Biology Excretion Notes

EXCRETION
What is Excretion?
Excretion is the process by which living organisms eliminate unwanted waste products
produced by metabolic processes.
This may sound similar to egestion, which is the process by which undigested material is
evacuated as a solid via the anus, however, they are two completely different processes.
Egestion doesn't get rid of any metabolic wastes, only solid undigested material from
digestion. Therefore, it cannot be classified as excretion.
Why is Excretion Important?

Well, without excretion to get rid of waste, the waste would build up within the organism.
Most waste products produced are harmful or toxic to the organism, so if they build up within
a cell, they will end up killing the cell.
Excretion in Plants
Plants, unlike humans and higher animals, lack a specialized/well-developed excretory
system. However, plants do need to excrete certain products, such as:
o the oxygen produced as a waste product in photosynthesis is excreted in the daytime

when photosynthesis occurs faster than respiration.
o the carbon dioxide produced as a waste product in respiration is excreted at night when
photosynthesis has stopped occurring or is occurring at a slower rate than respiration
o water, produced in respiration and excreted as water vapour at night when photosynthesis
is not occurring.
o organic waste products, including tannins, alkaloids, anthocyanins and the salts of organic
acids like calcium oxalate.
Oxygen, carbon dioxide and water diffuse out through the stomata of leaves and the lenticels
of bark-covered stems and roots. Organic waste products, however, are usually stored in dead
tissue like heartwood. Some plants convert them to insoluble substances (so that they cannot
affect metabolic processes and osmotic pressure) like oils or crystals (such as calcium ions
combining with the organic acid oxalic acid to form calcium oxalate crystals). These crystals
and insoluble substances are stored in the cells of leaves, bark, petals, fruits and seeds, which
are removed once the plant sheds those structures.
Excretion in Animals
Animals tend to have well-developed excretory systems. They produce several waste
products during metabolism, including:
o carbon dioxide, produced in respiration
o water, produced in respiration, lost in urine, sweat, and respiratory surfaces
o nitrogenous compounds, produced by the deamination of amino acids by the liver for
storage, such as urea, ammonia and uric acid
o bile pigments (like bilirubin) produced from breaking down haemoglobin from red blood
cells by the liver
o heat, produced from metabolism in general

Waste Products
These waste products are evacuated by specific organs in humans:
 The skin excretes water, salts, some urea and heat.
 The kidneys get rid of water, nitrogenous wastes and salts as urine.
 The lungs excrete carbon dioxide (and some water vapour) during exhalation/expiration.
 The liver excretes bile pigments and makes nitrogenous waste
The Excretory System

The excretory system consists of organs which remove metabolic wastes and toxins from the
body. In humans, this includes the removal of urea from the bloodstream and other wastes
produced by the body.
Excretory System Organs

