excretion [2015]
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EXCRETION &
OSMOREGULATION
OVERVIEW
A) DEFINITIONS & IMPORTANCE OF EXCRETION AND OSMOREGULATION
B) OSMOREGULATION IN A TERRESTRIAL INSECT
C) OSMOREGULATION IN BONY FISH (TELEOSTS)
D) THE HUMAN KIDNEY
Definitions
Excretion the elimination of waste metabolic substances
from the body which if permitted to accumulate would prevent the maintenance of a steady state
CO2
urea
Definitions
Egestion the elimination of waste substances, mainly
undigested food, which have never been involved in the metabolic activities of cells
Definitions
Secretion :
- the process involved in producing and releasing a substance which is useful, from the cell e.g.
digestive juices
hormones
sweat
milk
Definitions
Osmoregulation :
the maintenance of constant osmotic conditions in the body
the control of the gain and loss of:
water
solutes
Importance of excretion and osmoregulation
1) Removal of unwanted by-products of metabolic pathways
is important to prevent unbalancing chemical equilibria of reactions
e.g. A + B → C + D
- constant synthesis of C requires constant removal of D
Importance of excretion and osmoregulation
3) Regulation of ionic concentration of body fluids e.g. Na+, Ca2+
An albatross possesses nasal salt glands that
can secrete excess salt through ducts and out of
the nostrils.
Importance of excretion and osmoregulation
4) Regulation of water content of body fluids
California 2007 A woman who competed in a radio station’s contest to see how much water she could drink without going to the bathroom died of water intoxication, the coroner’s office said Saturday.
5) Regulation of the pH of the body fluids
Nitrogenous excretory products and environment
nitrogenous waste products are produced by the breakdown of:
proteins
nucleic acids
excess amino acids
A variety of animals also excrete small quantities of: creatine creatinine
Ammonia:
is the immediate nitrogenous waste of deamination
amino acid NH3
ammonia
Ammonia may be :
excreted immediately
converted into: urea or uric acid
The exact nature of the excretory product is determined by:
1. the availability of water to the organism
2. the extent to which the organism controls water loss
The relationship between excretory products and habitat of representative animal groups
Animal Excretory product Habitat Protozoan Ammonia Freshwater Insect Uric acid Very dry terrestrial Carp Ammonia Freshwater Cod Urea,
trimethylamine oxide
Marine
Bird Uric acid Terrestrial Dog Urea Dry terrestrial
Correlation with habitat
Ammonia: Freshwater
Uric acid: Terrestrial
Urea: Marine/Terrestrial
Ammonia:
• extremely soluble
• highly diffusible through water
• highly toxic
• cannot be stored in the body
• no energy is needed in its formation
• large volumes of water are needed for dilution to be excreted
Urea is less toxic & soluble than ammonia
Urea is about 100,000 times less toxic than ammonia.
Urea forms
during the Ornithine Cycle
Bird droppings =
faeces + nitrogenous waste
Car covered in bird droppings.
Uric acid
Why is uric acid an ideal excretory product for terrestrial organisms
(e.g. insects, reptiles & birds) which produce shelled eggs?
Uric acid can be stored in cells without producing toxic or harmful osmoregulatory
effects.
Uric acid is stored in the allantois.
Uric acid: is largely insoluble
in water can be excreted as
a paste with little water loss
Energy required for production: Amount of water required for excretion: Toxicity of waste :
Ammonia Urea Uric acid
None Moderate High
High Moderate Low
High Low Low
Question: DEC, 1987
The earthworm, although terrestrial, excretes mainly ammonia. How does this affect the
worm’s habitat preferences?
Must inhabit moist environments.
Question: DEC, 1987
Marine turtles have the ability to excrete all three nitrogenous waste products, yet they excrete mainly
ammonia. What are the advantages and disadvantages of excreting ammonia?
Advantages: no energy is needed to produce ammonia
Disadvantages: animal loses a lot of water to eliminate ammonia, risking dehydration
Percentage of waste nitrogen excreted as:
Ammonia Urea Uric acid
51 12 6
AMMONIA Tadpoles excrete:
UREA Frogs excrete:
Question: [MAY, 2010]
1. What are nitrogenous wastes? Name a biochemical process that produces nitrogenous waste. (2)
Nitrogenous wastes are substances produced as a result of metabolism that are not required by the body and contain nitrogen.
