Download Chapter 35 Excretion

Excretion
Chapter 35
Excretion
• Excretion – removal of cellular (metabolic)
waste products from an organism
• Metabolic wastes includes:
– Carbon dioxide & water (respiration)
– Water (Dehydration synthesis)
– Mineral salts (Metabolic processes)
– Nitrogenous wastes – (Protein metabolism)
– Heat – (Metabolic processes)
• Metabolic wastes may be toxic or nontoxic
Nitrogenous Wastes
• Formed by process of deamination.
• Deamination – removal of the amine group from
a protein
• Amine group converted into:
– Ammonia – highly toxic, very soluble. Requires large
amounts of water for removal
– Urea – less toxic, requires less water for removal
– Uric acid – harmless, water soluble, crystalline solid.
Requires very little or no water for removal
Types of Excretion
• Protozoa – simple diffusion out to
environment
• Plants – recycle gases (O2 & CO2), wall off
toxics in vacuoles
• Hydra – similar to Protozoa
• Earthworms – pairs of nephridia excrete
water, salts, & urea
• Nephridia filter the blood & interstitial fluids
Types of Excretion
• Insects – Use Malpighian tubules to filter
hemocoel
• Excrete uric acid & mineral salts w/
digestive wastes
• Uric acid is a water –conserving
mechanism developed in terrestrial
organisms
Major Excretory Organs
• Lungs – remove respiratory gas, some
heat & some water vapor
• Skin – remove heat, some water, very little
salts & urea
• Remove heat by evaporative cooling
• Evaporative cooling – use heat energy to
evaporate water off the skin surface
Major Excretory Organs
• Liver – converts ammonia into urea,
converts hemoglobin into bile, removes
toxins from the blood (Alcohol, some
drugs)
• Urea – ammonia + CO2
• Urea released back into the blood
• Removed by urinary system
Urinary System
• The urinary system maintains the chemical
composition of the blood and extracellular fluid
– Important for maintaining cellular metabolism
• Urinary system functions are performed through
three basic processes
1. Blood (or equivalent fluid) is filtered, removing water
and small dissolved substances
2. Nutrients are selectively reabsorbed from filtrate
3. Any remaining water and dissolved wastes are
excreted from the body
The Human Urinary System
• Several structures make up the human
urinary system
– Kidneys
– Ureters
– Bladder
– Urethra
The Kidneys
• Paired organs located on either side of the
spinal column, just above the waist
• Blood enters the kidneys by renal
arteries
• Blood leaves the kidneys by renal veins
The Ureters
• Urine flows from each kidney into a
muscular tube, called a ureter
• Rhythmic contractions of the ureter
transports urine to the bladder
The Bladder
• The ureters deposit urine in a hollow,
muscular chamber called the bladder
• The bladder wall is lined with smooth
muscle and is expandable
– Accommodates about 500 milliliters of urine
The Bladder
• Urine is contained within the bladder by
sphincter muscles at its base
– Internal sphincter: closest to bladder; under
involuntary control
– External sphincter: beneath internal
sphincter; under voluntary control
– When open, allow urine to flow into the
urethra
The Bladder
• When the bladder becomes distended, the
urination reflex is initiated
– The bladder contracts
– The internal sphincter opens involuntarily
– The external sphincter opens voluntarily
– Urine flows into the urethra
The Urethra
• A single tube that extends from the base
of the bladder to the external environment
– In males, it is about 8 inches long
– In females, it is about 1.5 inches long
The Anatomy of the Kidney
• Made of an outer layer of tissue (renal
cortex) overlying deeper tissues (renal
medulla)
• The subdivided inner chamber is called
the renal pelvis
– Collects urine and directs it into the ureter
• The tissues of each kidney are made up
of more than 1 million microscopic tubules
called nephrons
Urine Is Formed in the Nephrons
• Each nephron has three major parts
– Bowman’s capsule: cuplike structure that
contains a capillary glomerulus; blood is
filtered here
– Tubule: long twisted tube; composed of the
proximal tubule, loop of Henle, and distal
tubule
– Collecting ducts: collect fluid from many
nephrons and deposits it in the renal pelvis
Filtration in the Glomerulus
• The kidneys receive more than one liter of
blood per minute
• Blood flows from the renal artery, which
branches
• An arteriole directs blood to each nephron
• The arteriole branches within the
Bowman’s capsule into a capillary tuft,
called the glomerulus
Filtration in the Glomerulus
• Glomerular capillary walls are permeable
to water and small dissolved molecules
– Large molecules and cells cannot pass
• Blood pressure drives filtration of water
and small dissolved substances from
blood through glomerular walls
– Filtrate is deposited in the Bowman’s capsule
Filtration in the Glomerulus
• Blood remaining in the glomerulus
contains 20% less fluid
• Blood leaves the glomerulus by an
outgoing arteriole
• The arteriole branches to form porous
capillaries that intertwine the nephron
tubule
– These capillaries can reacquire nutrients from
the nephron filtrate
Urine Formation
• Filtrate in the Bowman’s capsule contains
water, wastes, and essential nutrients
• As filtrate flows through the nephron
