CHAPTER 2.

ANIMAL EXCRETION AND OSMOREGULATION

A. Excretion

      The nitrogenous waste materials produced in the animal body due to metabolic reactions

are of no use to the cell. These waste materials if allowed to accumulate in the body, may

become toxic. Therefore, they must be removed from the body. The process of elimination of

metabolic waster products from the animal body to regulate the composition of the body fluids

and tissues is called excretion.     So      Excretion is the elimination of waste products from the

body. The non - volatile products are mainly excreted in the urine. The urinary system consists of

those organs which form, store and void urine.

These form the principal excretory product. They are formed mainly as the end product

of protein metabolism. Some wastes are generated by the metabolism of nucleic acids.

The principal nitrogenous waste product varies in different animals. Types of metabolic

waste produced by living systems. They are 1) Digestive waste, 2) Respiratory waste, 3) Excess

water and salts (through osmoregulation) and 4) Nitrogenous waste (through excretion)

Picture 2.1. Types of metabolic waste produced by living systems

So the Excretory System Functions are :

1. Collect water and filter body fluids.

2. Remove and concentrate waste products from body fluids and return other substances to

body fluids as necessary for homeostasis.

3. Eliminate excretory products from the body.

Excretory systems regulate the chemical composition of body fluids by removing

metabolic wastes and retaining the proper amounts of water, salts, and nutrients. Components of

this system in vertebrates include the kidneys, liver, lungs, and skin.

Not all animals use the same routes or excrete their wastes the same way humans do.

Excretion applies to metabolic waste products that cross a plasma membrane. Elimination is the

removal of feces.

B. Types of Excretion

      Different animals expel different nitrogenous compounds. On the basis of the type of

nitrogenous end product. There are three main types of nitrogenous wastes: ammonia, urea, and

uric acid. The type of waste an animal excretes depends on its living environment, because

nitrogenous waste excretion is accompanied by a certain amount of water loss.

Animals groups according their nitrogenous end product are :

1. Amoniotelic : ammonia as nitrogenous end product

2. Uricotelic : uric acid as nitrogenous end product

3. Ureotelic : urea as nitrogenous end product

Ammonotelic (ammonia-excreting) animals generally live only in aquatic habitats,

because ammonia is extremely toxic, and a large volume of water is required to maintain the

excreted ammonia level lower than the body level. This is needed because ammonia excretion

relies on passive diffusion, so a gradient is required between the organism and the environment

in order for the ammonia to flow from high concentration to low concentration.

Types of nitrogenous wastes Deamination – protein and nucleic acid metabolism

Three main types differing in terms of:

1. Toxicity

2. Amount of water needed for excretion

3. Energy needed for synthesis

Picture 2.2. Types of nitrogenous waste

Nitrogen wastes are a by product of protein metabolism. Amino groups are removed from

amino acids prior to energy conversion. The NH2 (amino group) combines with a hydrogen ion

(proton) to form ammonia (NH3).

Ammonia is very toxic and usually is excreted directly by marine animals. Terrestrial

animals usually need to conserve water. Ammonia is converted to urea, a compound the body

can tolerate at higher concentrations than ammonia. Birds and insects secrete uric acid that they

make through large energy expenditure but little water loss. Amphibians and mammals secrete

urea that they form in their liver. Amino groups are turned into ammonia, which in turn is

converted to urea, dumped into the blood and concentrated by the kidneys.

Many invertebrates such as flatworms use a nephridium as their excretory organ. At the

end of each blind tubule of the nephridium is a ciliated flame cell. As fluid passes down the

tubule, solutes are reabsorbed and returned to the body fluids.

Insects have a network of Malpighian tubules extending throughout much of the body

cavity and attached to the alimentary canal between the midgut and the hindgut. The secretory

cells which line the walls of these long, thin tubules secrete KCl, NaCl, and phosphate from the

hemolymph (blood) into the lumen of the tubule. Smaller molecules, such as water, amino acids,

and sugars diffuse down their concentration gradient and into the lumen. The fluid then flows

along the tubule and into the gut. As the fluid passes through the hindgut, water and valuable

ions are transported back into the hemolymph, leaving behind a concentrated waste for excretion

from the body.