Deamination / break down of nucleic acids. 2. Name ONE organism that excretes nitrogenous wastes
as urea and ONE organism that excretes nitrogenous wastes as ammonia: (2) Urea: mammal / marine bony fish / frog Ammonia: protozoans / Amoeba / freshwater bony fish / tadpole
OVERVIEW
A) DEFINITIONS & IMPORTANCE OF EXCRETION AND OSMOREGULATION
B) OSMOREGULATION IN A TERRESTRIAL INSECT
C) OSMOREGULATION IN BONY FISH (TELEOSTS)
D) THE HUMAN KIDNEY
Malpighian Tubules in insects:
Function: excrete uric acid
Location:
- lie in the abdomen
- open into the hindgut at its junction with the midgut
Malpighian Tubules are:
blind-ending tubules of the hindgut of insects
Tubules vary in :
Shape:
long & slender or
short and compact
Number:
a pair - several hundred
Figure 44.12
Upper segment: Absorbs fluid from blood
Lower segment: Cells have microvilli
Rectal glands: Reabsorb water
Haemolymph
Mechanism
Faeces & uric acid
removed as semi-solid wastes
The excretory product is semi-solid
cockroach droppings
OVERVIEW
A) DEFINITIONS & IMPORTANCE OF EXCRETION AND OSMOREGULATION
B) OSMOREGULATION IN A TERRESTRIAL INSECT
C) OSMOREGULATION IN BONY FISH (TELEOSTS)
D) THE HUMAN KIDNEY
Fish are osmoregulators:
control concentration of body fluids
use energy to regulate
Osmoconformer
e.g. marine invertebrates osmoregulator
Hypertonic solution
Hypotonic solution
Isotonic solution
osmoregulator
Bony Fish can be:
FRESHWATER
eliminate ammonia
MARINE
eliminate urea &
trimethylamine oxide
Trimethylamine oxide = fish odour
Why is the nitrogenous waste product different in the two groups of fish?
FRESHWATER fish
afford to lose water
MARINE fish
cannot afford to lose water
Gills are permeable to:
1. Water
2. Ions
Freshwater Bony Fish:
Hypertonic body fluids
Not salty
Gills of freshwater bony fish:
Hypertonic body fluids
GAIN water
But this disturbs body fluid concentration!!
LOSE salts
gills
Freshwater Bony Fish:
Hypertonic body fluids
Water flows by osmosis through the gill surfaces
Water must be removed otherwise fish is no longer hypertonic.
How?
Freshwater Bony Fish:
Hypertonic body fluids
The fish discharges copious quantities of very dilute urine, few salts lost
Nitrogenous waste: Ammonia
No drinking
Kidneys of freshwater bony fish:
contain many large Malpighian bodies, with large
glomeruli
high rate of filtration produces a large
volume of glomerular filtrate
How does the fish remain hypertonic?
GAINS salts:
Selective reabsorption in kidney
Hypertonic body fluids
Selective uptake of Cl- at gills
In food
Marine Fish:
Hypotonic body fluids
SALTY
Marine Fish:
Hypotonic body fluids
Loss of water by osmosis
Compensatory mechanisms to
avoid dehydration MUST be present
At gills:
Gain of ions by diffusion
Marine Fish DRINK sea water:
Hypotonic body fluids
To replace water lost
But this means SALTS are gained too. What happens to salts?
Excess ions are:
Nitrogenous waste: Urea + Trimethylamine oxide
Hypotonic body fluids
actively secreted by special excretory cells in the gills
Some marine bony fish have NO glomeruli at all :
and so do not filter their blood urine is isotonic with the body fluids
Question: [SEP, 2010]
Fish do not need to convert ammonia into urea. Suggest ONE reason for this. (1)
Freshwater bony fish eliminate ammonia by adding large quantities of water to make it less toxic. As they can afford to lose a lot of water, they eliminate their nitrogenous waste in the form of ammonia rather than urea.
