Download Longitudinal Section of Kidney

Excretion
• Animals must regulate the chemical
composition of its body fluids by balancing
the uptake and loss of water and fluids.
• Management of the body’s water content and
solute composition, osmoregulation, is
largely based on controlling movements of
solutes between internal fluids and the
external environment.
•
Animals must also remove metabolic
wastes before they accumulate to
harmful levels:
1.
2.
3.
4.
5.
Water
Carbon dioxide
Salts
Bile pigments
Nitrogenous waste
a.
b.
c.
d.
Ammonia
Urea
Uric Acid
Creatine
Metabolic Wastes
• Because most metabolic wastes must be dissolved
in water when they are removed from the body, the
type and quantity of waste products may have a
large impact on water balance.
• In general, the kinds of nitrogenous wastes
excreted depend on an animal’s evolutionary
history and habitat - especially water availability.
• The amount of nitrogenous waste produced is
coupled to the energy budget and depends on how
much and what kind of food an animals eats.
• Animals that excrete nitrogenous wastes as
ammonia need access to lots of water.
– This is because ammonia is very soluble but can
only be tolerated at very low concentrations.
– Therefore, ammonia excretion is most common
in aquatic species.
– Many invertebrates release ammonia across the
whole body surface.
– In fish, most of the ammonia is lost as
ammonium ions (NH4+) at the gill epithelium.
• Ammonia excretion is much less suitable
for land animals and even many marine
fishes and turtles.
– Because ammonia is so toxic, it can only be
transported and excreted in large volumes of
very dilute solutions.
– Most terrestrial animals and many marine
organisms (which tend to lose water to their
environment by osmosis) do not have access
to sufficient water.
• Instead, mammals, most adult amphibians,
and many marine fishes and turtles excrete
mainly urea.
• Urea is synthesized in the liver by
combining ammonia with carbon dioxide
and excreted by the kidneys.
• The main advantage of urea is its low
toxicity, about 100,000 times less than that
of ammonia.
• Urea can be transported and stored safely
at high concentrations.
• This reduces the amount of water needed
for nitrogen excretion when releasing a
concentrated solution of urea rather than a
dilute solution of ammonia.
• The main disadvantage of urea is that
animals must expend energy to produce it
from ammonia.
• Land snails, insects, birds, and many
reptiles excrete uric acid as the main
nitrogenous waste.
• Like urea, uric acid is relatively nontoxic.
• But unlike either ammonia or urea, uric acid
is largely insoluble in water and can be
excreted as a semisolid paste with very
small water loss.
• While saving even more water than urea, it
is even more energetically expensive to
produce.
Human Excretory Organs
1.
2.
3.
4.
Lungs
Skin
Liver
Kidneys (Urinary system)
The Human Urinary System
• Mammals have a pair of bean-shaped kidneys.
• These are supplied with blood by a renal artery
and a renal vein.
• Urine exits each kidney through a duct called the
ureter, and both ureters drain to a common urinary
bladder.
• During urination, urine is expelled from the urinary
bladder through a tube called the urethra, which
empties to the outside near the vagina in females
or through the penis in males.
• Sphincter muscles near the junction of the urethra
and the bladder control urination.
Human Urinary System
• The kidney has two distinct regions, an
outer renal cortex and an inner renal
medulla.
• Both regions are packed with microscopic
excretory tubules, nephrons, and their
associated blood vessels.
• Each human kidney packs about a million
nephrons.
Longitudinal Section of Kidney
Nephron: Functional Unit of Kidney
1.
2.
3.
4.
5.
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting tubule
Circulatory System Interface with
Urinary System
1.
2.
3.
4.
5.
6.
7.
8.
Renal artery
Renal arterioles
Afferent renal arteriole
Glomerulus  Bowman’s Capsule
Efferent renal arteriole
Peritubular capillary network  tubules
Renal venules
Renal vein
Nephron
Peritubular capillary network
Stages of Urine Formation
1. Pressure Filtration : Glomerulus
Bowman’s Capsule
2. Selective Reabsorption: Proximal
convoluted tubule and Loop of Henle
Peritubular capillary network
3. Tubular Secretion: Peritubular capillary
network Distal convoluted tubule
4. Urine concentration: Collecting tubule
Pressure Filtration
• Filtration occurs as blood pressure forces
fluid from the blood in the glomerulus into
the lumen of Bowman’s capsule.
– The porous capillaries, along with specialized
capsule cells called podocytes, are permeable
to water and small solutes but not to blood
cells or large molecules such as plasma
proteins.
– The filtrate in Bowman’s capsule contains salt,
glucose, vitamins, nitrogenous wastes, and
other small molecules.
Glomerulus and Bowman’s
Capsule
Pressure Filtration
Selective Reabsorption
• From Bowman’s capsule, the filtrate passes
through the proximal convoluted tubule and
the loop of Henle, a hairpin turn with a
descending limb and an ascending limb.
• Selective reabsorption of molecules out of
filtrate in nephron into blood in peritubular
capillary network occurs in the proximal
tubule
•Valuable nutrients, including glucose,
amino acids, and vitamins are actively
or passively absorbed from filtrate into
blood.