tubule, urine is formed by 2 processes
– Tubular reabsorption
– Tubular secretion
Tubular Reabsorption
• Occurs primarily in the proximal tubule,
although water and other nutrients are
also reabsorbed in other tubule areas
• Tubule cells actively transport many
nutrients
– Examples: salts, amino acids, glucose
• Water follows nutrients by osmosis
– 99% of water reabsorbed from filtrate
Tubular Secretion
• Occurs primarily in the distal tubule
• Tubule cells actively transport wastes and
excess substances from blood into filtrate
– Examples: hydrogen and potassium ions,
ammonia, and many drugs
The Loop of Henle
• The loop of Henle allows urine to become
concentrated
• Mammal and bird urine has a higher
concentration of dissolved materials than the
blood
– Due to the actions of the nephron tubule and
collecting duct
• The loop of Henle (in the renal medulla) forms a
salt and urea concentration gradient around the
nephron tubule and collecting duct
The Loop of Henle
• The salt and urea gradient causes an
osmotic gradient between the filtrate and
surrounding fluids
• The longer the loop, the stronger the
gradient
• The most concentrated fluid surrounds the
bottom of the loop
The Loop of Henle
• As 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 when it enters the
collecting duct
– Urine can be more than four times as
concentrated as the blood
Role of Kidney in Homeostasis
• One drop of blood passes through a kidney
about 35 times per day
• Each time, the kidneys fine-tune blood
composition and help maintain homeostasis
• Regulating blood water content
• Regulating blood pressure and oxygen levels
• Monitoring and regulating dissolved substances
in blood
Water Content of the Blood
• If the kidneys did not reabsorb water, 50
gallons of urine would be produced daily
• The ability of kidneys to reabsorb water is
under the influence of antidiuretic
hormone (ADH)
– Produced by the posterior pituitary
– Makes the distal tubule and collecting ducts
more permeable to water
Water Content of the Blood
• ADH release is controlled by negative
feedback
– ADH is released when the hypothalamus
detects the increased blood osmotic
concentration (dehydration)
– ADH is released into the blood from the
posterior pituitary
– ADH increases the permeability of the distal
tubule and collecting duct to water
– More water is reabsorbed from urine
Blood Pressure
• When blood pressure falls, the kidneys release
the hormone renin into the bloodstream
• Renin catalyzes the formation of the hormone
angiotensin in the blood
• Angiotensin causes arterioles constriction,
elevating blood pressure
• Kidney arteriole constriction reduces filtration
– Results in less water removed from the blood and
adding to increase in blood pressure
Oxygen Levels
• When blood oxygen levels are low, the
kidneys release the hormone
erythropoietin
– Stimulates bone marrow to make more red
blood cells
– More red blood cells increase the oxygen
carrying capacity of the blood
Dissolved Substances
• The kidneys monitor blood composition
during filtration
• Tubular reabsorption and secretion rates
are adjusted to maintain blood
homeostasis
Dissolved Substances
• Substances the kidneys monitor include
– Glucose
– Amino acids
– Vitamins
– Urea
– Sodium ions
– Potassium ions
– Chloride ions
– Sulfate ions
Dissolved Substances
• Hydrogen and bicarbonate ions are
monitored and regulated to maintain blood
pH
• Harmful foreign substances are removed
from the blood by tubular secretion
– Examples: drugs, food additives, pesticides,
and nicotine
Vertebrate Kidney Adaptations
• Freshwater and saltwater environments
pose challenges to animals
• Animals are immersed in a solution with a
higher or lower osmolarity than their body
fluids
• Animals have evolved homeostatic
mechanisms, including kidney
adaptations, to maintain water and salt
within their bodies (osmoregulation)
Vertebrate Kidney Adaptations
• Freshwater fish live in a hypotonic
environment
– Water continuously leaks into their bodies by
osmosis
– Salts diffuse out
• To compensate, their kidneys can excrete
large quantities of extremely dilute urine
Vertebrate Kidney Adaptations
• Saltwater fish live in a hypertonic environment
– Water is constantly leaving their tissues by osmosis
– Salt constantly diffuses in
• Fish kidney tubules lack loops of Henle, and
cannot produce concentrated urine
• To conserve water, they excrete only small
quantities of urine containing salts not
eliminated by their gills
Vertebrate Kidney Adaptations
• Mammals have structurally different kidneys
depending on the availability of water in their
natural habitat
• If water availability is low, water can be
conserved by producing concentrated urine
– The longer the loop of Henle, the more concentrated
the urine
• Example: beaver nephrons
– Nephrons have only short loops of Henle
– Urine can be twice as concentrated as blood plasma
Vertebrate Kidney Adaptations
• Example: human nephrons
– Some nephrons have short loops, others
have long loops
– Urine can be four times as concentrated as
blood plasma
• Example: kangaroo rat nephrons
– Live in deserts
– Nephrons have only very long loops of Henle
– Urine can be fourteen times as concentrated
as blood plasma