OVERVIEW
A) DEFINITIONS & IMPORTANCE OF EXCRETION AND OSMOREGULATION
B) OSMOREGULATION IN A TERRESTRIAL INSECT
C) OSMOREGULATION IN BONY FISH (TELEOSTS)
D) THE HUMAN KIDNEY
The kidneys contribute to homeostasis
Let us see how:
Functions of the Human Kidney:
1. Removal of metabolic waste products
2. Regulation of the water content
3. Regulation of the pH of body fluids
4. Regulation of the chemical composition of body fluids by removal of substances which are in excess of immediate requirements
Position and structure of kidneys
External structure of a Pig Kidney
Kidneys are surrounded by a fibrous capsule:
Kidneys are surrounded by a fibrous capsule:
LS through human kidney
medulla
cortex
LS through human kidney
the apex of each pyramid
papilla
The renal artery branches inside kidney
Renal artery
Ureter
Renal vein
Each capillary supplies blood to hundreds of thousands of tiny filtration units called nephrons
Detail of a nephron
Two types of nephron:
Juxtamedullary nephron
Cortical nephron
Juxta = close to
CORTEX
MEDULLA
Loop of Henle
Collecting duct
Nephron is the basic structural & functional unit of the kidney
Deal with the control of blood volume under
normal conditions of water
availability
Increase water retention when
water is in short supply
Cortical nephron Juxtamedullary nephron
Nephron Structure
vasa recta
Slow blood flow:
important to produce a
concentrated urine
The nephron 1.5 million per kidney
collecting duct
Bowman’s capsule
distal tubule
loop of Henle
proximal tubule
The nephron blood supply
peritubular capillaries
Vasa Recta
glomerulus
branch of renal artery
afferent arterioles
efferent arterioles
branch of renal vein
The glomerular capillaries drain into efferent arterioles not venules. ‘Portal System’
Three key process in urine formation:
Ultrafiltration
Excretion =
ultrafiltration – reabsorption + secretion
Selective reabsorption
Ultrafiltration
takes place in the renal capsule
is filtration under pressure
pressure comes from blood pressure (hydrostatic pressure)
Glomerular Filtrate (GF): is the filtered fluid
chemical composition is similar to blood plasma, containing:-
Glucose
Amino acids
Vitamins
Ions
Nitrogenous waste
Some hormones
Water
Glomerular filtrate
Key Words!!
Nephron: structure in the kidney that acts as a microscopic filtration unit
Glomerulus:
dense mass of very fine blood capillaries at the nephron that act as a filter
Key Words!!
Bowman’s capsule: cup-shaped part of the nephron that holds a glomerulus and collects the products of filtration from it
Glomerular filtrate:
liquid removed from the blood by filtration in the kidney
Explain why proteins & RBC are not found in urine.
Too large to be filtered.
But can blood ever be detected in urine?
YES. But, this shows that
something is wrong .
Ultrafiltration takes place through three layers:
1) Endothelium of the blood capillary
2) Basement membrane of the blood capillaries
3) Epithelium of the renal capsule
Cells lining the Bowman’s capsule:
Podocyte
Squamous epithelium
Podocytes (modified squamous epithelial
cells):
highly modified for filtration
Podocytes:
each podocyte has many foot-like extensions projecting from its surface
the projections interlink with extensions from neighbouring cells
they fit together loosely, leaving slits called SLIT PORES or FILTRATION SLITS
The basement membrane is
the main filtration barrier
endothelial fenestration
Some types of capillaries have ‘fenestrations’
Fenestrated capillary has holes to facilitate filtration Continuous capillary
Filtrate passes through the
basement membrane & not across cells
Filtration through podocytes
Basement membrane
Fenestrated capillaries
(capillaries with windows) Permeable to substances < 100 nm
endothelial cell
fenestration
nucleus
Filtration Barrier
mesangial cells
podocyte
slit pore
glucose amino acids
(basement membrane)
podocyte slit pore
Na+
- -
-
-
-
- - -
- -
-
-
-
-
-
-
- -
- -
- -
-
-
- -
- -
-
Limited permeability to molecules between
7000 > mwt > 70000 Da 4 nm > diameter > 8 nm
Freely permeable to small molecules mwt < 7000 Da
diameter < 4 nm
Not permeable to large molecules mwt > 70000 Da diameter > 8 nm
Water Permeable albumin 60000 Da
completely excluded…
because of –ve charge
endothelial cell fenestration
Bowman’s Capsule
Bowman’s Space
Proximal Tubule
petesmif@liv.ac.uk
The net filtration pressure =
1.3 kPa
8 kPa
2.7 kPa 4 kPa
