FEDERAL UNIVERSITY OF OWERRI

JUBEP BIOLOGY 004 NOTES

4.0 EXCRETION

Excretion is the expulsion from the body of the waste products of metabolism
such as Carbon dioxide (CO2), urea, uric acid, ammonia, excess water, excess
mineral salts, bile pigments etc. These wastes are expelled from the body
because they can be toxic if they remain in the body and become harmful. In
mammals, the organ of excretion are the Lungs, kidneys, liver and skin.

Osmoregulation

This is the control of water and salt balance so that the concentration of
dissolved substances in the body fluids remains constant, especially
concentration of various ions e.g. Na2+, K+, Cl- and water content. Animals are in
two groups based on their form of osmoregulation. They are either Osmotic
conformers or Osmotic regulators.

a)Osmotic conformers (Osmo conformers): These are animals whose Osmotic

concentration of body fluids fluctuates according to that of the environment.
E.g. fresh water lower animals.

b) Osmotic regulators (Osmo regulators): Animals that maintain or

regulate within narrow limits the internal body osmolarity despite
environmental changes. E.g. Most marine vertebrates, higher fresh water
animals (they remain hyperosmotic)

Animals can either be tolerant of wide variations in salt concentrations in their

environment ( Euryhaline) or be tolerant of narrow/ limited variations in salt
concentrations in their environment (Stenohaline).

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Based on these descriptions, based on their osmoregulation, animals can
belong to either of the following categories:

i)Euryhaline osmotic conformers (tissue tolerant species): species that

tolerate wide external and therefore internal osmotic fluctuations.

ii)Stenohaline osmotic conformers: species that tolerate only limited external

and therefore internal osmotic fluctuations. Such organisms’ habitats are limited
to environments of constant concentration e.g. the hagfish is strictly marine and
stenohaline, its body fluids are iso-osmotic (have same concentrations as sea
water)

ii)Euryhaline Osmotic regulators: species that maintain within narrow limits

the internal body osmolarity over a wide range of environmental changes. E.g.
migratory fish like eel (Anguilla bengalensis) which migrate from fresh water
to sea water, Salmon (Salmo fario) which migrate from sea to fresh water for
spawning,
iv)Stenohaline osmotic conformers: species that regulate the internal body
osmolarity over a narrow range of external environmental changes.
Excretion and osmoregulation are both aspects of homeostasis carried out by
the kidney in mammals.
Functions of excretion and osmoregulation are:

• The removal of metabolic waste substances that would otherwise be

poisonous to the body.
• Regulate the ionic content of the body fluids such as sodium, potassium,
chloride and so on to ensure efficient metabolic and physiological process.
• Regulate the water content of the body fluids.
• Regulate the pH of the body fluids

Homeostatis
This is the Maintenance by the body of internal environment within narrow
range of conditions, regardless of the conditions in the external environment. It

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 involves the maintenance of the concentration of blood glucose, core body
temperature, blood PH (acid-base balance), concentration of oxygen and
Carbon dioxide.

Morphology of the excretory system

Animal Excretory Structures

Platyhelminthes e.g. planaria, Flame cells (solenocytes)
liverfluke, tapeworm
Annelids Nephridia
Insects, millipedes Malpighian tubules
Arachnids Book lungs
Fish Gills and kidneys
Amphibians Lungs, kidneys, liver and gills
Birds and Reptiles Lungs, kidneys and liver
Mammals Lungs, kidneys, liver and skin
Unicellular organisms Cell surface membrane
Crustaceans Antennal glands
Roundworm Excretory cell

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 Fig 4.1 The mammalian excretory system

The mammalian urinary system is composed of two Kidneys, that control the
composition of the body fluids by selecting, and removing unwanted substance
from blood; ureter, convey urine from the kidneys to the urinary bladder;
urinary bladder, stores urine temporarily before it is passed out of the body
the urethra.

Fig 4.2 Structure of the Kidney

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The outer layer is called the capsule. Inside the kidney, there are two zones- the
outer zone is the cortex and the inner zone is the medulla. The cortex extends in
between the medullary pyramids as renal columns called columns of Bertin.

Urinary Bladder

The urinary bladder is a muscular sac-like structure, which stores urine. It is

emptied by the process of micturition, i.e. the act of urination.

Urethra

The urethra is a tube which arises from the urinary bladder and helps to expel
urine out of the body. In males, it acts as the common route for sperms and
urine. Its opening is guarded by sphincter muscles.

