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Chapter 15
The Urinary System
Functions of the Urinary System
 Elimination of waste products
 Nitrogenous wastes
 Toxins
 Drugs
 Regulate aspects of homeostasis
 Volume and chemical makeup of
the blood
 Water and electrolyte balance
 Acid-base balance in the blood
 Produce hormones
 Renin: regulates blood pressure
and kidney function
 Erythropoietin: red blood cell
production
Organs of the Urinary system
 Kidneys

Filters about 200 liters of fluid
daily (47 gallons!)

Major excretory organs
 Ureters

Transport urine from kidneys to
bladder
 Urinary bladder

Temporary storage reservoir for
urine
 Urethra

Transports urine from bladder to
the external environment
Location of the Kidneys

Bean-shaped organ

Lies in the superior lumbar region

Extends from T12 to L3

Right kidney is slightly lower than
the left

Average dimensions (about the size
of a bar of soap)

12 cm long; 6 cm wide; 3 cm thick

Lateral surface is convex

Medial surface is concave

Renal hilum


The ureter, blood vessels,
lymphatic vessels and
nerves all join the kidney
here
Atop each kidney is an adrenal
gland
Regions of the Kidney
 Renal cortex

Outer region
 Renal medulla

Inside the cortex

Exhibit medullary pyramids

Renal columns separate
the pyramids
 Renal pelvis

Inner funnel-shaped tube

Continuous with the ureter
leaving the hilum
Figure 15.2b
Kidney Structures
 Medullary pyramids
 Triangular regions of
tissue in the medulla
 Calyces
 Cup-shaped structures
that collect urine from
the medullary pyramids
and empty it into the
renal pelvis
 Major calyces and minor
calyces
Blood Flow in the Kidneys

As each renal artery approaches a kidney, it divides into five segmental
arteries

Each segmental artery branches further to form lobar arteries and then
interlobar arteries

The interlobar arteries branch into the arcuate arteries that arch over the
bases of the medullary pyramids

Small interlobular arteries radiate outward from the arcuate arteries to
supply the cortical tissue

Afferent arterioles branching from the interlobular arteries turn into
microscopic blood vessels called the glomerulus, which is the key element
of kidney function
Blood Flow in the Kidneys

Veins trace the pathway of arterial supply in reverse

Blood leaving the renal cortex (efferent arteriole) drains into the
interlobular veins, arcuate veins, interlobar vein and then renal
vein(notice no segmental veins) and then the renal vein empties into the
inferior vena cava
Blood Flow in the Kidneys
Nephrons
 The structural and functional units of the kidneys
 Kidneys contains over 1 million of these tiny blood-processing units
 Responsible for forming urine
 Each nephron consists of a glomerulus (capillaries) and renal tubule
 The renal tubule has a cup-shaped end called the glomerular
capsule or Bowman’s capsule
Glomerulus
 A specialized capillary bed
 Attached to arterioles on both sides
(maintains high pressure)
 Large afferent arteriole
 Narrow efferent arteriole
 Endothelium of the capillaries is very
porous
 This allows large amounts of solute-rich,
protein free fluid to pass from the blood to
the glomerular capsule
 Filtrate contains everything found in blood
plasma except proteins
 Urine contains mostly metabolic wastes
and unneeded substances
Figure 15.3c
Renal Tubule

Four parts to the renal tubules

Glomerular (Bowman’s) capsule

Proximal convoluted tubule (PCT)


Walls are cuboidal epithelial cells
with dense microvilli

Increases the surface area to
reabsorb water and solutes from
the filtrate
Loop of Henle

Descending end walls are similar
to PCT

Distal convoluted tubule (DCT)

Empties into a collecting duct

The length enhances its filtrate
processing capabilities
Figure 15.3b
Renal Tubule
 Collecting ducts

Receive filtrate from
many nephrons

Run through the
medullary pyramids

As they reach the renal
pelvis, a couple fuse
together and deliver urine
into the minor calyces
Figure 15.3b
Nephron Capillary Beds
 The renal tubule of every
nephron is closely
associated with two capillary
beds
 Glomerulus
 Produces the filtrate
 Peritubular capillaries
 Reclaims most of the
filtrate
Nephron Capillary Beds

Glomerulus is specialized for
filtration

Blood pressure in glomerulus
is extremely high to easily
force fluids and solutes out of
the blood

Afferent arteriole is feeding the
glomerulus and it is larger in
diameter than the efferent
arteriole draining the bed

Between the blood and
glomerular capsule lies a
filtration membrane

Porous membrane that allows
free passage of all plasma
components (water and
solutes) but not blood cells
Nephron Capillary Beds