Arterial pressure 8 kPa
Plasma osmotic pressure 4 kPa
Glomerular capsule pressure 2.7 kPa 6.7 kPa
Arterial pressure - Plasma osmotic pressure
Glomerular capsule pressure +
Net outward pressure = 8 – (4 + 2.7) = 1.3 kPa
Three ways to increase the filtration rate:
1. Raising blood pressure
2. Dilating the afferent arterioles (to decrease the resistance to the flow of blood into the glomerulus)
3. Constricting the efferent arterioles
Efferent arteriole
Afferent arteriole
Filtration pressure
GFR maintained
Afferent arteriole narrow
LOW pressure
HIGH pressure
Efferent arteriole
wide
Dilating the afferent arterioles &
Constricting the efferent arterioles
BUT when arterial pressure falls too low,
however, the kidney fails to produce urine
Arterial pressure 8 kPa
Plasma osmotic pressure 4 kPa
Glomerular capsule pressure 2.7 kPa 6.7 kPa
Net outward pressure = 8 – (4 + 2.7) = 1.3 kPa
The Proximal Convoluted Tubule
longest (14 mm) and widest (60 m) part of the nephron
carries filtrate from Bowman’s capsule to loop of Henle
CORTEX
MEDULLA
Function of the nephron is to :
actively secrete
waste substances from the blood capillaries to
the tubules
selectively reabsorb
substances useful to the body
Proximal Convoluted Tubule is composed of:
a single layer of cuboidal epithelial cells with extensive microvilli forming a ‘brush border’ on the inside surface of the tubule
Figure 44.9
Proximal Convoluted Tubule is adapted for reabsorption in three ways:
1. large surface area due to:
Figure 44.9
Microvilli
Basal channels
BLOOD FILTRATE
Tight junction
Epithelial cell
Proximal Convoluted Tubule is adapted for reabsorption:
Figure 44.9
2. numerous mitochondria (M)
Proximal Convoluted Tubule is adapted for reabsorption:
Figure 44.9
3.closeness of blood capillaries
blood capillary Glomerular filtrate
Microvilli Cuboidal epithelium
Over 80% of filtrate is reabsorbed in the proximal tubule
REABSORBED
all the glucose, amino acids, vitamins, hormones
about 80% water
about 80% sodium
about 80% chloride
about 80% potassium
about 40-50% urea
MECHANISM
diffusion + active transport
osmosis
diffusion
+ active transport
diffusion
Fig. 15 Selective reabsorption of sodium in the proximal convoluted tubule
Figure 44.9
1
Selective reabsorption of glucose in the proximal convoluted tubule
Figure 44.9
Secondary Active
Transport
Na+
glucose Na+
ATP ADP
Blood
Urine
Proximal tubule epithelial cell
petesmif@liv.ac.uk
Selective reabsorption in the proximal convoluted tubule
In humans:
Glomerular filtrate production: 125 cm3 min-1
Urine production: 1 cm3 min-1
24 cm3
100 cm3
Urine 1 cm3
125 cm3
Question: MAY, 2012
Briefly describe the following processes in the context of urine formation in humans.
a) Ultrafiltration. (2)
Filtration of blood occurs under high pressure provided by the heart. Small molecules which can cross the glomerular lining, end up as glomerular filtrate inside the Bowman’s capsule.
b) Selective reabsorption of glucose. (3)
Occurs in the proximal convoluted tubule. All glucose is reabsorbed in a normal person but appears in urine in a diabetic one. Secondary active transport is involved in the reabsorption of glucose. A symport binds sodium ions and glucose to transport them from the lumen of the proximal convoluted tubule into the epithelial cells. Glucose leaves the cell by facilitated diffusion through a carrier protein. Glucose diffuses into the blood capillary.
THE LOOP OF HENLE
Function: to conserve water
the concentration of urine produced is directly related to the:
length of the loop of Henle
thickness of the medulla relative to the cortex
The longer the loop of Henle, the more concentrated the urine that can be produced
BEAVER (abundant water)
RABBIT (moderate water)
SAND RAT (scarce water)
Question: [MAY, 2010]
Use your knowledge of biology to describe the selective advantage of the following adaptation.
Desert rats have a long loop of Henle. (5) The loop of Henle acts as a counter-current multiplier.
Fluid moves in opposite directions in the descending and ascending limbs. The ascending limb is permeable to salts which contribute towards a concentrated medulla. As water moves down the descending limb, it moves out into the vasa recta.
Desert rats need to conserve water. Thus having a long loop of Henle enables them to extract as much water as possible out of the glomerular filtrate as there is more time for reabsorption.