Body fluids are drawn into the Malphigian tubules by osmosis due to large concentrations

of potassium inside the tubule. Body fluids pass back into the body, nitrogenous wastes empty

into the insect's gut. Water is reabsorbed and waste is expelled from the insect. Study

invertebrate excretion like protozoa, porifera, plathyhelminthes,annelid,insect,mollusk and

arthropoda !

C. Kidney and urine formation

         All vertebrates have paired kidneys. Excretion is not the primary function of kidneys.

Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.

Kidney is main organ in excretory. The nephron or uriniferous tubule is the functional unit of the

kidney. A nephron consists of a twisted tubule closed at one end, open at the other with a

network of associated blood vessels. Each kidney of man is formed of about one million

nephrons.

The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus,

and a long renal tube. Blood flows into the kidney through the renal artery, which branches into

capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the

blood to filter into the capsule. Fluid flows through the proximal tubule, which include the loop

of Henle, and then into the distal tubule. The distal tubule empties into a collecting duct. Fluids

and solutes are returned to the capillaries that surround the nephron tubule.

The nephron has three functions:

1. Glomerular filtration of water and solutes from the blood.

2. Tubular reabsorption of water and conserved molecules back into the blood.

3. Tubular secretion of ions and other waste products from surrounding capillaries into the

distal tubule.

Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component

16 times each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5

liters are reabsorbed. The remaining 1.5 liters forms urine. The haemodialyzer or artificial

kidney is a machine used to filter the blood of a person whose kidneys are damaged. The process

is called haemodialysis.

      Grafting a kidney from a donor who is compatible to the recipient to restore kidney

functions in a recipient is called renal transplantation. All patients with renal failure are

considered eligible for kidney transplantation.

     .

     The liver is a multipurpose,largest gland in vertebrates. It plays a vital role in excretion of

many substances.

     Skin plays an important role in excretion in the mammals. Skin of mammals is glandular and

has 2 types of glands, e.g., sebaceous glands and sudoriferous or sweat glands.

Components of The Nephron are :

Glomerulus: mechanically filters blood

Bowman's Capsule: mechanically filters blood

Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino

acids

Loop of Henle: Countercurrent exchange, which maintains the concentration gradient

Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.

In some cases, excess wastes crystallize as kidney stones. They grow and can become a

painful irritant that may require surgery or ultrasound treatments. Some stones are small enough

to be forced into the urethra, others are the size of huge, massive boulders.

A kidney receives the blood from the renal artery that arises from the dorsal aorta. In the

kidney, the renal artery divides and subdivides to send an afferent arteriole into each Bowman's

capsule. Here the afferent arteriole forms a bunch of about 50 parallel capillaries called

glomerulus. These rejoin to form the efferent arteriole. The efferent arterioles forms a network in

the cortex around the proximal and distal convoluted tubules of the nephron.

Kidneys perform a number of homeostatic functions:

1. Maintain volume of extracellular fluid

2. Maintain ionic balance in extracellular fluid

3. Maintain pH and osmotic concentration of the extracellular fluid.

4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid. \

Infection, environmental toxins such as mercury, and genetic disease can have

devastating results by causing disruption of kidney function. Many kidney problems can be

treated by dialysis, where a machine acts as a kidney. Kidney transplants are an alternative to

dialysis.

Kidney is role in pH regulation in animal body. Regulation of pH is governed by the

carbon dioxide/bicarbonate buffering system in the body, which consists of three steps:

CO2 + H2O <==> H2CO3 <==> HCO3- + H+

CO2 + OH- + H+ <==> HCO3- + H+

HOH <==> OH- + H+

The excretion of acid by the kidney is one of the two major factors which influence this

system (the other being the excretion of carbon dioxide by the lungs). The excretion of hydrogen

ions (acid) in the urine is primarily responsible for maintaining the plasma HCO3- concentration.