Peritubular capillaries

Arise from efferent arteriole of
the glomerulus

Cling close to the renal tubule
and empty into nearby venules

Normal, low pressure
capillaries

Readily absorb solutes and
water from collecting tubes

Most of the resulting filtrate
(99%) is reabsorbed by the
renal tubule and returned to
the blood in the peritubular
capillaries
Kidney Physiology: Urine Formation

A: Interlobular artery

F: Proximal convoluted tubule

B: Afferent arteriole

G: Loop of Henle

C: Efferent arteriole

H: Distal convoluted tubule

D: Bowman’s (glomerular) capsule

I: Collecting Duct

E: Glomerulus
Kidney Physiology: Urine Formation
 The total plasma filters into the renal tubules about every 22 minutes
 All of our plasma would be drained away as urine in less than 30
minutes were it not for the fact that most of the tubule contents are
quickly reclaimed and returned to the blood
Kidney Physiology: Urine Formation
1. Renal artery brings blood into each kidney.
2. Blood vessels branch off the main artery until they form the glomerulus
(specialized capillary bed)
3. Water and other small substances such as glucose, salts, amino acids and
urea are filtered out of the glomerulus and into the Bowman’s capsule.
4. As the filtrate flows through the renal tubule (PCT, loop of Henle and DCT)
most of the water and nutrients are reabsorbed back into the peritubular
capillaries that wrap around the nephrons.
5. Some materials are secreted back into the tubules from the blood.
6. The cleaned blood, which has slightly less water and much less waste
material, leaves each kidney in the renal vein to the inferior vena cava.
7. The yellow fluid that remains in the tubule is called urine.
8. Urine leaves each kidney through the ureter and flows into the urinary
bladder, where urine is stored.
Kidney Physiology: Urine Formation
• Pathway of Urine
• Bowman’s capsule (filtrate)Proximal convoluted tubule (filtrate) 
loop of Henle (filtrate) distal convoluted tubule (filtrate) 
collecting duct (urine)  minor calyces (urine)  major calyces
(urine)  ureter (urine)  bladder (urine)  urethra(urine)
Kidney Physiology: Urine Formation

Kidneys form urine in the nephrons and adjust
the blood composition with three major
processes


Glomerular filtration (#1)

Dump filtrate into renal tubules

Filters about 200 L daily and only 1.5L
leaves the body as urine
Tubular reabsorption (#2)

Kidneys reclaim what the body needs

Almost all the filtrate (99%)


Water, salt, glucose and amino acids

Not reabsorbed is uric acid, creatinine, urea

Anything not reabsorbed becomes urine
Tubular secretion (#3)

Fine-tuning the body’s chemical balance
Kidney Physiology: Urine Formation
 Step 1: Glomerular Filtration
 Passive, nonselective
process
 Glomerular blood pressure
is extremely high
 Pressure forces fluids and
solutes through a
membrane
 Small molecules such as
water, salts, bicarbonate,
hydrogen ions, urea,
glucose, amino acids and
some drugs
 Blood cells and large
molecules cannot pass
through the wall
Kidney Physiology: Urine Formation
 Step 2: Tubular Reabsorption
 Selective process
 Begins as soon as the filtrate
enters the proximal tubules
 Most organic nutrients are
completely reabsorbed
 Hormones regulate the
reabsorption of water and
many ions
 Depending on substances
transported, the reabsorption
can be passive or active
 Sodium ions are the single
most abundant cation in the
filtrate
Kidney Physiology: Urine Formation

Step 2: Tubular Reabsorption




Reabsorptive abilities of regions
of the renal tubules
Proximal convoluted tubule (PCT)

Most active reabsorbing area

Sodium (Na+), bicarbonate (HCO3-),
chlorine (Cl-) and water
Loop of Henle

Water is salts are reabsorbed

Vital role in kidneys ability to form
dilute or concentrated urine
Distal convoluted tubule (DCT)

NaCl and water

Most reabsorption at this time
depends on the body’s needs
Kidney Physiology: Urine Formation
 Step 2: Tubular Reabsorption

Regulated by hormones

Aldosterone


Released when blood
pressure decreases or Na+
concentration drops
Antidiuretic hormone (ADH)

Reabsorption of water
Kidney Physiology: Urine Formation
 Step 3: Tubular Secretion

Substances such as H+, NH4+, creatinine, organic acids move from the
capillaries into the renal tubule

Important for disposing substances, such as drugs or poisons that can not be
filtered

Eliminates undesirable substances or end products that have been reabsorbed
by passive processes like urea and uric acid
Physical Characteristics of Urine
 Color

Clear to deep yellow

Yellow color is due to the pigment urochrome (from the destruction of
hemoglobin)

More concentrated the urine, the deeper the yellow color

An abnormal color such as pink or brown may result from eating certain
foods (beets, rhubarb), the presence of bile pigments or blood, or from
some commonly prescribed drugs and vitamins