 A pair of kidneys
 A pair of ureters
 A urinary bladder
 A urethra
Ureter
A pair of thin muscular tubes called the ureter comes out of each kidney extending from the
renal pelvis. It carries urine from the kidney to the urinary bladder.
Urinary Bladder
It is a muscular sac-like structure, which stores urine. The urinary bladder is emptied by the
process of micturition, i.e. the act of urination.
Urethra
This tube arises from the urinary bladder and helps to expel urine from the body. In males, it
acts as the common route for sperm and urine. Its opening is guarded by sphincter muscles.
Kidneys
Kidneys are bean-shaped structures located on either side of the backbone and are protected
by the ribs and muscles of the back. Each human adult kidney has a length of 10-12 cm, a
width of 5-7 cm and weighs around 120-170g.
The kidneys have an inner concave structure. The blood vessels, ureters and nerves enter the
kidneys through the hilum, which is a notch at the inner concave surface of the kidney. The
renal pelvis, a large funnel-shaped space present inner to the hilum, has many projections
known as calyces.
Structure of Kidney
Capsule
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.
Nephrons
Nephrons are the functional units of the kidney. Each nephron has two parts- the glomerulus
and renal tubule. The glomerulus consists of a bunch of capillaries formed by afferent
arterioles. Blood from the glomerulus is carried away by efferent arterioles.
The renal tubule starts with a cup-like structure called Bowman’s capsule and this encloses
the glomerulus. The highly coiled structure in the tubule next to Bowman’s capsule is the
proximal convoluted tubule.
Henle’s Loop
The next part of the tubule is Henle’s loop which has an ascending and a descending limb.
The ascending loop continues as a distal convoluted tubule. The distal convoluted tubules of
many nephrons open into the collecting duct. The cortical region of the kidney comprises of
Malpighian corpuscle, proximal convoluted tubule and distal convoluted tubule and the
medullary region contains a loop of Henle.
Excretion in Humans
Excretion is the process where all metabolic wastes are removed from the body. Excretion in
humans is carried through different body parts and internal organs in a series of processes.
Diffusion is the most common process of excretion in lower organisms. The human body is
an exceptional machine, where different life processes (respiration, circulation, digestion,
etc.) take place simultaneously. As a result, many waste products produced in our bodies are
in various forms that include carbon dioxide, water, and nitrogenous products like urea,
ammonia, and uric acid.
In addition to these, chemicals and other toxic compounds from medications and hormonal
products are also produced. Simple diffusion is not sufficient to eliminate these wastes from
our bodies. We need more complex and specific processes to eliminate waste products.
Blood contains both useful and harmful substances. Hence, we have kidneys which separate
useful substances by reabsorption and toxic substances by producing urine.
The kidney has a structural filtration unit called a nephron where the blood is filtered. Each
kidney contains a million nephrons.
Capillaries of the kidneys filter the blood and the essential substances like glucose, amino
acids, salts, and the required amount of water get reabsorbed, and the blood goes into
circulation.
Excess water and nitrogenous waste in humans are converted to urine. Urine thus produced is
passed to the urinary bladder via the ureters. The urinary bladder is under the control of
the Central Nervous System. The brain signals the urinary bladder to contract and through the
urinary opening called the urethra, we excrete the urine.
Mechanism of Excretion in Humans – Urine Formation
The urine is formed in the nephrons and involves the following steps:
 Glomerular Filtration
 Tubular Reabsorption
 Secretion
Glomerular Filtration
It is the primary step in urine formation. In this process, the excess fluid and waste products
from the kidney are filtered out of the blood into the urine collection tubules of the kidney
and eliminated from the body.
Tubular Reabsorption
It is the absorption of ions and molecules such as sodium ions, glucose, amino acids, water
etc. Water involves passive absorption, while glucose and sodium ions are absorbed by an
active process.
Secretion
Potassium ions, hydrogen ions, and ammonia are secreted out to maintain the equilibrium
between the body fluids.
Functions of the Parts of the Nephron
 Glomerulus- filters the blood
 Proximal Convoluted Tubules – Eighty per cent of water is reabsorbed here, along with
ions and nutrients. They remove toxins and help in maintaining the ionic balance and pH
of the body fluids. Descending Loop of Henle- is permeable to water and the filtrate gets
concentrated as it is impermeable to electrolytes.
 Ascending Loop of Henle- it is impermeable to water and permeable to electrolytes. The

filtrate gets diluted due to the movement of electrolytes from the filtrate to the medullary
fluid.
 Distal Convoluted Tubule - allows reabsorption of water and sodium ions according to
the body’s needs. It also helps in maintaining pH and ionic balance by secretion and
reabsorption of ions.
 Collecting Duct- a large amount of water is reabsorbed from the filtrate by the collecting
duct. The fluid in the collecting duct is more concentrated than the glomerular filtrate. It
contains mostly water, urea and salts, and is now called urine.
Kidney Failure and Dialysis

Holding urine, a physical blow, diabetes and hypertension can all lead to kidney failure. The
nephrons stop functioning properly, resulting in the blood volume and concentration not
being regulated. Wastes build up in the blood, and high concentrations can lead to death.
Dialysis is the process of filtering and purifying blood with the help of a machine. This
balances our fluids and electrolytes when the kidneys stop functioning. It is used to treat
people suffering from kidney disorders.

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EXCRETION

Why is Excretion Important?

o the oxygen produced as a waste product in photosynthesis is excreted in the daytime

o water, produced in respiration, lost in urine, sweat, and respiratory surfaces

o heat, produced from metabolism in general

The Excretory System

Excretory System Organs

Mechanism of Excretion in Humans – Urine Formation

Functions of the Parts of the Nephron

 Glomerulus- filters the blood

 Ascending Loop of Henle- it is impermeable to water and permeable to electrolytes. The

Kidney Failure and Dialysis

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