Transcript Urinary, ch. 35

Chapter 35
The Urinary System
Basic Functions

Urinary systems help maintain homeostasis—
the relatively constant internal environment



Composition of blood and extracellular fluid
Control the concentration or osmolarity, of dissolved
substances in cells and their extracellular
environment
Excretion - removal of unwanted substances

Produces urine - contains waste products of cellular
metabolism
Urinary System Functions

Three basic processes

Blood or extracellular fluid is filtered, removing water
and small dissolved substances

Nutrients are selectively reabsorbed back into the
filtered fluid

Excess water, excess nutrients, and dissolved wastes
are excreted from the body in urine
Flatworm Urine Systems

The earliest excretory system served to maintain
water balance, the primary function of the simple
excretory system of flatworms

This system consists of protonephridia - tubules that branch
throughout extracellular fluid surrounding flatworm tissues

Collects excess water from the extracellular fluid using ciliated
“flame cells” and forces the fluid out through excretory pores

The large body surface of flatworms also serves as an excretory
structure through which cellular wastes diffuse
Flatworms Use Protonephridia
excretory pore
eye spot
tubule
extracellular
fluid
cilia
excretory
pore
nucleus
flame
cell
(a) Flatworms use protonephridia
Insect Urinary Systems

Insects have an open circulatory system where hemolymph fills the
hemocoel and bathes internal tissues directly

Insect excretory systems are Malpighian tubules that extend
outward from the intestine and end blindly within hemolymph

Wastes and nutrients move from the hemolymph into the tubules by
diffusion and active transport, water follows by osmosis

Urine is conducted into the intestine, solutes are secreted into the
hemolymph by active transport

Produces concentrated urine, which is excreted along with feces
Insects Use Malphigian Tubules
abdomen
Malpighian tubules
intestine
hemocoel
(filled with
hemolymph)
rectum
cellular and
digestive wastes
(b) Insects use Malpighian tubules
Earthworm Excretory Systems

In earthworms, mollusks, and other invertebrates, excretion is
performed by tubular structures called nephridia

The body cavity is filled with extracellular fluid into which wastes and
nutrients diffuse

Each nephridium begins with a funnel-like opening, the nephrostome,
ringed with cilia that direct extracellular fluid into a narrow, twisted
tubule surrounded by capillaries

As the fluid traverses the tubule, salts and nutrients are reabsorbed
back into the capillary blood, leaving the wastes and water behind

Urine is excreted through a nephridiopore

Each segment in an earthworm’s body contains a pair of nephridia
Earthworms Use Nephridia
coelom (filled with
extracellular fluid)
nephridium
capillary
bed
nephrostome
nephridiopore
(c) Earthworms use nephridia
Vertebrate Urinary Systems

Kidneys - organs of the vertebrate urinary system,
where blood is filtered and urine is produced

Because vertebrates live in a wide variety of habitats, vertebrate
kidneys face different challenges in maintaining constant
conditions within their bodies
Homeostatic Kidney Functions

The mammalian urinary system consists of kidneys, ureters,
bladder, urethra

These organs filter the blood, collecting, then excreting dissolved
waste products in urine

During filtration, water and dissolved molecules are forced out of the
blood

The kidneys return nearly all of the water and nutrients required by
the body to the blood

The urine retains wastes, which are expelled
Mammalian Urinary Systems

Helps maintain homeostasis in several ways:

Regulate blood levels of ions - sodium, potassium, chloride, calcium

Maintain proper pH of the blood by regulating hydrogen and
bicarbonate ion concentrations

Regulate water content of the blood

Retain important nutrients - glucose and amino acids in the blood

Eliminate cellular waste products as urea

Secrete substances that help regulate blood pressure and oxygen
levels
Urea

A waste product of protein digestion

Eliminate nitrogenous wastes that are formed when cells break
down amino acids

Nitrogenous wastes from cells enter the blood as ammonia
(NH3), toxic

The livers of humans and other mammals convert ammonia into
urea, which is less toxic

Urea is filtered from the blood by kidneys and excreted in urine
Urea Formation and Excretion
1 Proteins in food are digested
4 The liver converts ammonia
to urea, which is less toxic
urea
2 Amino acids are carried in
the blood to body cells
amino acid
3 The cells convert the
amino groups (-NH2) to
ammonia, which is carried
in the blood to the liver
ammonia
NH3
5 Urea is carried in the blood
to the kidneys
6 In kidney nephrons, urea
is filtered into the urine
Human Urinary System