Cloudy urine may indicated a urinary tract infection
 Odor

Slightly aromatic

If allowed to stand, it develops an ammonia odor as bacteria metabolize
its urea solutes
Physical Characteristics of Urine
 pH

Slightly acidic (around pH 6)

Acidic diet that contains large amounts of protein and whole wheat
products, diabetes mellitus and starvation produces acidic urine

Vegetarian diet, prolonged vomiting, and bacterial infection of the urinary
tract all can cause the urine to become alkaline
 Specific gravity

Ratio of the mass of a substance to the mass of an equal volume of distilled water

Urine is water plus solutes

Distilled water specific gravity is 1.00

Urine specific gravity ranges from 1.001 to 1.035 depending on its solutes
Chemical Composition of Urine
 Urine is 95% water and 5% solutes
 Solutes

Urea (largest component)


Uric acid


Derived from the normal
breakdown of amino acids
End product of nucleic acid
metabolism
Creatinine

Metabolite for creatinine
phosphate which stores energy
for the regeneration of ATP
 Normal solute concentrations in
urine from high to low

Urea  sodium  potassium 
phosphate  sulfate  creatinine
 uric acid
Chemical Composition of Urine
 Abnormal Urinary Constituents
 Glucose (glycosuria) Benedict’s solution and heat

Causes: diabetes mellitus
 Proteins (proteinuria) Biuret’s solution

Causes: Non-pathological: excessive physical
exertion, pregnancy, high-protein diet; Pathological:
heart failure, severe hypertension, renal disease
Chemical Composition of Urine
 Abnormal Urinary Constituents
 Hemoglobin (hemoglobinuria)

Causes: transfusion reaction, hemolytic
anemia, severe burns, etc.
 Bile pigments (bilirubinuria)

Causes: liver disease (hepatitis, cirrhosis)
 Erythrocytes (hematuria)

Causes: bleeding (due to trauma, kidney
stones, or infection)
 Leukocytes (pyuria)

Causes: urinary tract infection
Ureters
 Slender tubes that carry urine from the kidneys to the bladder

Composed of transitional epithelium
 Peristalsis aids gravity in urine transport
 Homeostatic Imbalance

Kidney stones

Calcium, magnesium, or uric acid salts in urine may crystallize and
precipitate in the renal pelvis

Most are under 5 mm in diameter and pass through the urinary
tract without causing problems

Larger stones can obstruct a ureter and block urine drainage

Increasing pressure in the kidney causes excruciating pain

Treatment includes shock wave lithiotripsy a noninvasive
procedure that uses ultrasonic shock waves to shatter the stone
Urinary Bladder
 Smooth, collapsible, muscular sac that temporarily stores urine
 Located on the pelvic floor just posterior to the pubic symphysis
 Interior has three openings called trigone
 Two from the ureters
 One to the urethra
Urinary Bladder

When empty, the bladder collapses and its walls are thick and have folds
(rugae)

Bladder can expand significantly


A full bladder is about 12 cm (5 inches) long and holds approximately
500 mL (1 pint) of urine, but it can hold nearly double that if necessary

Maximum capacity of the bladder is 800-1000 mL and when it is
overdistended, it may burst
Urine is formed continuously by the kidneys but it is stored in the bladder
until it is convenient to release
Urethra
 Thin-walled muscular tube that drains urine from the bladder to the
outside of the body
 Release of urine (micturition or voiding)is controlled by two
sphincters
 Internal urethral sphincter (involuntary)
 External urethral sphincter (voluntary)
Urethra Gender Differences

Length and function of the urethra differ in the two sexes
 Females
 Length: only 3–4 cm (1.5 inches)
 Function: carries only urine out of the body
 Males
 Length: Urethra is 20 cm (8 inches) long
 Double function: carries semen and urine out of the body
Maintaining Water Balance
 Normal amount of water in the human body
 Young adult females – 50% because more body fat
 Young adult males – 60% because more muscles
 Babies – 75% because of low body fat and low bone mass
 Old age – 45%
 Water is necessary for many body functions and levels
must be maintained
Distribution of Body Fluid
 Total body water volume is 40 L or 60%
of body weight
 Water occupies two main fluid
compartments within the body
 Intracellular fluid (inside cells)
 About 25 L or 40% body weight
 Extracellular fluid (outside cells)
 About 15 L or 20% body weight
 Divided into two subcompartments

Interstitial fluid (fluid in the
microscopic spaces between tissue
cells)

Blood plasma (fluid portion of
blodd)
Composition of Body Fluids

Water is the universal solvent in which a variety of solutes are dissolved

Solutes can be classified into electrolytes and nonelectrolytes

Nonelectrolytes have bonds and cannot dissociate in solution


Organic molecules such as glucose, lipids, creatinine and urea
Electrolytes are chemical compounds that do dissociated into ions in
water