Structure and function of the nephron -Ultrafiltration

The nephron are microscopic structures in the kidney that consists of a

glomerulus, a renal corpuscle and a renal tubule through which the glomerular
filtrate passes before it emerges as urine. here are millions of nephrons in each
human kidney.

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Fig 4.3 Structure of a Nephron

The mammalian nephron is a long tube-like structure, its length varying from
35–55 mm long. At one end, the tube is closed, folded and expanded, into a
double-walled, a cuplike structure called the Bowman’s capsule or renal
corpuscular capsule, which encloses a cluster of microscopic blood vessels
called the glomerulus. This capsule and glomerulus together constitute the renal
corpuscle. The nephron comprises two major portions:

1. Renal Tubule
2. Renal Corpuscle

Renal Tubule
The renal tubule is a long and convoluted structure that emerges from the
glomerulus and can be divided into three parts based on function.

 The first part is called the proximal convoluted tubule (PCT) due to its
proximity to the glomerulus; it stays in the renal cortex.

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  The second part is called the loop of Henle, or nephritic loop because it
forms a loop (with descending and ascending limbs) that goes through the
renal medulla.
 The third part of the renal tubule is called the distal convoluted tubule
(DCT) and this part is also restricted to the renal cortex.
The capillaries of the glomerulus are enclosed by a cup-like structure called
Bowman’s capsule. This structure extends to form highly coiled tubules called
PCT. PCT continues to form the loop of Henle which ascends to DCT, which in
turn opens into the collecting duct.

The major function of tubules is reabsorption and the process can either be
through active transport or passive transport. In addition, secretions by tubules
help in the urine formation without affecting the electrolyte balance of the body.

 Proximal Convoluted Tubule (PCT)

The blood brought by the renal artery is filtered by the glomerulus and then
passed to the PCT. Maximum reabsorption takes place in PCT of the
nephron.PCT is the region of renal tubule where reabsorption of essential
substances like glucose, proteins, amino acids, a major portion of electrolytes
and water takes place. The surface area for reabsorption is facilitated by the
lining of the simple cuboidal epithelium in them. Reabsorption takes place at
the expense of energy, i.e., the process is active.PCT selectively secretes ions
such as hydrogen, ammonia, and potassium into the filtrate and absorbs HCO 3–
from it. Thus, PCT maintains the electrolyte and acid-base balance of the body
fluids.

 Henle’s Loop
Henle’s loop has a descending and an ascending limb. Being parts of the same
loop, both the descending and ascending limbs show different permeability. The
descending limb is permeable to water but impermeable to an electrolyte, while
the ascending limb is permeable to electrolytes but impermeable to water. Since
the electrolytes get reabsorbed at the ascending loop of Henle, the filtrate gets
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diluted as it moves towards the ascending limb. But reabsorption is limited in
this segment.

 Distal Convoluted Tubule (DCT)

The DCT, which is the last part of the nephron, connects and empties its
contents into collecting ducts that line the medullary pyramids. The collecting
ducts amass contents from multiple nephrons and fuse together as they enter the
papillae of the renal medulla.

Similar to PCT, DCT also secretes ions such as hydrogen, potassium, and
NH3 into the filtrate while reabsorbing the HCO3–from the filtrate. Conditional
reabsorption of sodium ions and water takes place in DCT. Thus, it maintains
the pH and sodium-potassium level in the blood cells.

Collecting Duct

Collecting duct is a long, straight tube where H+ and K+ ions are secreted to
maintain the electrolyte balance of the blood. This is also the region where the
maximum reabsorption of water takes place to produce concentrated urine.

Renal Corpuscle

The renal corpuscle consists of a glomerulus surrounded by a Bowman’s

capsule. The glomerulus arises from an afferent arteriole and empties into an
efferent arteriole. The smaller diameter of an efferent arteriole helps to maintain
high blood pressure in the glomerulus.

The Bowman’s capsule is divided into three layers:

1. Outer Parietal layer: It is made up of epithelial cells with minute pores

of diameter 12nm.
2. Middle Basement membrane: This layer is selectively permeable.

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 3. Inner Visceral Layer: It consists of large nucleated cells called
podocytes which bear finger-like projections called podocel.

Types of Nephron

There are two types of nephron:

 Cortical nephron
These are the nephrons present within the cortex. These are short and extend
only into the medulla. They comprise about 80% of the total nephrons.

 Juxtamedullary nephron
These have long loops of Henle and extend into the medulla. These comprise
about 20% of the total nephrons.