Kidneys - paired organs located on either side of the spinal column,
just above the waist

Blood enters each kidney through the renal artery, after blood has
been filtered, it exits through the renal vein

Urine leaves the kidney through the ureter - a narrow, muscular tube

Rhythmic contractions of the ureter transports urine to the bladder,
a hollow, muscular chamber that collects and stores urine

The bladder wall is lined with smooth muscle and is capable of
considerable expansion, accommodating up to a pint of urine
The Human Urinary System
left renal artery
left kidney
left renal vein
aorta
left ureter
vena cava
urinary
bladder
urethra
(in penis)
Bladder Sphincters

Urine is contained within the bladder by two sphincter muscles

The internal sphincter, where the bladder joins the urethra,
opens automatically during the reflexive contractions of the
smooth muscle

The external sphincter, located slightly below the internal
sphincter, is under voluntary control, allowing the brain to
suppress urination unless the bladder becomes overly full

When open, the sphincters allow urine to flow into the urethra, a
single narrow tube that conducts urine outside the body
Animation: Kidney Overview
Kidney Structure

The structure of the kidney supports its function
of producing urine

Each kidney contains a solid outer layer - the renal
cortex and the inner layer - the renal medulla

The renal medulla surrounds a branched, funnel-like
chamber - the renal pelvis, which collects urine and
funnels it into the ureter
Cross-Section of a Kidney
renal pelvis
(cut away to
show the
path of urine)
renal
artery
renal
cortex
renal
medulla
renal
pelvis
renal
vein
ureter
collecting
duct
nephron
enlargement of a
single nephron and
collecting duct
renal
medulla
renal
cortex
urine
to the
bladder
Nephrons

The renal cortex is made up of more than 1
million microscopic filters or nephrons

Two major parts –

The glomerulus, a dense knot of capillaries where fluid is
filtered out of the blood through the porous capillary walls

A long, twisted tubule, where urine formation occurs
Nephron

Bowman’s capsule, a cup-like chamber that surrounds the glomerulus and
receives fluid filtered out of the blood from the glomerular capillaries

Collected fluid is conducted to the proximal tubule

The loop of Henle carries the filtered fluid from the cortex, deep into the
medulla and back to the cortex

The distal tubule - in the cortex - collects the filtrate from the loop of Henle
and passes it to the collecting duct

The collecting duct is not part of the nephron, but collects fluid from many
nephrons and deposits it in the renal pelvis
Individual Nephron and Blood Supply
collecting duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
venule
branch of
the renal
branch of the artery
renal vein
loop of Henle
capillaries
Animation: Parts of the Nephron
The kidney’s blood supply

The kidneys have an enormous blood supply, receiving more than
one quart of blood every minute

Blood flows to each kidney from the renal artery, which branches
into arterioles that each supply nephrons with blood for filtration

The arterioles branch into capillaries and form the glomerulus of
each nephron
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The capillaries empty into an outgoing arteriole that branches into
capillaries that surround the tubule

The capillaries carry blood into a venule that takes the blood to the
renal vein and then the inferior vena cava
Individual Nephron and Blood Supply
collecting duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
venule
branch of
the renal
branch of the artery
renal vein
loop of Henle
capillaries
Urine Production – 3 Stages
1.
Filtration - water and small dissolved molecules are
filtered out of the blood
2.
Tubular reabsorption - water and necessary nutrients
are restored to the blood
3.
Tubular secretion - wastes and excess ions remaining
in the blood are secreted into the urine
Urine Formation and Concentration
1 Filtration: Water, nutrients,
and wastes are filtered from the
glomerular capillaries into the
Bowman’s capsule of the nephron
2 Tubular reabsorption: In the
proximal tubule, most water and nutrients
are reabsorbed into the blood
proximal
tubule
blood
leaving the
glomerulus
3 Tubular secretion:
Additional wastes are
actively transported into
the proximal and distal
tubules from the blood
collecting
duct
distal tubule
blood entering
the glomerulus
Bowman’s
capsule
loop of
Henle
4 Concentration: The loop of
Henle produces a salt concentration
gradient in the extracellular fluid;
in the collecting duct, urine may
become more concentrated than the
blood as water leaves by osmosis
Filtration

