March 4, 1954 at
Peter Bent Brigham Hospital in Boston. The surgery was performed by Dr.
Joseph E. Murray, who was awarded the Nobel Prize in Medicine in 1990
for this feat.

There are two types of kidney transplants: living donor transplant and a
cadaveric (dead donor) transplant. When a kidney from a living donor,
usually a blood relative, is transplanted into the patient\'s body, the
donor\'s blood group and tissue type must be judged compatible with the
patient\'s, and extensive medical tests are done to determine the health
of the donor. Before a cadaveric donor\'s organs can be transplanted, a
series of medical tests have to be done to determine if the organs are
healthy. Also, in some countries, the family of the donor must give its
consent for the organ donation. In both cases, the recipient of the new
organ needs to take drugs to suppress their immune system to help
prevent their body from rejecting the new kidney.

## Review Questions

Answers for these questions can be found here

1\. While reading a blood test I notice a high level of creatinine, I
could assume from this that

:   A\) There is a possibility of a UTI
:   B\) There is a possibility of diabetes
:   C\) There is a possibility of kidney failure
:   D\) There is nothing wrong, this is normal

2\. Direct control of water excretion in the kidneys is controlled by

:   A\) Anti-diuretic hormone
:   B\) The medulla oblongata
:   C\) Blood plasma
:   D\) Sodium amounts in the blood

3\. Nephrons

:   A\) Eliminate wastes from the body
:   B\) Regulate blood volume and pressure
:   C\) Control levels of electrolytes and metabolites
:   D\) Regulate blood pH
:   E\) All of the above

4\. If I am dehydrated, my body will increase

:   A\) ATP
:   B\) ADP
:   C\) Diluted urine
:   D\) ADH

5\. Which part of the nephron removes water, ions and nutrients from the
blood?

:   A ) vasa recta
:   B ) loop of henle
:   C ) proximal convoluted tubule
:   D ) peritubular capillaries
:   E ) glomerulus

6\. Kidneys have a direct effect on which of the following

:   A ) Blood pressure
:   B ) How much water a person excretes
:   C ) Total blood volume
:   D ) pH
:   E ) all of the above

7\. Why do substances in the glomerulus enter the Bowman\'s capsule?

:   A ) the magnetic charge of the Bowman\'s capsule attracts the
    substances
:   B ) the substances are actively transported into the Bowman\'s
    capsule
:   C ) blood pressure of the glomerulus is so great that most
    substances in blood move into capsule
:   D ) little green men force it in with their ray guns

8\. What happens in tubular excretion?

:   A ) urine bonds are formed between the wastes
:   B ) wastes are diffused from the tubule
:   C ) wastes move into the distal convoluted tubule from the blood
:   D ) blood pressure forces wastes away from the kidney

9\. The countercurrent exchange system
includes\_\_\_\_\_\_\_\_\_and\_\_\_\_\_\_\_\_\_.

:   A ) glomerulus and macula densa
:   B ) proximal convoluted tubule and distal convoluted tubule
:   C ) loop of Henle and collecting tubule
:   D ) afferent arteriole and efferent arteriole
:   E ) ureters and bladder

10\. The function of the loop of the nephron in the process of urine
formation is:

:   A ) reabsorption of water
:   B ) production of filtrate
:   C ) reabsorption of solutes
:   D ) secretion of solutes

11\. Name the six important roles of the kidneys.

## Glossary

**Antidiuretic:** lessening or decreasing of urine production or an
agent that decreases the release of urine.

**Catheterisation:** a catheter is a tube that can be inserted into a
body cavity, duct or vessel. Catheters thereby allow drainage or
injection of fluids or access by surgical instruments. The process of
inserting a catheter is catheterisation. In most uses a catheter is a
thin, flexible tube: a \"soft\" catheter; in some uses, it is a larger,
solid tube: a \"hard\" catheter.

**Dehydration:** condition resulting from excessive loss of body fluid.

**Diabetes:** a general term for a disease characterized by the
beginning stages and onset of renal failure. It is derived from the
Greek word diabaínein, that literally means \"passing through,\" or
\"siphon\", a reference to one of diabetes\' major symptoms---excessive
urine production.

**Diuresis:** secretion and passage of large amounts of urine.

**Diuretic:** increasing of urine production, or an agent that increases
the production of urine.

**Erythropoietin:** hormone that stimulates stem cells in the bone
marrow to produce red blood cells

**Fibrous Capsule:** the kidney\'s loose connective tissue

**Glomerulus:** capillary tuft that receives its blood supply from an
afferent arteriole of the renal circulation.

**Gluconeogenesis:** the cycle of producing a glucose form fat or
protein; preformed by the kidney in times of long fasting, initially
gluconeogenesis is preformed by the liver

**Juxtaglomerular (JG) cells:** Renin-secreting cells that are in
contact with the macula densa and the afferent arterioles of the renal
nephron.

**Juxtaglomerular apparatus (JGA):** A site of juxtaglomerular cells
connecting with the macula densa where renin is secreted and sensor for
control of secretion of golmerular filtration rate.

**Loop of Henle/ Nephron Loop:** u-shaped tube that consists of a
descending limb and ascending limb; primary role is to concentrate the
salt in the interstitium, the tissue surrounding the loop

**Medullary Pyramids or Renal Pyramids:** the cone shaped masses in the
kidney

**Micturition:** another name for excretions

**Nephron:** basic structural and functional unit of the kidney; chief
function is to regulate water and soluble substances by filtering the
blood, reabsorbing what is needed and excreting the rest as urine

**Podocytes:** filtration membrane, in the visceral layer of the
bowman\'s capsule

**Renal Calculi:** kidney stones, solid crystals of dissolved minerals
in urine found inside the kidneys

**Renal Cortex:** outer portion of the kidney

**Renal Lobe:** each pyramid together with the associated overlying
cortex

**Renal Pelvis:** a central space, or cavity that transmits urine to the
urinary bladder via the ureter

**Renin:** hormone released by the Juxtaglomerular (JG) cells of the
kidneys when blood pressure falls

**TURP:** transurethral resection of the prostate. During TURP, an
instrument is inserted up the urethra to remove the section of the
prostate that is blocking urine flow. This is most commonly caused by
benign prostatic hyperplasia (BPH). A TURP usually requires
hospitalization and is done using a general or spinal anesthetic. It is
now the most common surgery used to remove part of an enlarged prostate.

**Urethra:** a muscular tube that connects the bladder with the outside
of the body

**Ureters:** two tubes that drain urine from the kidneys to the bladder

**Urine:** liquid produced by the kidneys, collected in the bladder and
excreted through the urethra

**Urinary Bladder:** a hollow, muscular and distensible or elastic organ
that sits on the pelvic floor

**Urinary System:** a group of organs in the body concerned with
filtering out excess fluid and other substances from the bloodstream

## References

-   Graaff, Van De (2002). \"Human Anatomy, Sixth Edition\". New York:
    McGraw-Hill.
-   Mader, Sylvia S. (2004). *Human Biology*. New York: McGraw-Hill.
-   Smith, Peter (1998). Internet reference,
    \[<http://www.liv.ac.uk/~petesmif/teaching/1bds_mb/notes/homeo/kidney.htm>\|*The
    Role of the Kidney*\]. Department of Clinical Dental Sciences,The
    University of Liverpool.
-   McCance, Katherine L., Heuther, Sue E. (1994). \"Pathophysiology:
    The Biological Basis for Disease In Adults and Children, Second
    Edition\". Mosby-Year Book, Inc.
