Controlling the Internal
Environment
Chapter 51
The Big Picture
• The excretory system is a regulatory system that
•
•
•
helps to maintain homeostasis within the body
Homeostasis = the dynamic constancy of the
internal environment
The excretory system eliminates wastes and
functions in osmoregulation.
Osmoregulation= the control of salt and water
balance
Osmoconformers vs. Osmoregulators
• Osmoconformers
– Do not actively adjust their internal osmolarity
• Osmoregulators
– Animals whose body fluids are not isotonic
with the external environment must
manipulate solute concentrations in their body
fluids
• Which one are we??
Passive vs. Active Transport
• Passive transport = does not require an
input of energy by the cell
• Active transport = requires an input of
energy by the cell
The Vertebrate Excretory System
• Kidneys, ureters, urinary bladder, urethra,
renal artery, renal vein
• Also…
– Lungs: excrete carbon dioxide
– Skin: excretes water, salts, and a small
amount of urea (in sweat)
Anatomy of the Vertebrate
Excretory System
• Kidneys = bean-shaped organs, about the size
•
•
•
•
of your fist in humans
Blood enters the kidney through the renal
artery, and exits the kidney through the renal
vein
Urine exits the kidney through the ureter
The ureter from each kidney goes to the urinary
bladder
Urine leaves the body through the urethra
Human Excretory System
Blood Supply to the Kidney
• Into the kidney:
– Renal artery to afferent arteriole to capillaries of the
glomerulus
• Capillaries converge into efferent arteriole,
•
•
•
which subdivides into peritubular capillaries
Vasa recta = looped capillary system that serves
the loop of Henle
Out of the Kidney: renal vein
Kidneys receive ~20% of the blood pumped
with each heartbeat
Kidney Structure
• Divided into renal cortex (outer portion)
and renal medulla (inner portion)
• Renal pelvis: where the ureter meets the
kidney
The Nephron
• The functional unit of the kidney (ie. The
•
•
•
smallest part of the kidney that can still perform
all the tasks associated with it)
There are approximately 1 million nephrons in
each kidney
Nephron consists of a single long tubule and a
ball of capillaries (glomerulus)
Made up of Bowman’s Capsule, Proximal Tubule,
Loop of Henle, Distal Tubule, (Collecting Duct)
The Nephron
The Nephron
• 80% of nephrons are cortical nephrons
(located in the renal cortex)
– Have reduced loops of Henle
• 20% of nephrons are juxtamedullary
nephrons (extend into renal medulla)
– Have well-developed loops of Henle
– Allow for the conservation of water
Regulation of Blood Composition
• Filtration:
– Blood pressure forces fluid from the capillaries
of the glomerulus into Bowman’s Capsule
• Capillaries function as a filter: permeable to water
and small solutes but not to blood cells or large
molecules like plasma proteins
• Nonselective with regard to small molecules
Regulation of Blood Composition
• Secretion:
– Substances are transported into the tubule from the
surrounding interstitial fluid (ISF)
– Occurs mostly in the proximal and distal tubules
– Net effect is the addition of plasma solutes to the
filtrate within the tubule
– A very selective process involving both passive and
active transport
– Responsible for maintenance of a constant pH level
(secretion of H+ ions)
Regulation of Blood Composition
• Reabsorption:
– The selective transport of substances across the
epithelium of the excretory tubule from the filtrate to
the ISF
– Because filtration is nonselective, it’s important that
small molecules essential to the body be returned to
the ISF and blood plasma
– Occurs in the proximal tubule, loop of Henle, distal
tubule, and collecting duct
– Involves both passive and active transport
Bowman’s Capsule
• Tube of nephron that surrounds the
glomerulus, which is a ball of capillaries
• Blood pressure forces water, urea, salts
and other small solutes from blood in the
glomerulus into Bowman’s Capsule; this
fluid is now called filtrate
Proximal Tubule
• Reabsorption and secretion
• Reabsorption:
– Nutrients (glucose, amino acids)
– Water *
– NaCl *
• Secretion:
– Drugs and other poisons from the liver
– Ammonia
• Maintains a constant pH: H+ and HCO3
Descending Loop of Henle
• Permeable to water but not to salt
• ISF is hypertonic to filtrate, and water
moves out of the tubule by osmosis
• NaCl becomes more and more
concentrated in the tubule
Ascending Loop of Henle
• Permeable to salt, but impermeable to
water
• Two regions:
– Thin: NaCl diffuses out into the ISF
– Thick: NaCl is actively transported into the ISF
• Result: filtrate becomes more and more
dilute because it loses salt without losing
water (ie. salt concentration decreases)
Distal Tubule
• Site of selective secretion and absorption
– Secretion:
• K+
– Reabsorption:
• Water
• NaCl
• Regulates pH levels
– Secretion of H+
– Reabsorption of HCO3-
Collecting Duct
• Shared by many nephrons
• Carries the filtrate back towards the
medulla and renal pelvis
• Permeable to water but impermeable to
salt
• Filtrate becomes more and more
concentrated again as water is lost by
osmosis to the hypertonic ISF
The Nephron
An Overview of
Kidney Function
Why do animals have to get rid of
nitrogen?
• The metabolism of proteins and nucleic acids
•
produces toxic by-products
Ammonia:
ammonia
– a small and very toxic molecule
– can be transported and excreted only in a very dilute
solution
• Many animals convert ammonia to urea or uric
acid, which are much less toxic, but require ATP
to produce