Mammalian urine is mildly acidic, with a pH of about 6, and contains no bicarbonate. However,

the initial glomerular filtrate has a high bicarbonate concentration and a low hydrogen ion

concentration. Therefore, in the process of urine formation, acid must be added to the filtrate,

and bicarbonate must be removed.   Therefore, the excretion of H+  and the recovery of HCO3- are

both important mechanisms by which the kidneys help the body regulate pH.

This process is accomplished by special cells in the distal tubule and collecting duct,

called A-type cells and B-type cells. The A-type cells are acid-secreting cells that have a proton

ATPase in the apical membrane and a Cl-/ HCO3- exchange system in the basolateral membrane.

The cells also contain carbonic anhydrase, which hydrates carbon dioxide passing through the

membrane to form protons and bicarbonate ions. The protons formed are pumped back into the

lumen and can react with the bicarbonate in the filtrate to form carbon dioxide and water, which

can diffuse back into the cell, and create an uptake of bicarbonate back into the blood.

B-type cells, on the other hand, are base-secreting cells. They have a different form of

chloride/bicarbonate exchanger in the apical membrane than the A-type cells, and secrete

bicarbonate into the lumen of the tubule in exchange for chloride ions.

Regulation of pH is accomplished then, by altering the activity of A and B-type cells,

which determines whether bicarbonate is reabsorbed or secreted.

Another mechanism used in pH regulation is the uptake of H+ by HPO4- and NH3 in the

lumen to trap excess H+ in the filtrate. This occurs in order to bind H+ with something so that

these protons will not move back into the epithelial cells and the blood, which would lower pH.

      The urine is a pale yellow coloured fluid. The yellow colour is due to the presence of

urochrome pigment formed from the haemoglobin of dead RBC's in the liver cells. It is acidic in

nature and has a pH of 6.0. It has a faint anamalic odour due to the presence of urinod. It soon

gets a strong smell of ammonia which forms as a result of degradation of urea.

The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron,

an evolutionary modification of the nephridium, is the kidney's functional unit. Waste is filtered

from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and

collects in the bladder. The bladder can distend to store urine that eventually leaves through the

urethra.

Urine Production involve 1)Filtration in the glomerulus and nephron capsule,

2)Reabsorption in the proximal tubule and 3)Tubular secretion in the Loop of Henle.

Picture 2.3.. Urine formation

Function of urine formation are Dispose of metabolic wastes, Regulate solute concentrations

in the body and Transport epithelia arranged in tubes

4 major processes in urine formation are

1. Filtration, pressure-filtering of body fluids producing a filtrate (water, salts,

sugars, amino acids, N-wastes)

2. Reabsorption, reclaiming valuable solutes (glucose, salts, amino acids) from the

filtrate

3. Secretion, addition of larger molecules like toxins and other excess solutes from

the body fluids to the filtrate

4. Excretion, the filtrate leaves the system

Picture 2.4. Kidney and urine formation

     A partial or total inability of kidneys to carry on excretion and salt - water regulatory

functions is called as kidney or renal failure.

     Mammals and birds secrete a hypertonic urine. This is a main adaptation of land rentilates for

the conservation of water. There are 2 counter current mechanisms operating inside the kidneys.

Picture 2.5. Counter current mechanism

H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

300

300 100

400

600

900

1200

700

400

200

100

Activetransport

Passivetransport

OUTERMEDULLA

INNERMEDULLA

CORTEX

H2O

Urea

H2O

Urea

H2O

Urea

H2O

H2O

H2O

H2O

1200

1200

900

600

400

300

600

400

300

Osmolarity of interstitial

fluid(mosm/L)

300

Picture 2.6. Active and passive transport in counter current mechanism

D. OSMOREGULATION

Osmo regulation is the process which regulates the concentration and osmotic pressure of

blood by regulating the water contents of blood plasma. It is an important process as excessive

loss of water may cause dehydration whereas excess of water intake may dilute the body fluids

Osmoregulation is the regulation of water and ion concentrations in the body. Keeping

this regulation precise is critical in maintaining life in a cell. Balance of water and ions is partly

linked to excretion, the removal of metabolic wastes from the body.

