Biology 20
Excretion
General, Structure & Function,
Four Steps to Urine Formation
Hormones, Composition of Urine
Introduction
• Humans are 70% water
– 1/3 of this is in plasma
• Blood – carries nutrients, picks up waste
• Wastes need to be removed
• Composition of fluids need to be kept in
balance
• Excretion: monitor, analyze, select, reject
EXCRETION
• Excretion is the process of removing
metabolic wastes from the body.
• During the metabolic processes of the
body, waste products are removed from
the site of production by the blood.
• As these wastes accumulate, the kidney
removes them from the blood and
excretes them to the environment.
• The excretory product becomes urine.
Excretion
• Process of removing cellular
waste
• Balance pH of blood
• Maintain water balance
• Happens in Kidney
FUNCTIONS OF THE
EXCRETORY SYSTEM
• Functions:
• To maintain homeostasis
• Regulates and stabilise the internal environment
by controlling 4 groups of chemicals
– Water:
– Excretion of metabolic wastes: Elimination of
poisonous by-products of chemical reactions:
– Osmoregulation fluid and salt regulation: regulation of
hydrogen, sodium, potassium, calcium, chloride ions:
– Regulation of body fluid composition: Removal of
essential nutrients that dangerous in excess:
WASTE PRODUCTS
• The principle metabolic wastes in most
animals are:
o Carbon Dioxide – is excreted through the
respiratory surfaces
o Water – excreted through respiratory
surfaces, skin as sweat as well as kidneys
o Nitrogenous Wastes – products of protein
and nucleic acid digestion
Waste
Products
NITROGENOUS WASTES
• Ammonia – The first metabolic product of
amino acid deamination – hydrolysis
(protein digestion)
o highly toxic
o cannot accumulate in body
o must be converted into less toxic uric acid and
urea
• Uric Acid – produced from ammonia
o not very soluble – can be excreted as a paste
with little water loss
o non - toxic
Deamination & Urea
• Proteins – contain a nitrogen molecule
– Amino acid – building blocks of protein
• Nitrogenous base
• Removal of N and H
• Occurs in the liver
• Byproduct – ammonia
– Toxic substance
• Ammonia combines with CO2 to form urea
• Urea –less toxic
• Uric acid – waste product from the breakdown of
nucleic acids (DNA)
NITROGENOUS WASTES
• Urea – converted from ammonia
o less toxic than ammonia
o produced in the liver
o can be excreted in concentrated form
o requires more water to excrete than uric acid
Urinary System
1. Vena cava
2.Right Kidney
3. Ureter
4. Bladder
5. Urethra
6. Aorta
7. Left Kidney
8. Renal Vein
9. Renal Artery
Urinary System
Adrenal gland
Kidney
Ureters
Bladder
Urethra
• ORGAN: Function
•
•
•
•
•
•
Kidney: site of blood filtration
Renal artery: brings blood to kidney
Renal vein: brings blood back to heart
Ureter: Brings waste TO bladder
Bladder: Temporary urine storage site
Urethra: Brings waste FROM bladder,
out of system
The Kidney
Structure
Function
Cortex
Outer layer
Where filtration occurs
Medulla
Inner area
Water reabsorption
Renal Pelvis
Collects waste
Joins kidney to ureter
STRUCTURE AND FUNCTION
OF KIDNEY
• Three distinct regions of the kidney
o Cortex – outer region
o Medulla – just below cortex
o Pelvis – a hollow chamber within the medulla
• The cortex and medulla of each kidney are
made up of a approximately one million
nephrons
The
Kidney
STRUCTURE AND FUNCTION
OF KIDNEY
• NEPHRON
– the structural and functional unit of the
kidney
Label the following diagram
Bowman’s
Capsule
Glomerulus
Renal
artery
Distal tubule
Proximal
tubule
Collecting
duct
Renal vein
Capillary bed
Loop of Henle
STRUCTURE
FUNCTION
Afferent arteriole
Brings blood to glomerulus
Efferent Arteriole
Leaves glomerulus, goes to vein
Glomerulus
Ball of capillaries – site of filtration
Bowman’s Capsule
Filter
Proximal Tubule
Distal Tubule
First tube in nephron. From BC.
Reabsorption of Water
Leads to collecting duct
Collecting duct
Empties waste into renal pelvis
Renal pelvis
Collecting site for all nephrons –
waste out to ureter
STRUCTURE AND FUNCTION
OF KIDNEY
• NEPHRON
o the structural and functional unit of the
kidney
• BOWMAN’S CAPSULE
o a double walled chamber – start of the
tubule.
The Nephron
STRUCTURE AND FUNCTION
OF KIDNEY
• GLOMERULUS
o network of capillaries within the Bowman’s
capsule
o high pressure (4x higher than in capillaries)
The Nephron
The
Nephron
STRUCTURE AND FUNCTION
OF KIDNEY
• PROXIMAL TUBULE
– active transport of many valuable substances
back into blood network
• glucose
• amino acids
• sodium
The Nephron
STRUCTURE AND FUNCTION
OF KIDNEY
• PROXIMAL TUBULE
• What doesn’t get absorbed?
o urea
o other toxic substances
o some salt
o much of the water
STRUCTURE AND FUNCTION
OF KIDNEY
• LOOP OF HENLE
o the long hair-pin turn!!