Functions of the Nephron

The primary function of nephron is removing all waste products including the
solid wastes, and other excess water from the blood, converting blood into the
urine, reabsorption, secretion, and excretion of numerous substances.

As the blood passes through the glomerulus with high pressure, the small
molecules are moved into the glomerular capsules and travel through a winding
series of tubules.

The cell present in each tube absorbs different molecules excluding the glucose,
water, and other beneficial molecules which are called as the ultrafiltrate. As
the ultrafiltrate molecules travel down the tubules they become more and more
hypertonic, which results in more amount of water to be extracted from the
ultrafiltrate before it exits the nephrons.

The blood surrounding the nephron travels back into the body through the
renal blood vessels, which are free of toxins and other excess substances. The

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obtained ultrafiltrate is urine, which travels down via the collecting duct to the
bladder, where it will be stored and released through the urethra.

How Urine is formed in the Kidney

• Ultra-filtration: All small molecules. such as water, glucose and urea, are
filtered out of blood plasma in the glomerulus and produce a filtrate in
Bowman’s capsule which passes into the tubule of the nephron. The high
filtration pressure results from the fact that in each Bowman’s capsule the
afferent arteriole has a larger diameter than efferent arteriole. As a result of
this pressure, substances are forced out of blood into capsule space. Big
molecules like proteins and blood cells are retained in blood by the basement
membrane of the glomerular capillaries and do not appear in the filtrate.

• Selective re-absorption: All substances useful to the body and required to

maintain the water and salt composition of the body fluid at a stead state are
removed from the filtrate and re-absorbed into the blood capillaries,

- Glucose, water, salts, amino acids are absorbed in the proximal

convoluted tubule.
- Water is mainly absorbed in the loop of Henle
- In the distal convoluted tubules substances like water and salts are
absorbed according to need as fine adjustment of body fluids.
- Collecting duct mainly absorb water

• Secretion: Substances such as ammonia, uric acid and urea not required by
the body that were not filtered are secreted out into the filtrate, by cells of
the nephron before it leaves the kidney as urine to maintain the equilibrium
between the body fluids. For example K+, H+ and NH4+ in the distal
convoluted tubes. Potassium ions are secreted and re-absorbed: their
concentration in the blood depends on the balance between the two opposing
processes.

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 Basic mechanism of absorption and secretion in the tubules
• Active transport: e.g. glucose is absorbed from proximal tubules by active
transport.

• Differential permeability: Various regions of the nephron are selectively

permeable to ions, water and urea, for example the distal tubules are
relatively impermeable compared to proximal tubule; the permeability of the
collecting duct can be modified by hormones.

• Concentration gradients: A concentration gradient varying from

300mosmkg-1 of water in the cortex to 1200 mOsmkg-1 of water at the
papillae, is maintained within the interstitial region of the medulla in human
and results in passive absorption of water.

• Passive diffusion and osmosis: Sodium and chlorine ions and urea
molecules will diffuse either into or out of the filtrate, according to the
concentration gradients, wherever will pass out of the filtrate into a
concentrated fluid in the interstitial region of the kidney wherever the
nephron is permeable to water.

• Hormonal control: The regulation of water balance in the body and salt
excretion is achieved by the effect of hormones acting on the distal tubule
and collecting duct, such as anti-diuretic hormone, aldosterone, etc.

The functions of the loop of Henle in production of concentration urine.

The loop of Henle concentrates urine by a counter current flow mechanism.
Here urine flows down the descending loop and then exactly in opposite
direction in ascending loop.

(i) The Glomerulus filters the blood.

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 (ii) Proximal Convoluted Tubules (PCT) reabsorbs water, ions and
nutrients. They remove toxins and help in maintaining the ionic
balance and pH of the body fluids by secretion of potassium, hydrogen
and ammonia to filtrate and reabsorbing bicarbonate ions from the
filtrate.
(iii) The descending loop of Henle is fully permeable to water and sodium
chloride here water is absorbed by osmosis.
(iv) The ascending loop of Henle is impermeable to water thus less water
is absorbed from the filtrate. However, sodium and chloride ions are
removed by active transport and returned to the medulla. This coupled
with sluggish medullary blood, that reduces the rate of removal of
salts from the medulla, maintain a high salt concentration in the
medulla enabling maximum absorption water from the descending
loop of Henle and the collecting duct.
(v) Distal Convoluted Tubule (DCT) allows reabsorption of water and
sodium ions. It also helps in maintaining pH and ionic balance by
secretion and reabsorption of ions like PCT.
(vi) Collecting Duct reabsorbs a large amount of water from the filtrate.