Question: [MAY, 2002]
The table below gives the thickness of the medulla in relation to the rest of the kidney in a number of mammals. The maximum urine concentration for each mammal is also given. The data suggest that maximum urine concentration increases with relative thickness of the medulla.
Mammal Relative thickness of medulla
Maximum urine concentration in arbitrary units
Beaver 1.0 52
Human 2.6 140
Kangaroo rat
7.8 550
Species X 9.8 940
a) Why is such a relation between urine concentration and the relative thickness of the medulla observed?
(1)
The thicker the medulla, the higher the urine concentration produced due to more chance for water reabsorption.
Mammal Relative thickness of medulla
Maximum urine concentration in arbitrary units
Beaver 1.0 52
Human 2.6 140
Kangaroo rat 7.8 550
Species X 9.8 940
b) What habitat is species X likely to inhabit? (1)
Desert / dry habitat
Mammal Relative thickness of medulla
Maximum urine concentration in arbitrary units
Beaver 1.0 52
Human 2.6 140
Kangaroo rat
7.8 550
Species X 9.8 940
Birds & Mammals are the only vertebrates:
which can produce a urine which is more concentrated than the blood
[hypertonic]
with loops of Henle
Loop of Henle
The loop of Henle creates a concentration gradient
humans can produce urine that is 4x more
concentrated than their blood plasma
11200/ 300 = 4
Pelvis Medulla
Cortex
a countercurrent multiplier mechanism made possible by the anatomical arrangement of the loops of Henle
The concentrating ability of the mammalian kidney arises from
hairpin turn of the loop of Henle
countercurrent refers to the opposing directions in which the tubule fluid in the descending and the ascending limbs flows
multiplier refers to the ability of this system to create a solute concentration gradient in the renal medulla
Medulla
Cortex
The loops of Henle do not themselves produce concentrated urine;
rather they increase the osmolarity of the extracellular fluid in the medulla in a graduated way:
from 300 to 1,200 mosm/l
Osmolarity is :
a measure of solute concentration
the osmolarity of a solution is the number of moles of active solutes per litre of solvent
osmole [Osm or osmol]
[For your information only]
The loops produce this effect as explained below
Three distinct regions in the loop of Henle
Thin ascending limb
Descending limb
Thick ascending limb
Thin walls
Thick walls
Permeability of the loop of Henle to water:
Highly permeable
Descending limb
Almost
totally impermeable
to water Thin ascending limb
Thick ascending limb
Permeability of the loop of Henle to Na+ & Cl-ions:
Not very permeable
Descending limb
Thin ascending limb
Thick ascending limb
Permeable
Active secretion
What happens to the concentration of the fluid in the ascending limb as it reaches the
distal convoluted tubule?
The fluid becomes very dilute
Distil convoluted tubule
Reason: IONS are lost
WHY it is vital for ions to move out of the tubule?
ions
To create an Osmotic Gradient From Cortex to Medulla
Pelvis Medulla
Cortex
The outer layer of the kidney is isotonic with the blood:
~300 milliosmoles/liter
The innermost layer (medulla) is very hypertonic: ~1200 milliosmoles/liter
The concentration gradient allows:
water to move out by
osmosis from the
descending loop of Henle
Vasa recta as countercurrent exchangers
• the countercurrent exchange of salt occurs in the vasa recta
1. Blood flowing into the medulla in the descending limb picks up salt from the hypertonic medulla.
2. As the surrounding medullary fluid becomes more salty toward the papilla, more salt is picked up by the descending vasa recta limb.
Vasa recta as countercurrent exchangers
3. But as the blood heads back up to the cortex in the ascending limb of the vasa recta, the interstitial fluid becomes less and less salty
4. This causes the gradient to reverse and salt diffuses back out of the vasa recta into the medulla
What is the importance of the vasa recta as an exchanger of salts?
1. to help conserve salt
2. keep the medulla hypertonic
Countercurrents exist when :
fluids flow in opposite directions in parallel and adjacent tubes
Fig. 20 Three Countercurrents:
1. the two limbs of the Henle's loop
Fig. 20 Three Countercurrents:
1. the two limbs of the Henle's loop
2. the two limbs of the vasa recta
Fig. 20 Three Countercurrents:
1. the two limbs of the Henle's loop
2. the two limbs of the vasa recta
3. the descending limb of Henle with the ascending limb of the vasa recta;
the ascending limb of Henle and the descending vasa recta
Question: [SEP, 2009]
Briefly describe the role of each of the following in osmoregulation in humans:
i) The descending limb of the Loop of Henle; (2)
Is permeable to water. Functions towards water conservation.
ii) The ascending limb of the Loop of Henle; (2)
Is relatively impermeable to water but permeable to salts. The tissue fluid inside the medulla becomes concentrated as salts move out of the ascending limb. This causes water to be drawn out of the descending limb.
