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Excretory products and their elimination
The process of removal of toxic nitrogenous waste from the body is called
excretion
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Ammonotelism
It is the process of excreting ammonia
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Ammonia is the most toxic form and requires a large amount of water for its
elimination
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Kidneys do not play any significant role in removal of ammonia
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It requires a moderate amount of water for its removal
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Ammonia produced by metabolism is converted into urea in the liver and
released into the blood
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Some
amount may be retained in the kidney matrix to maintain osmolarity
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Uric acid is least toxic and can be removed with a minimum loss of water
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Organs of Excretion
(1)Protonephridia or flame cells are excretory structures in
platyhelminthes, rotifers, some annelids and the cephalochordates
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(2)Nephridia are tubular structures in earthworms and other annelids
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(3)Malpighian tubules are present in most of the insects
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(4)Antenatal glands or green glands are excretory organs in crustaceans
like prawns
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HUMAN EXCRETORY SYSTEM
● Human excretory system consists of a pair of kidneys, a pair of ureter,
urinary bladder and urethra
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● It is situated between the levels of the last thoracic and third lumbar
vertebra, close to the dorsal inner wall of the abdominal cavity
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● The concave surface of the kidney has a notch called hilum through
which ureter, blood vessels and nerves enter
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● Inside the kidney, there are two zones: an outer cortex and an inner
medulla
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● The cortex extends in between the medullary pyramids as renal
columns called columns of Bertini
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● Nephron is the structural and functional unit of the kidney
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● Glomerulus is a soft mass of blood capillaries made up of wide afferent
capillaries and narrow efferent capillaries
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● The renal tubule begins with a double walled cup like structures called
Bowman’s capsule, It encloses the glomerulus
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● The tubule continues further to form a highly coiled network called
proximal convoluted tubule (PCT)
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It is differentiated into descending limb and ascending
limb
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● The DCT;s of many nephrons open into a straight tube called collecting
duct
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● The malpighian corpuscle, PCT and DCT of the nephron are situated in
the cortical region of the kidney
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● The majority of nephrons, Henle’s loop is very long and runs deep into
the medulla
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● The efferent arteriole emerging from the glomerulus forms a fine
capillary network around the renal tubule called the peritubular
capillaries
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Vasa recta is absent or highly reduced
in cortical nephrons
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It includes three stages: ultrafiltration,
tubular reabsorption and tubular secretion
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● Filtration of blood is carried out by the glomerulus of the kidney and
hence called glomerular filtration
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● The filtration of blood occurs through 3 layers in the glomerulus
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(2) the epithelium
of Bowman’s capsule
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● The glomerular capillary blood pressure causes filtration of blood
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In between
podocytes there are nubyte spaces called filtration slit or slit pores
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● The filtrate passes into the lumen of the Bowman’s capsule
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● The amount of the filtrate formed by the kidneys per minute is called
glomerular filtration rate (GFR)
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● i
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TUBULAR REABSORPTION
● The process of absorption of substance from filtrate is called tubular
reabsorption
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● 99% of glomerular filtrate is reabsorbed by the renal tubules
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But at the end of reabsorption only 1
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● Substances like glucose, amino acids, Na+ etc in the filtrate are
reabsorbed actively
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● PCT is lined by a simple cuboidal brush border epithelium, increases
the surface area for reabsorption
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● Henle’s loop: Reabsorption is minimum in this segment
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The ascending limb is
impermeable to water but allows transport of electrolytes actively or
passively
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● DCT - Conditional absorption of Na⁺ and water takes place in this
segment
...
It allows passage of small
amounts of urea into the medullary interstitium to keep up the
osmolarity
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● PCT - Helps to maintain the pH and ionic balance of the body fluids by
selective secretion of hydrogen ions, ammonia and potassium ions into
the filtrate and by absorption of HCO₃
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● Collecting Duct: It helps in maintenance of pH and ionic balance of
blood by the selective secretion of H⁺ and K⁺ ions
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● Henle’s loop and Vasa recta play a significant role in this process
...
This forms a counter current, blood in the two limbs of Vasa
recta flows in a counter current manner
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The osmolarity in the cortex will be around 300m osmol/
L, whereas in the inner medulla it will be around 1200m Osmol/ L
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● Nacl is transported by the ascending limb of Henle;s loop into the
descending limb of Vasa recta, From Vasa recta, Nacl is transferred into
the interstitium
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● This whole process is described as a counter current mechanism
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● Human kidneys have the capacity to produce urine nearly 4 times
concentrated than the initial filtrate
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● Excessive loss of fluid from the body can activate and stimulate the
hypothalamus to release antidiuretic hormone (ADH) or Vasopressin
from the neurohypophysis of the pituitary gland
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● Increase in body fluids volume switch off the osmoreceptors and
suppress the ADH release
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● ADH can affect the kidney function by constricting the blood vessels
(Vasa constrictions)
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JUXTAGLOMERULAR APPARATUS (JGA)
Decrease in the glomerular blood pressure activates the JG cells to
release renin
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Angiotensin II is a vase constrictor and hence increases the glomerular
blood pressure
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Aldosterone helps in reabsorption of NA+ and water from the distal
part of the tubule
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This mechanism is called the Renin angiotensin mechanism
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This causes vasodilation and
thereby decreases the blood pressure
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MICTURITION
● The process of release of urine is called micturition
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● As the bladder expands due to accumulation of urine, the stretch
receptors on the walls send signals to CNS
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Simultaneously
urethral sphincter relaxes causing the release of urine
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● Ina single day, an adult human excretes 1 to 1
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COMPOSITION OF URINE
It is light yellow coloured with slightly acidic pH (6
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25-30 gm of urea is excreted out per day
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ROLE OF OTHER ORGANS IN EXCRETION
The other organs which helps in excretion are
(1)Lungs: removes CO₂ and small quantities of water
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(3)Sweat glands: Eliminates water with Nacl, small amounts of urea, lactic
acid etc
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(4)Sebaceous glands : Eliminates sterols, hydrocarbons\and waxes through
sebum
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DISORDERS OF THE EXCRETORY SYSTEM
Uremia (accumulation of urea in blood)
● This happens when there is malfunctioning of the kidneys
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● Hemodialysis is done in such conditions
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● Blood is mixed with anticoagulants (e
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heparin) before passing it into
the dialysing unit
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● The porous cellophane membrane allows the diffusion of nitrogenous
wastes from blood into dialysing fluid based on concentration gradient
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It is also
mixed with anti heparin
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Transplantation is normally done under this condition
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Renal calculi/kidney stones
● Stone or insoluble mass of crystallised salts like calcium, oxalates etc
are formed within the kidney
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