•The epithelial cells actively transport
Na+ out of the filtrate into the blood.
•This transfer of positive charge is
balanced by the passive transport of Clout of the filtrate into the blood.
•As salt moves from the filtrate to the
blood, water follows by osmosis.
Selective Reabsorption in
Proximal Tubule
• The reabsorption of water continues as the
filtrate moves into the descending limb of
the loop of Henle.
– The membrane of the descending limb is freely
permeable to water but not very permeable to
salt and other small solutes.
– For water to move out of the tubule by
osmosis, the interstitial tissue fluid bathing the
tubule must be hypertonic to the filtrate.
– Because the osmolarity of the interstitial tissue
fluid does become progressively greater from
the outer cortex to the inner medulla, the
filtrate moving within the descending loop of
Henle continues to loose water.
Selective Reabsorption in the
Loop of Henle
• In contrast to the descending limb, the
membrane of the ascending limb is
permeable to salt, not water.
– As filtrate ascends the thin segment of the
ascending limb, NaCl diffuses out of the
permeable tubule into the interstitial tissue
fluid, increasing the osmolarity of the medulla.
– The active transport of salt from the filtrate into
the interstitial tissue fluid continues in the thick
segment of the ascending limb, increasing the
osmolarity of the medulla
– This reinforces the water loss from the filtrate
in the descending limb (counter-current effect)
Selective Reabsorption in the
Loop of Henle
Tubular Secretion and Urine
Concentration
• The distal tubule plays a key role in
regulating the K+ and NaCl concentrations
in body fluids by varying the amount of K+
that is secreted into the filtrate and the
amount of NaCl reabsorbed from the
filtrate.
• The distal tubule also contributes to pH
regulation by controlled secretion of H+ and
the reabsorption of bicarbonate (HCO3-).
• As the collecting duct traverses the gradient
of osmolarity in the kidney from cortex to
medulla, the filtrate becomes increasingly
concentrated as it loses more and more
water by osmosis to the hypertonic
interstitial tissue fluid.
• In the inner medulla, the collecting duct
becomes permeable to urea, contributing to
hypertonic interstitial tissue fluid and
enabling the kidney to conserve water by
excreting a hypertonic urine.
Tubular Secretion and Urine
Concentration
K+
H+
Summary of Urine Formation
Summary of Urine Formation
Hormones of the Kidney
If blood pressure/ volume is too low:
• Anti-diuretic Hormone (ADH)
• Renin, Angiotensin II, Aldosterone
If blood pressure/ volume is too high:
• Natriuretic Peptide Hormones
Antidiuretic Hormone (ADH)
• One hormone important in regulating water
balance is antidiuretic hormone (ADH).
• ADH is produced in hypothalamus of
the brain and stored in and released
from the pituitary gland, which lies just
below the hypothalamus.
• Osmoreceptor cells in the
hypothalamus monitor the osmolarity
of the blood.
Antidiuretic Hormone (ADH)
No ADH Present - Collecting tubule is NOT permeable
to water and large volume of urine is produced
Collecting tubule
ADH Present - Collecting tubule is permeable to water
and a small volume of urine is produced
Collecting tubule
Renin
One of the functions of the kidney is to monitor
blood pressure at the juxtaglomerular apparatus
and take corrective action. If blood pressure
should drop:
1. The juxtaglomerular apparatus of the kidney
secretes the enzyme renin
2. Renin catalyzes the conversion of the plasma
protein angiotensinogen to angiotensin I
3. Angiotensin converting enzyme (secreted by
blood vessels) catalyzes the conversion of
angiotensin I to angiotensin II
4. Angiotensin II
a. constricts the walls of arterioles  increase blood
pressure
b. stimulates the proximal convoluted tubules to
reabsorb sodium ions  water follows by osmosis
 increase blood pressure
c. stimulates the adrenal cortex to release the
hormone aldosterone
d. aldosterone causes the kidneys to reclaim still more
sodium ions water follows by osmosis  increase
blood pressure
e. increases the strength of the heartbeat
f. stimulates the pituitary glands to release ADH
Action of Renin, Angiotensin II, and
Aldosterone
Angiotensin I
Natriuretic Peptide Hormones
1. In response to a rise in blood pressure,
the heart releases two peptides:
a. A-type Natriuretic Peptide (ANP) This
hormone of 28 amino acids is released from
stretched atria (hence the "A")
b. B-type Natriuretic Peptide (BNP) This
hormone (29 amino acids) is released from
the ventricles. (It was first discovered in brain
tissue; hence the "B")
2. Both hormones lower blood pressure by
a. dilating arterioles
b. inhibiting the secretion of renin and
aldosterone
c. inhibiting the reabsorption of sodium ions by
the kidneys
d. The latter two effects reduce the
reabsorption of water by the kidneys, so the
volume of urine increases as does the
amount of sodium excreted in it. The net
effect of these actions is to reduce blood
pressure by reducing the volume of blood in
the circulatory system