The excretory system is responsible for regulating water balance in various body fluids.

Osmoregulation refers to the state aquatic animals are in: they are surrounded by freshwater and

must constantly deal with the influx of water. Animals, such as crabs, have an internal salt

Proximal tubule – secretion and reabsorption

FiltrateH2OSalts (NaCl and others)HCO3

–

H+

UreaGlucose; amino acidsSome drugs>> Same concentration of substances in blood plasma

Key

Active transport

Passive transport

CORTEX

OUTERMEDULLA

INNERMEDULLA

Descending limbof loop ofHenle – reabsorptionPermeable to water but not to salt

Thick segmentof ascendinglimb – reabsorption- Impermeable to water but permeable to salt

Thin segmentof ascendinglimb

CollectingDuct – permeable to water but not to salt, bottom portion is permeable to urea

NaCl

NaCl

NaCl

Distal tubule – secretion and reabsorption

NaCl Nutrients

Urea

H2O

NaClH2O

H2OHCO3 K+

H+ NH3

HCO3

K+ H+

H2O

1 4

32

3 5

concentration very similar to that of the surrounding ocean. Such animals are known as

osmoconformers, as there is little water transport between the inside of the animal and the

isotonic outside environment.

Osmoregulation function are 1) balance of uptake and loss of water and solutes and

2)Controlled movement of solutes between internal fluids and environment. Mechanism of

osmoregulatin on fish are deferent between osmoconformer and osmoregulation.

Osmoconformer involve marine animals that isoosmotic with environment. Osmoregulator

involve freshwater, marine, and terrestrial animals that adjust internal osmolarity.

Picture 2.7. Osmoregulation on saltwater fish and freshwater fish

Marine vertebrates, however, have internal concentrations of salt that are about one-third

of the surrounding seawater. They are said to be osmoregulators. Osmoregulators face two

problems: prevention of water loss from the body and prevention of salts diffusing into the body.

Fish deal with this by passing water out of their tissues through their gills by osmosis and salt

through their gills by active transport. Cartilaginous fish have a greater salt concentration than

seawater, causing water to move into the shark by osmosis; this water is used for excretion.

Freshwater fish must prevent water gain and salt loss. They do not drink water, and have their

skin covered by a thin mucus. Water enters and leaves through the gills and the fish excretory

system produces large amounts of dilute urine.

Terrestrial animals use a variety of methods to reduce water loss: living in moist

environments, developing impermeable body coverings, production of more concentrated urine.

Water loss can be considerable: a person in a 100 degree F temperature loses 1 liter of water per

hour.

E. Hormone Control of Water and Salt

Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative

feedback. ADH is released from the pituitary gland in the brain. Dropping levels of fluid in the

blood signal the hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to

increase water absorption in the kidneys. This puts more water back in the blood, increasing the

concentration of the urine. When too much fluid is present in the blood, sensors in the heart

signal the hypothalamus to cause a reduction of the amounts of ADH in the blood. This increases

the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.

Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from

the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the

blood, causing more sodium to pass from the nephron to the blood. This causes water to flow

into the blood by osmosis. Renin is released into the blood to control aldosterone.

DISCUSSION

How do animals regulate their water intake in different environments?

Fish and insects regulate osmotic balance

The sources of water gain and loss

The major problems that animals face with regard to osmoregulation

Why and how do organisms excrete metabolic wastes (particularly nitrogenous wastes)?

How does the mammalian kidney produce urine?

the lumen of the nephron to become part of the filtrate. 

How do the kidneys regulate pH?

DISKUSI :Bagaimana hewan mengatur asupan air mereka dalam lingkungan yang berbeda?Ikan dan serangga mengatur keseimbangan osmotikSumber air keuntungan dan kerugianMayor masalah yang dihadapi hewan berkaitan dengan osmoregulasi Mengapa dan bagaimana organisme mengekskresikan limbah metabolik (limbah terutama nitrogen)?Bagaimana ginjal mamalia menghasilkan urin?lumen nefron untuk menjadi bagian dari filtrat.Bagaimana ginjal mengatur pH?