The Nephron
STRUCTURE AND FUNCTION
OF KIDNEY
• LOOP OF HENLE
o the long hair-pin turn!!
o some of the remaining water and salt will be returned
to the blood
o lies in the medulla which is relatively salty
(hypertonic)
STRUCTURE AND FUNCTION
OF KIDNEY
• DISTAL TUBULE and COLLECTING
DUCTS
The Nephron
The Nephron
STRUCTURE AND FUNCTION
OF KIDNEY
• DISTAL TUBULE and COLLECTING
DUCTS
o More water reabsorption
• This depends on the presence of certain
hormones
• (ADH) Anti diuretic Hormone
o Exact amounts of substances are reclaimed
to the blood
• very precise
Urine Formation
• Depends on three functions:
– Filtration
• Movement from blood – Bowman’s capsule
– Reabsorption
• Transfer of needed nutrient back INTO blood
• Tubules
– Secretion
• Movement of material from blood back into
nephron
Four Steps to Urine Formation
1)
FILTRATION
– Occurs at the junction of the glomerulus and
the wall of the Bowman’s capsule
– Each glomerulus receives blood from an
afferent arteriole and discharges its blood into
an efferent arteriole (hypertonic).
– Fluid and dissolved materials (nutrients,
wastes, ions) in the blood plasma pass from the
glomerulus into Bowman’s capsule
• due to a local increase in blood pressure within the
glomerulus
Four Steps to Urine Formation
1) Filtration (con’t)
– this material is then called nephric filtrate
– blood cells, plasma proteins and platelets are
too large to pass through the wall of the
capillary and therefore remain within the
capillary.
Four Steps to Urine Formation
2)
REABSORPTION
– in the proximal tubule
– returns about 99% of filtrate to the blood
– efferent arteriole feeds second capillary network
that surrounds the tubule
– this network receives reabsorbed
substances
• eventually leads to renal vein
Four Steps to Urine Formation
2) REABSORPTION (con’t)
• Water rushes into the blood because of
osmosis
Problem
• Not enough water is returned this way.
Solution
• Just actively transport water into the blood
right?
Wrong!!!
There is no way of ACTIVELY
transporting water
So how can we transport more
water into the blood?
Solution
2) Reabsorption (con’t)
o active transport of solutes into the capillary
bed
• glucose
• amino acids
• vitamins
• inorganic ions (Na+)
o water is passively reabsorbed from the
proximal tubule as these solutes are
actively removed from the filtrate
Four Steps to Urine Formation
2) Reabsorption (con’t)
• Reabsorption and the distal tubule.
– A more selective, precisely regulated
reabsorption occurs in the distal tubules
– Additional quantities of salts and water
may be reabsorbed
– The exact amount of each substance
reclaimed occurs in the distal tubules.
– excess is excreted in urine
• e.g. glucose and diabetes
Four Steps to Urine Formation
3) SECRETION
– This is the last chance for anything to leave
the blood and enter the urine
– Active transport
– Occurs in the distal tubule and collecting duct
– Hydrogen ion secretion – helps regulate
blood pH
• Distal tubule
• Na+ moves into the blood and H+ moves into the
tubule filtrate
– blood pH increases (ranges 7.3 - 7.4)
– urine pH decreases (ranges 4.5 - 8.5)
Four Steps to Urine Formation
– potassium secretion
• prevents accumulation of potassium that can
create neural and muscular problems
– some drugs are removed from the body by
secretion
– substances eliminated in this manner are
• creatine – by product of protein metabolism
• potassium
• penicillin
Four Steps to Urine Formation
4) Elimination
• Pathway
o collecting duct
o renal pelvis
o ureter
o urinary bladder
o urethra
o environment
What is in urine?
•
•
•
•
•
Excess sugars
Excess salts
Excess H+ ions
Urea and uric acid
Excess H2O
Control of Excretion
Disorders and Treatments
HORMONES
• ADH
• The Antidiuretic hormone
– influences the RATE of water reabsorption into blood
from collecting ducts
– released by the pituitary gland in the brain
• Osmoreceptors in brain stimulated by low blood
volume and increased osmotic pressure
– both of those means that there is not enough water in
the blood.
– rate of ADH secretion is increased. ADH saves water.
– More ADH = more H2O absorption = increased urine
concentration
– Very yellow concentrated urine
• Most water reabsorbed in proximal tubule
– permeable
• Rest of tubule permeable ONLY IF ADH is
present
– Ascending loop
– Distal tubule
Aldosterone and Sodium
• Aldosterone – hormone
– Produced in adrenal gland
– Increased Na+ uptake in nephron
– More water may also move out
• Osmosis (high to low)
• Blood pressure
– Less fluid – lower blood pressure
– Angiotensin produced
• Constricts blood vessels, increase blood pressure
• Causes release of aldosterone
– Aldosterone acts on distal tubule and
collecting duct
• More sodium reabsorbed
• Fluid level increase
• BP increases
Kidney Disease
• Diabetes Insipidus
– Problems with ADH production
– No ADH, no H2O reabsorption
– Huge urine output
• Up to 20 L/day
• Kidney Stones
–
–
–
–
Minerals forming solid crystals (Ca+, Na+)
Get lodged in pelvis or ureter
Can tear tissues as it moves out
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DTHEPAIN!!!!!!!!!!!!!!!!
• Stone removal
– Surgery
– Ultrasound
– catheter
Kidney Stones
• Kidney Dialysis
– Cleaning of blood
– Treatment of kidney failure
– Blood goes through a filter
– Concentration gradients remove waste
Continuous Dialysis
• Kidney Donation
– Human system built in twos
– Extra kidney for backup
– One kidney – can do all the work
– With less than 20% kidney function, problems
occur
• Requires kidney dialysis
– If problem gets really bad, might need a new
one  TRANSPLANT