Micturition
The urinary bladder is stretched and gets filled with urine formed in the
nephrons. The receptors present on the walls of the urinary bladder send signals
to the Central Nervous System, thereby, allowing the relaxation of sphincter
muscles to release urine. This is known as micturition.

Osmoregulation in the Kidney

The amount of water reabsorbed is geared to the body’s needs. When a person
loses a lot of water or takes excess salt, osmotic pressure of blood rises and

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detected by osmoreceptors of the hypothalamus. This causes Antidiuretic
hormone (ADH) to be released. ADH causes reabsorption of water from distal
convoluted tubule thus diluting blood. When the required dilution is not
achieved, drinking of water is initiated by osmoreceptors in the hypothalamus.

Drinking, particularly if excessive, results in the osmotic pressure of the blood

to fall below normal value. The osmoreceptors are less stimulated leading to
less ADH being produced. Less water is thus reabsorbed from the tubule leading
to copious dilute urine (diuresis).
Inability of a person to produce insufficient amounts ADH causes diuresis; the
disease condition is called diabetes insipidus.

Production of hypertonic/hypotonic urine is determined by the distal

convoluted tubules by the presence of absence of ADH. Presence of ADH leads
to reabsorption of water from the filtrate leading to hypertonic urine.

Control of sodium levels in blood

Another hormone called aldosterone responsible for maintaining a more or less
constant sodium level in the plasma has a secondary effect on water
reabsorption.

Any loss of sodium which causes a decrease in in blood volume causes a group
of secretory cells called juxtaglomerular complex between the afferent
arteriole and the distal convoluted tubule to release an enzyme renin.
- Renin causes a plasma globulin produced by the liver to form a hormone
angiotensin.
- Angiotensin stimulates the release of aldosterone from adrenal cortex.
- Aldosterone stimulate active uptake of sodium ions and consequently
water from glomerular filtrate restoring the volume of blood.

pH control
Proteins, hydrogen carbonate and phosphate buffers in blood prevent excess
hydrogen ions (H+), produced by metabolic activities from decreasing the pH of

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blood. Carbon dioxide released into the blood during respiration is regulated by
this system and prevented from causing changes in blood pH prior to excretion
in the lungs.

However, changes in pH of blood is counteracted by the distal convoluted

tubules. In the distal convoluted tubules, carbon dioxide reacts with water to
form hydrogen ions and hydrogen carbonate.
H2O + CO2 H+ + HCO3-
When the blood pH decrease, the distal convoluted tubules secretes H + into
tubules and retain HCO3-. H+ react with ammonia for form ammonium ions
(NH4+) that is excreted in urine.

When the pH of blood rises, the tubules secretes HCO 3- ions and retain H+
lowering pH.

Other Excretory organs

Skin

Skin is known to be the largest organ in the human body. It contains sweat
glands and the sebaceous glands. The sweat glands produce sweat which
contains sodium chloride, amino acids, water, and glucose. The sebaceous
glands excrete excess fat such as sterol and wax.

Lungs

Lungs facilitate gaseous exchange. They help in inhaling oxygen and

eliminating carbon dioxide and water vapour from the body.

Around 18L of carbon dioxide per hour and 400ml water per day is eliminated
from the body.

The effects of weather on excretion

In the hot/ dry season, humans to be outside and more active. We sweat more
(to lose heat) and it’s easy to become dehydrated if we don’t drink enough

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water. This impacts the amount of free fluid our body is willing to excrete, and
our urine volume is often reduced because of this. In cold weather, humans are
often indoors, around water sources, so we are more likely to be hydrated, less
active, and to sweat less. As such, we tend to have more free fluid to excrete via
our urine.

If we become cold very quickly, the body protects our internal organs in a
number of ways. One is an increase in urine excretion (cold-induced diuresis) in
response to the cold. Initially, blood is diverted away from the skin to avoid
losing its heat to the outside air. This means more blood ends up flushing
through your internal organs. In particular, blood rushes to your kidneys in a
greater volume and at a higher pressure. This increases the amount the kidneys
need to filter. As a result, your rate of urine excretion increases.

The composition of urine also changes in the cold/ wet season. The body
excretes a higher amount of calcium in the urine during cold/ wet season. This
is more likely due to the fact that humans tend to be less active in the cold / wet
season. This means there can be a higher risk of developing kidney
stones during the cold/ wet season for people who are susceptible. So it’s
important to stay warm, and hydrated, even when the weather is cold.

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