Small organic nutrients - amino acids and glucose - are
filtered out and returned to the blood

Large quantities of water and ions are filtered out, but the return rate
is adjusted to meet changing needs


Ions include sodium (Na+), chloride (Cl–), potassium (K+), calcium (Ca++),
hydrogen (H+), and bicarbonate
Urine is formed in the glomerulus and tubule of the nephron

Filtration - when water carrying small dissolved molecules and ions
is forced through the walls of the capillaries that form the glomerulus


Blood cells and large proteins are too large to leave the capillaries, so
remain in the blood
The fluid filtered out of the glomerular capillaries – the filtrate –
collected in Bowman’s capsule and continues through the tubule
Tubular Reaborption

Occurs primarily in the proximal tubule, water and other
nutrients are reabsorbed in other tubule areas



Remaining wastes and excess ions move from the blood
into the proximal and distal tubules



Returns organic nutrients - glucose, amino acids, vitamins, ions
(Na+, Cl–, K+, Ca2+, H+ and HCO3–) to the blood
Restores most of the water, water follows the nutrients and ions
by osmosis through aquaporins - proteins that form water pores
Excess K+ and H+, small quantities of ammonia, drugs, food
additives, pesticides, and toxins (ie: nicotine)
Tubular secretion occurs primarily by active transport and takes
place in both the proximal and distal tubules
When the filtrate leaves the distal tubule, it is urine
The Loop of Henle

Creates an extracellular concentration gradient in the
renal medulla

The functions of the loop of Henle


Some water and salt is reabsorbed from the filtrate as it passes
through the loop
Most importantly, it creates a high salt and urea concentration in
the extracellular fluid within the medulla
Water Regulation

Why is a high salt concentration is important? Water
regulation
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The kidneys help maintain water content in body
tissues by producing….
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

dilute, watery urine when fluid intake is high
concentrated urine when fluid intake is low
Water is conserved by allowing it to move out of the
collecting duct by osmosis and down its concentration
gradient

The more concentrated the extracellular fluid, the more water
leaves the urine as it moves through the collecting duct
Summary

The loop of Henle produces and maintains a high salt
concentration gradient in the extracellular fluid of the
medulla by transporting salt out of the filtrate

The salt and urea gradient causes an osmotic gradient
between the filtrate and the surrounding extracellular fluid

The most concentrated fluid surrounds the bottom of the
loop

The collecting duct passes through this gradient as it
conducts urine from the distal tubule in the renal cortex
into the renal pelvis
Details of Urine Formation
TUBULAR REABSORPTION
& TUBULAR SECRETION
FILTRATION
HCO3– H+
Ca2+ NH3
Cl– some
Na+
nutrients H2O K+ drugs
URINE
CONCENTRATION
H+
K+
NaCl some H2O*
Ca2+ drugs
1
H2O*
7
6 distal
tubule
2 proximal
tubule
Bowman’s
capsule
renal cortex
NaCI
H2 O
renal medulla
5
NaCI
H2 O
NaCI
3
H2 O
4
(extracellular fluid)
NaCI
urea
8
H2O*
H2 O
osmosis
active transport
loop of Henle
diffusion
collecting duct
Concentration

As the filtrate descends into the loop of Henle and
collecting duct…
 It is exposed to the osmotic gradient surrounding the
nephron
 Water leaves the filtrate by osmosis and enters the
surrounding capillaries

Filtrate becomes urine once it enters the collecting duct
and can be more than four times as concentrated as
blood
Kidneys Regulate Osmolarity of Blood

Kidneys regulate the water content of the blood

Human kidneys filter out 1/2 cup of fluid from the
blood each minute



Fine-tuning the composition of blood and helping maintain
homeostasis
If the kidneys did not reabsorb this water, the rate of
filtration would require that we drink 50 gallons of
water a day
The urinary system needs to restore nearly all of
the water that is initially filtered out of the
glomeruli
Antidiuretic Hormone

Antidiuretic hormone (ADH) – Regulates reabsorption
and influences the ability of kidneys to reabsorb water



