1. Chapter 35
The Urinary System

2. 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

3. 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

4. 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

5. Flatworms Use Protonephridia
excretory pore
eye spot
tubule
extracellular
fluid
cilia
nucleus
flame
cell
excretory
pore
(a) Flatworms use protonephridia

6. 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

7. Insects Use Malphigian Tubules
abdomen
Malpighian tubules
intestine
hemocoel
(filled with
hemolymph)
rectum
cellular and
digestive wastes
(b) Insects use Malpighian tubules

8. 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

9. Earthworms Use Nephridia
coelom (filled with
extracellular fluid)
nephridium
capillary
bed
nephrostome
nephridiopore
(c) Earthworms use nephridia

10. 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

11. 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

12. 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

13. 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

14. Urea Formation and Excretion1
Proteins in food are digested 4
The liver converts ammonia
to urea, which is less toxic
urea
Amino acids are carried in
the blood to body cells 2
amino acid
Urea is carried in the blood
to the kidneys 5
The cells convert the
amino groups (-NH2) to
ammonia, which is carried
in the blood to the liver 3
In kidney nephrons, urea
is filtered into the urine 6
ammonia
NH3

15. 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

16. The Human Urinary System
left renal artery
left kidney
left renal vein
aorta
left ureter
vena cava
urinary
bladder
urethra
(in penis)

17. 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

18. Animation: Kidney Overview

19. 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

20. 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
collecting
duct
ureter
urine
to the
bladder
nephron
renal
medulla
renal
cortex
enlargement of a
single nephron and
collecting duct

21. 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

22. 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

23. Individual Nephron and Blood Supply
collecting duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
venule
branch of
the renal
artery
branch of the
renal vein
loop of Henle
capillaries

24. Animation: Parts of the Nephron

25. 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
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

26. Individual Nephron and Blood Supply
collecting duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
venule
branch of
the renal
artery
branch of the
renal vein
loop of Henle
capillaries

27. Urine Production – 3 Stages
Filtration - water and small dissolved molecules are filtered out of the blood
Tubular reabsorption - water and necessary nutrients are restored to the blood
Tubular secretion - wastes and excess ions remaining in the blood are secreted into the urine

28. Urine Formation and Concentration
Filtration: Water, nutrients,
and wastes are filtered from the
glomerular capillaries into the
Bowman’s capsule of the nephron 1
Tubular reabsorption: In the
proximal tubule, most water and nutrients
are reabsorbed into the blood 2
Tubular secretion:
Additional wastes are
actively transported into
the proximal and distal
tubules from the blood 3
proximal
tubule
collecting
duct
blood
leaving the
glomerulus
distal tubule
Bowman’s
capsule
blood entering
the glomerulus
loop of
Henle
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 4

29. 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

30. Tubular Reaborption
Occurs primarily in the proximal tubule, water and other nutrients are reabsorbed in other tubule areas
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
Remaining wastes and excess ions move from the blood into the proximal and distal tubules
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

31. 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

32. Water Regulation
Why is a high salt concentration is important? Water regulation
The kidneys help maintain water content in body tissues by producing….
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

33. 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

34. Details of Urine Formation
TUBULAR REABSORPTION
& TUBULAR SECRETION
URINE
CONCENTRATION
FILTRATION
HCO3–
Ca2+
Cl– K+ H+
NH3
some
drugs H+ K+
some
drugs
Na+
nutrients
NaCl
Ca2+
H2O*
H2O 1
H2O* 7 2
proximal
tubule 6
distal
tubule
Bowman’s
capsule
H2O
NaCI
renal cortex
renal medulla 5
NaCI
H2O 3
NaCI
H2O 4
(extracellular fluid)
urea
NaCI 8
H2O*
H2O
osmosis
loop of Henle
active transport
diffusion
collecting duct

35. 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

36. 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

37. 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

38. Animation: Urine Formation

39. 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

40. Dehydration Stimulates ADH Release and Water Retention1
Heat causes water loss and
dehydration through sweating
Receptors in the hypothalamus
detect the increased blood
osmolarity and signal the pituitary
gland 2
The pituitary gland
releases ADH into the
bloodstream 3 4
ADH increases the
permeability of the distal tubule
and the collecting duct, allowing
more water to be reabsorbed
into the blood
Water is retained in the body
and concentrated urine is
produced 5

41. 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

42. 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

43. 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

44. 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

45. A Well-Adapted Desert Dweller

46. 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

47. 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

48. 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
The kidneys conserve salt
and excrete large amounts
of dilute urine
(a) Freshwater fish

49. 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

50. 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
Some salt is excreted in
small quantities of urine
water
salt
(b) Saltwater fish

51. 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