UNIT 2: EXCRETION

AND OSMOREGULATION

CAMPBELL & REECE (2010) CHAPTER 44

1. OSMOREGULATION AND EXCRETION

• Relative concentrations of water and solutes must be maintained within fairly narrow limits

• Osmoregulation regulates solute concentrations and balances the gain and loss of water.

• Freshwater animals show adaptations that reduce water uptake and conserve solutes

OSMOREGULATION AND EXCRETION

• Desert and marine animals face desiccating environments that can quickly deplete body water.

• Excretion gets rid of nitrogenous metabolites and other waste products.

OSMOREGULATION OF MARINE BONY FISH

• Marine bony fishes are hypoosmotic (have less solutes/salt than) to sea water.

• They lose water by osmosis and gain salt by diffusion and from food.• They balance water loss by drinking

seawater and excreting salts

OSMOREGULATION OF MARINE BONY FISH

OSMOREGULATION OF FRESH WATER FISH

Freshwater animals constantly take in water by osmosis from their hypoosmotic environment

They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine

Salts lost by diffusion are replaced in foods and by uptake across the gills

OSMOREGULATION OF FRESH WATER FISH

2. WASTE PRODUCTS UREA

AMMONIA URIC ACID

The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat

The amount of nitrogenous waste is coupled to the animal’s energy budget

AMMONIA Form when an amino group (NH2) adds a

third hydrogen ion. = NH3. Ammonia is rather toxic. If enough water is available to wash it from

the body it can be a nitrogenous excretory product.

Do not require a lot of energy for production.

Ammonia is excreted by most fish whose gills and skin are in direct contact with the water of the environment.

UREA Requires a lot of energy for production of Urea. Carrier molecules take up carbon dioxide and 2

molecules of ammonia to form and excrete urea. Urea is less toxic than ammonia. Can be excreted as a concentrated solution. –

Important allows terrestrial animals to conserve water.

Sharks, frogs and mammals excrete urea as their main nitrogenous waste.

URIC ACID Requires an extreme amount of ATP (energy) for

the production of Uric acid. Uric acid is not very toxic and do not dissolve in

water readily, therefore is excreted very concentrated – to conserve water in the body.

Uric acid is routinely excreted by insects, reptiles and birds.

3. EXCRETORY ORGANS AMONG INVERTEBRATES

Most animals have tubular excretory organs that regulate the water-salt balance of the body and excrete metabolic wastes into the environment.

3 examples amongst invertebrates:Planarians – use Flame cells.Earthworms – use Nephridia.Insects – use Malpighian tubules.

On occasion, excretion also involves other organs, such as the rectum in the earthworm and the gills in crayfish.

PLANARIANS (FLATWORMS)with flame cells

Flatworms use flame cells (bud-like outgrowths in a branched tubular excretion system) with cilia to filter out waste and get rid of excess water.

Earthworm with nephridiaEarthworms have nephridia with a ciliated opening, called the nephridiostome, that leads to a coiled tubule surrounded by blood capillaries. Waste move through the tubules and is excreted through the nephridiopore.

Insects with Malpighian tubules

Insects use thin tubules called Malpighian tubules attached to the gut for excretion

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4. HUMAN EXCRETORY ORGANS

Humans use several excretory organs: Lungs – Carbon dioxide (waste product from

cellular respiration) Kidneys and bladder - Ammonia, uric acid,

excess water, excess salt and urea (Waste products in deamination and food)

Liver - Colorants, alcohol, poisons(Waste products from food and drink)

Alimentary canal - Feaces (undigested food), cellulose.

Skin - Excess water and salt, ammonia.

5. HUMAN URINARY SYSTEM Consist of paired kidneys, renal artery, renal

vein, ureter, bladder and urethra. Each kidney is supplied with blood by a

renal artery (impure, oxygenated blood) and drained by a renal vein (pure, deoxygenated blood).

Urine exits each kidney through a duct called the urether.

Both urethers drain into a common urinary bladder, and urine is expelled through a urethra.

HUMAN URINARY SYSTEM

STRUCTURE OF A KIDNEYThe mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla

STRUCTURE OF A KIDNEY

NEPHRON Microscopically each kidney is composed of

over 1 million tiny tubules called nephrons. The nephrons of the kidneys produce urine. Some nephrons are located primarily in the

renal cortex, but others dip down into the renal medulla.