Inorganic and organic acids and bases and some proteins

Have the greatest ability to make fluid shifts down their gradients

Most abundant solutes in body fluids

Extracellular fluids have high sodium and chloride ions

Intracellular fluids contains only small amounts of sodium and chloride
ions; its most abundant cation is potassium anion is phosphate (HPO42) as well as high amounts of proteins
Fluid Moving Among Compartments
 Continuous exchange and mixing of fluids are regulated by
osmotic and hydrostatic pressures
 Water moves freely between the compartments along osmotic
gradients
 Solutes are unequally distributed because of their size,
electrical charge, or dependence on transport proteins
 Changes in electrolyte balance causes water to move from one
compartment to another
Maintaining Water Balance
 Body must remain properly hydrated water intake must equal water
output
 Water intake is typically about 2500 mL a day in adults
 Water enters the body through ingested liquids (60%), solid
foods (30%) and produced from metabolic processes (10%
 Water output occurs by several routes

Vaporization out of the lungs and skin (28%)

Perspiration of skin (8%)

Leaves the body in the feces (4%)

The balance (about 60%) is excreted by the kidneys in urine
Maintaining Water Balance
 A rise in plasma concentration causes thirst (prompts us to drink
water) and release of antidiuretic hormone (ADH) which causes the
kidneys to conserve water and excrete concentrated urine
 A decline in plasma concentration inhibits thirst and ADH release
and causes output of large volumes of dilute urine
Regulation of Water
 Water intake is controlled by the thirst mechanism
 An increase in plasma concentrations
 A dry mouth occurs
 Less saliva production
 Decrease in blood pressure
 Water Output of certain amounts of water is unavoidable
 Reason why we cannot live without drinking
 Solute concentration and volume of urine excreted depend on fluid
intake, diet and water loss via other avenues
 Regulation is primarily by hormones
 Antidiuretic hormone (ADH) prevents excessive water loss in urine
 Aldosterone regulates sodium ion content of extracellular fluid
 Osmoreceptor cells in the kidneys are active monitors
Regulation of Water
 Dehydration

When water output exceeds intake over a period of time and the body is
in negative fluid balance

Commonly follows hemorrhage, severe burns, prolonged vomiting or
diarrhea, profuse sweating, and diuretic abuse

Signs are sticky oral mucosa, thirst, dry flushed skin, and decreased
water output (oliguria)
Electrolyte Balance

Refers to the salt balance in the body

Important in controlling fluid movements and crucial for cellular activity

Salts enter the body in foods and fluids

Salts are lost from the body in perspiration, feces and urine

Sodium holds a central position in fluid and electrolyte balance and overall
body homeostasis

Water follows salt


A change in plasma sodium levels affects not only plasma volume and
blood pressure but also the ICF and IF volumes
Regulation is linked to blood pressure and aldosterone

When aldosterone is high all the sodium is reabsorbed in the DCT

Water follows sodium and maintains blood pressure

When aldosterone is inhibited none of the sodium is reabsorbed

Goal of aldosterone is to decrease urinary output and increase blood
volume
Acid-Base Balance
 All biochemical reactions are influenced by the pH of their fluid
environment
 The acid-base balance of body fluids is closely regulated
 pH measures the amount of H+ ions in solution
 Acids are proton donors
 Blood normally ranges between pH 7.35 and pH 7.45
 If the pH rises above 7.45 a person has alkalosis
 If the pH drops below 7.35 a person has acidosis
 H+ concentration regulation
 Chemical buffers resist changes within a fraction of a second
 Respiratory rate changes within 1-3 minutes
 Kidneys requires hours to a day to effect changes in blood pH
Acid-Base Balance
 Respiratory acidosis


Most common cause of acid-base imbalance

Caused when a person breathes shallow or when gas exchange is
hampered by diseases

CO2 accumulates in the blood and causes the pH to fall
Respiratory alkalosis



Results from carbon dioxide being eliminated faster than it is produced
otherwise known as hyperventilation
Metabolic acidosis

Second most common cause of acid-base imbalance

Low blood pH and HCO3- levels

Caused when a person ingests too much alcohol and excessive loss of
HCO3- as a result of excessive diarrhea
Metabolic alkalosis

Rising blood pH and HCO3- levels

Caused by vomiting and intake of excess base
Acid-Base Balance
 Effects of acidosis and alkalosis

Absolute blood pH limits for life are a low of 7.0 and a high of 7.8

When the pH falls below 7.0 the CNS is so depressed that the person goes
into coma and death

When blood pH rises above 7.8, the nervous system is overexcited and
leads to muscle spasms, extreme nervousness, and convulsions; death
usually results from respiratory arrest