Question: MAY, 2012
The diagram below shows the simplified structure of a human nephron. the loop of Henle
Substance Quantity passing
through P
Quantity passing
through Q
%
reabsorbed
Water 180 dm3 1.5 dm3 99.17%
Glucose 180 g 0 g 100%
Urea 53 g 25 g 52.8%
The table below represents the quantities of water, glucose and urea passing through P and Q over a period of time, while the last column shows the percentage reabsorption during the same period of time.
Question: MAY, 2012
a) Relate the role of structure R to the filtrate composition as it passes through Q. (5)
Structure Q is permeable to water. Water is reabsorbed by the vasa recta as fluid passes through Q. This is possible because the ascending limb creates the ideal concentration gradient within the medulla by losing ions. The thin ascending limb of Structure R is permeable to ions but impermeable to water. The thick ascending limb of Structure R allows ions to move actively out of it and is also impermeable to water. Loss of ions from the whole ascending limb, creates an ever increasing salt concentration on moving deeper into the medulla.
Question: MAY, 2012
Substance Quantity passing
through P
Quantity passing
through Q
%
reabsorbed
Water 180 dm3 1.5 dm3 99.17%
Glucose 180 g 0 g 100%
Urea 53 g 25 g 52.8%
b) Explain the biological significance of the percentage reabsorption of water and urea. (3)
Most of the water is reabsorbed to avoid dehydration.
Only half of the urea is reabsorbed so that it contributes to the concentration of solutes in the medulla. A high solute concentration is needed to ensure reabsorption of water from the loop of Henle.
THE DISTAL CONVOLUTED TUBULE AND COLLECTING DUCT
Functions:
1. fine tuning of the body fluid composition
2. control blood pH
Cell structure of the distal tubule:
similar structure to those of the proximal tubule, with:
microvilli
mitochondria
REGULATION OF KIDNEY FUNCTIONS
Several regulatory mechanisms act on the kidneys to maintain:
blood pressure
blood osmolarity
blood composition
Glomerular filtration rate is regulated
if the kidneys stop filtering blood, they cannot accomplish any of their functions
the maintenance of a constant GLOMERULAR FILTRATION RATE (GFR) depends on:
an adequate blood supply to the kidneys
at an adequate blood pressure
High pressure results from TWO ways:
1. renal arteries that deliver blood to the kidneys at high pressure because they are early branches off the aorta
2. AUTOREGULATORY mechanisms ensure adequate: blood supply blood pressure
regardless of what is happening elsewhere
in the body
Autoregulatory mechanisms involve:
The dilation of the afferent renal
arterioles when blood pressure falls
The release of the enzyme RENIN from the kidney into the blood, if arteriole filtration
does not keep the GFR from falling
1
2
Renin is released:
From: a group of secretory cells,
the JUXTAGLOMERULAR COMPLEX situated between the: distal convoluted afferent arteriole
When: the blood pressure volume
decrease
Role of renin:
angiotensinogen
[made by liver]
Renin
converts a circulating protein produced by the liver, ANGIOTENSINOGEN, into ANGIOTENSIN I
angiotensin I
angiotensin II or angiotensin
enzyme
1. It constricts the efferent arterioles.
2. It constricts peripheral blood vessels all over the body.
3. It stimulates the adrenal cortex to release the hormone ALDOSTERONE.
Efferent arteriole
Afferent arteriole
Filtration pressure
GFR maintained
Angiotensin has several effects that help restore the GFR to normal:
4. It acts on the brain to stimulate thirst.
Effect of aldosterone on the distal convoluted tubule:
Stimulates the Na+/K+ pumps in the cells of the tubule
Decrease in Na+
Results in a low blood volume & pressure (as less water enters by osmosis)
Activates angiotensinogen to become ANGIOTENSIN I
RENIN is released
ALDOSTERONE is released
Causes Na/K pump in distal tubule to take up Na+ into the blood
Water enters the blood
ANGIOTENSIN I changes into ANGIOTENSIN II
OSMOREGULATION, ADH & URINE FORMATION
In this topic we mention TWO hormones that affect the kidneys:
Urine
ADH (antidiuretic
hormone)
Posterior pituitary
Adrenal cortex
Aldosterone
ADH brings about the precise control of
solute potential in TWO ways:
1. increasing the permeability of the distal convoluted tubule and collecting duct to water
2. increasing the permeability of the collecting duct to urea
1. Urea moves into
medulla
2. Medulla becomes concentrated
RESULT: 3. Water moves out of
descending limb
ADH is released when osmoreceptors: detect a low level of water in blood
kidney
Water Salts
Fig. 23 The effect of ADH on the permeability of the distal convoluted tubule and collecting duct to water
Blood too concentrated
ADH level high
Blood too dilute
ADH level low
Dilute
urine
Urine concentrated
Water Salts
Fig. 22 Aquaporins
H2O
H2O
H2O
Release of ADH from the posterior pituitary is inhibited by drinking
alcohol & caffeine.
How would this affect urination?
Increases
ADH
Failure to release sufficient ADH leads to a condition: DIABETES INSIPIDUS
large quantities of dilute urine are produced
Water level regulation by negative feedback control
Water content of the blood normal
Water content of the blood HIGH
Water content of the blood LOW
Too much water drunk
Too much salt or sweating
Brain produces More ADH
Urine output LOW
Brain produces Less ADH
Urine output HIGH
High volume of water reabsorbed by kidney
Low volume of water reabsorbed by kidney
(small volume of Concentrated urine)
(large volume of dilute urine)
Question: [MAY, 2011]
1. The human kidney, in association with various hormones, plays a central role in the regulation of the chemical and physical characteristics of blood.
a) List THREE ways through which the human kidney may affect
the chemical composition of blood. (3) 1) Through aldosterone, the kidney determines the amount of
sodium and potassium in the blood. 2) Through ADH, the kidney plays a role in the amount of
water in the blood. 3) The kidney helps to keep the blood pH constant by secreting
H+ or OH-.
b) Complete the table below by filling in the empty spaces with the appropriate answers: (3)
Hormone Site of
production
Effect
Antidiuretic
hormone
Adrenal
cortex
Stimulates excretion of
potassium ions and
reabsorption of sodium
ions in the nephron
b) Complete the table below by filling in the empty spaces with the appropriate answers: (3)
Hormone Site of
production
Effect
Antidiuretic
hormone
Hypothalamus
Stimulates distal
convoluted tubule and
collecting duct to
reabsorb water
Aldosterone
Adrenal
cortex
Stimulates excretion of
potassium ions and
reabsorption of sodium
ions in the nephron
Question: [MAY, 2011]
c) Briefly describe how vasoconstriction and vasodilation of blood vessels may affect blood pressure. (4)
When blood vessels dilate, the blood pressure is lowered as there is less resistance to blood flow. When blood vessels constrict, the blood pressure becomes higher as cross-sectional area decreases.
Control of Blood pH Blood pH:
maintained at 7.4
Urine pH varies:
4.5 - 8.2
Abrupt changes in blood pH
are prevented by
Plasma proteins
Phosphate
Hydrogen carbonate
buffers
Longer-term adjustments in the ion balance of the blood
are made in the distal convoluted
tubule
Falls below 7.4:
distal tubule cells secrete H+ into the
urine
Rises:
distal tubule cells secrete OH- & HCO3
- into the urine
H+ HCO3
-
OH-
If the pH:
Essay Titles
1. Give an overview of the role of the mammalian kidney in excretion and osmoregulation.
[SEP, 2000]
2. Evaluate the role of the human kidney in excretion and osmoregulation. [SEP, 2002]
3. The mammalian kidney is a homeostatic organ. Discuss. [SEP, 2004]
Essay Titles
4. Write an account on biological countercurrent systems. [SEP, 2013]
Gills in bony fish – blood & seawater flow
Thermoregulation – blood flow in artery & vein in a limb
Excretion – loop of Henle; vasa recta
Pregnant female - blood of embryo & uterus in certain mammals like rabbits, but not humans
Erythropoietin [EPO] :
Made by: Kidneys
Released when:
O2 levels in blood are low
Causes:
RBC formation
Negative Feedback Control
EPO is abused by certain athletes. What is the benefit?
Blood carries more oxygen.
Manneken Piss [Brussels, Belgium]
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