Secreted by the posterior pituitary gland, carried in the blood
It stimulates cells of the distal tubule and collecting ducts to insert more
aquaporin proteins into their membranes
The abundance of aquaporin membranes determines the permeability
of the membranes to water

Normally some ADH is always present in the blood

Within the hypothalamus, receptors monitor blood
osmolarity, which increases when water is lost
Animation: Urine Formation
An Example

When water is lost during dehydration:

If blood osmolarity exceeds optimal level, the
hypothalamus stimulates the pituitary gland to release
ADH into the bloodstream

Cells of the distal tubule and collecting duct insert
more aquaporins into their membranes, increasing
permeability to water

The more concentrated extracellular fluid draws water
out by osmosis, restoring water to the blood through
nearby capillaries
Dehydration Stimulates ADH Release and
Water Retention
1 Heat causes water loss and
dehydration through sweating
2 Receptors in the hypothalamus
detect the increased blood
osmolarity and signal the pituitary
gland
3 The pituitary gland
releases ADH into the
bloodstream
4 ADH increases the
permeability of the distal tubule
and the collecting duct, allowing
more water to be reabsorbed
into the blood
5 Water is retained in the body
and concentrated urine is
produced
Kidneys regulate BP and Oxygen

Kidneys release substances that help regulate
blood pressure and oxygen levels

When blood pressure falls, kidneys release renin

Renin catalyzes the formation of the hormone
angiotensin in the blood
Angiotensin

Combats low blood pressure in three ways

It stimulates the proximal tubules of nephrons to
reabsorb more Na+ into the blood, causing water to
follow by osmosis (increase blood volume)

It stimulates ADH release, causing more water to be
reclaimed from the distal tubule and collecting duct

It causes arterioles throughout the body to constrict,
which directly increases blood pressure
Erythropoietin

When blood oxygen levels are low, kidneys release
erythropoietin

Stimulates the bone marrow to make more red blood cells

The higher number of red blood cells increases the oxygen
carrying capacity of the blood
Vertebrate kidneys are adapted to diverse
environments

Mammals have structurally different nephrons,
depending upon the availability of water in their natural
habitat

Mammals adapted to dry climates have long loops of
Henle

Longer loops allow a higher concentration of salt to be
produced in the extracellular fluid of the medulla, so
more water is reclaimed from the collecting duct

A mammal with very long-looped nephrons is the desert
kangaroo rat
A Well-Adapted Desert Dweller
More Mammal Adaptations

Mammals adapted to habitats with an abundance of
fresh water have short loops of Henle


Beavers, which live along streams, can only concentrate their
urine to about twice their blood osmolarity
Humans have a mixture of long- and short-looped
nephrons, and can concentrate urine up to four times the
osmolarity of blood
Freshwater Fish

Animals have evolved homeostatic mechanisms,
including kidney adaptations, to maintain water and salt
within their bodies – osmoregulation



Freshwater fish live in a hypotonic environment
 Water continuously moves into their bodies by
osmosis, salts diffuse out
Freshwater fish acquire salt from food and through
their gills but never drink
Their kidneys retain salt and excrete large quantities
of extremely dilute urine
Osmoregulation in Fish
fresh water
water
salt
Water moves in by
osmosis; salt diffuses out
Salt is pumped in
by active transport
Salt and some
water enters
in food
(a) Freshwater fish
The kidneys conserve salt
and excrete large amounts
of dilute urine
Saltwater Fish

Saltwater fish live in a hypertonic environment; seawater
has a solute concentration of two to three times that of
their body fluids

Water is constantly leaving their tissues by osmosis,
and salt is constantly diffusing in and being taken in
with food

To compensate, saltwater fish drink to restore their
lost water, and excess salt they take in is excreted by
active transport through their gills
Osmoregulation in Fish
salt water
Salt and water enter
in food and by drinking
seawater
Water moves out by
osmosis; salt diffuses in
Salt is pumped out
by active transport
water
salt
(b) Saltwater fish
Some salt is excreted in
small quantities of urine

Fish nephrons completely lack loops of Henle, and so
they cannot produce concentrated urine

To conserve water, the kidneys of saltwater fish
excrete small quantities of urine containing salts not
eliminated by their gills