Topic 11.3
The Kidney and osmoregulation
Bellwork:
On your own whiteboard and without
looking at a drawing, draw the human
excretory system. Label kidney,
bladder, ureter, and urethra.
Understanding:
Animals are either osmoregulators or osmoconformers
Osmolarity = solute concentration of a solution
Osmoregulator = an animal who maintains constant internal solute concentration
(even when living in salt water!)
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e.g. all terrestrial & freshwater animals and some marine (bony fish)
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Maintain body solute concentration of ~1/3 seawater & 10x freshwater
Osmoconformer = an animal whose internal solute conc. is SAME as environment
e.g. marine invertebrates (starfish, scallops, mussels, lobster, jellyfish) & sharks
Osmoregulator or osmoconformer?
Understanding:
The Malpighian tubule system in insects & the kidney carry
out osmoregulation & removal of nitrogenous wastes.
Osmoregulation
• Osmoregulation = form of homeostasis where concentration of blood
or hemolymph is kept within narrow range
• Hemolymph = combo of blood & lymph (tissue fluid); in arthopods,
including insects
• Malpighian tubules = tubes that branch off intestinal tract
1. Cells actively transport ions & uric acid from hemolymph into
tubule lumen
2. Water passively transported from hemolymph into tubule lumen
3. Tubules empty contents into gut
4. Water & ions reabsorbed
5. Uric acid excreted with feces
In mammals…
Insects have Malpighian tubules!
Birds have a cloaca!
What’s in a bird dropping?
Skill: Drawing & labelling a diagram of human kidney
Skill: Annotation of diagrams of the nephron
• Nephron = functional unit of kidney
• Tube whose walls are made of one layer of cells
• Structure & function:
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Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
Blood vessels
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Afferent arteriole
Glomerulus
Efferent arteriole
Peritubular capillaries
Vasa recta
venules
Understanding: The composition of blood in the renal artery is
different from that in the renal vein.
Renal Artery
Renal Vein
blood enters kidney
blood leaves kidney
before osmoregulation
after osmoregulation
variable water (from cell resp or
food/drink) or salt content (from
food)
more constant water or salt
content
higher amount of toxins & other
substances not metabolized by
body (pigments, drugs)
lower amount of toxins & other
unwanted substances
more urea
more O2
less urea
less O2
less CO2
more CO2
more glucose
same concentration of plasma
proteins
less glucose
same concentration of plasma
proteins
Understanding: Ultrastructure of the glomerulus &
Bowman’s capsule facilitate ultrafiltration.
Glomerulus = ball of capillaries in nephron where ultrafiltration takes place
•
Very high pressure & permeable (volume of fluid forced out is ~100x greater
than in other capillaries)
Ultrafiltration = separation of particles differing in size by only a few nm’s
Permeability to larger molecules depends on their shape & charge.
Too large: most proteins, and blood cells
Structure of glomerulus
3 parts to ultrafiltration system
1. Fenestrations = pores (100nm diameter) between cells of capillaries allowing
fluid to escape, but not blood cells
2. Basement membrane = the filter that covers & supports wall of capillaries
3. Podocytes = strangely shaped cells with finger-like extensions which wrap
around capillaries & provide support; on inner wall of Bowman’s capsule
• If molecules pass through all 3 parts, they become part of the glomerular filtrate!
Understanding: The proximal convoluted tubule selectively
reabsorbs useful substances by active transport.
• Almost all glomerular filtrate gets reabsorbed back into blood!
• Most reabsorption happens in the PCT.
• Substances reabsorbed:
• All glucose (co-transport)
• All amino acids
• 80% water (osmosis)
• 80% sodium ions (active transport)
• Chloride ions
Sample Exam Question: Explain how the structure of
the PCT is adapted to carry out selective reabsorption:
Reabsorption vs secretion
Understanding: The loop of Henle maintains
hypertonic conditions in the medulla.
Countercurrent mechanism in kidney
• Expends energy to create a concentration gradient
• Flow of fluid in descending & ascending limb is in opposite directions
• Causes steeper gradient of solute concentration
• Loop of Henle = countercurrent multiplication system
• Vasa recta = countercurrent exchange system – allows vasa recta to
supply kidney with nutrients without messing up the conc. gradient
Understanding: Length of the loop of Henle is positively
correlated with the need for water conservation in animals.
• Loops of Henle found within medulla of kidney (short ones are just in
cortex)
• The longer the loop of Henle, the more water volume will be reclaimed
• Dryer environment = longer loops of Henle = thicker medulla
Understanding: ADH controls reabsorption of
water in the collecting duct.
• The kidney functions in osmoregulation = keeps the relative amounts of
water and solutes in balance
• If solute concentration in blood too high, hypothalamus detects and causes
pituitary gland to secrete hormone ADH
• ADH = antidiuretic hormone
• ADH causes walls of DCT & collecting duct to become
more permeable to water (aquaporins allow H2O out)
• In the presence of ADH, 99% of H2O is reabsorbed!
• Adult beverages inhibit ADH! (close aquaporins)
Understanding: The type of N waste in animals is
correlated with evolutionary history & habitat.
Application: Consequences of dehydration & overhydration!
Dehydration
• from exercise, insufficient drinking of water, diarrhea
• Sign: dark yellow urine due to increased solute concentration
• Water needed for removal of wastes, so dehydration can cause:
• Tiredness & lethargy due to decreased efficiency of muscle function &
increased tissue exposure to waste
• Decreased BP due to low blood volume
• Increased heart rate due to low BP
• Lower ability to sweat, therefore difficult body temp regulation
Overhydration
• Less common
• Over-consumption of water, such as after exercise, without replacing
electrolytes lost at same time
• Causes dilution of blood solutes
• Swelling of cells due to osmosis
• Headache
• nerve function disruption
Application: Treatment of kidney failure by
hemodialysis or kidney transplant.
• Most commonly occurs as complication of diabetes or hypertension
• Dialysis machine = artificial kidney
Application: Blood cells, glucose, proteins and
drugs are detected in urinary tests.
• Urinalysis = analysis of urine composition
• Tests for pH, glucose and protein
• High levels of glucose & protein may indicate diabetes
• High protein may indicate kidney damage
• Drug test = monoclonal antibody test: looks for presence of drugs
• Microscopic examination:
• presence of wbc’s may indicate UTI
• Presence of rbc’s may indicate kidney stone or tumor