The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus.

STRUCTURE OF A NEPHRON

STRUCTURE OF A NEPHRON

6. THE FUNCTIONING OF THE KIDNEY: URINE FORMATION

An average person produces between 1 and 2 liters of urine daily. Urine production requires three distinct processes:

Glomerular filtration at the Malpighian body (Glomerulus and Capsule of Bowman.)

Tubular reabsorption at the convoluted tubules.

Tubular secretion at the convoluted tubules.

Glomerular filtrationBlood enters the kidney via the renal artery. This branches many times eventually forming

many afferent arterioles, each of which delivers blood to an individual kidney nephron.

The diameter of the afferent (incoming) arteriole is greater than the diameter of the efferent arteriole (by which blood leaves the glomerulus).

The pressure of the blood inside the glomerulus is increased due to the difference in diameter of the incoming and out-going arterioles.

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Glomerular filtrationThis increased blood pressure helps to force

the following components of the blood out of the glomerular capillaries into the cavity of the capsule of Bowman:

Most of the water; Most/all of the salts; Most/all of the glucose; Most/all of the urea.

Glomerular filtration• These substances are filtered in preference to

other components of blood based on particle size. • Blood cells and plasma proteins are not filtered

through the Glomerular capillaries because they are relatively larger in physical size.

• The water and salts that have been forced out of the glomerular capillaries pass into the Bowman's Capsule and are called the glomerular filtrate.

• It contains all the materials present in the blood except blood cells and most proteins - which are too large to cross the basement membrane of the Glomerular.

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Tubular reabsorptionOnly about 1% of the glomerular filtrate

actually leaves the body because the rest (the other 99% - useful substances) is reabsorbed into the blood while it passes through the renal tubules and ducts.

This is called Tubular reabsorption and occurs via three mechanisms. They are:

Osmosis DiffusionActive Transport

Tubular reabsorptionReabsorption varies according to the body's

needs, enabling the body to retain most of its nutrients.

The processes of tubular reabsorption occur in the following order:

Proximal convoluted tubule (PCT). Loop of Henle Distal convoluted tubule (DCT).

Tubular reabsorption: Proximal convoluted tubule

Most of the volume of the filtrate solution is reabsorbed in the proximal convoluted tubule.

This includes some water and most/all of the glucose.

Solutes are selectively moved from the glomerular filtrate to the plasma by active transport.

However, almost all glucose and amino acids, and high amounts of ions, are reabsorbed again later.

Following the movement of solutes (including Na+ ), water is then also reabsorbed by osmosis.

Tubular reabsorption:In the loop of henle The water, urea, and salts contained within the

ascending limb of Henle eventually pass into the distal convoluted tubule (DCT).

The DCT reacts to the amount of anti-diuretic hormone (ADH) in the blood:

The more ADH is present in the blood, the more water is re-absorbed into it. This happens because the presence of ADH in the blood causes the cells of the DCT to become more permeable to water, therefore they allow more water to pass from the tubular fluid back into the blood. This results in more concentrated urine.

Tubular reabsorption:In the loop of henle

If the level of ADH in the blood is reduced then the cells in the DCT becomes less permeable to water therefore less water is able to pass from the tubular fluid back into the blood - which results in less concentrated urine.

Tubular secretion• Involves substances being added to the tubular fluid.• This removes excessive quantities of certain dissolved

substances from the body.• Also maintains the blood at a normal healthy pH (pH 7.35

to pH 7.45)• The substances that are secreted into the tubular fluid

(for removal from the body) include:• Potassium ions (K+), • Hydrogen ions (H+), • Ammonium ions (NH4

+), • creatinine, • urea, • some hormones, and • some drugs (e.g. penicillin).

7. THE KIDNEY AND HOMEOSTASIS

The kidneys are organs of homeostasis for four main reasons:

1. They excrete metabolic waste such as urea, which is the primary metabolic waste of humans.

2. They maintain the water salt balance in the body.3. They maintain the acid-base (pH) balance in the

body.4. They secrete hormones.

THE KIDNEY AND HOMEOSTASIS

Through the hormone ADH the water is kept constant. (Refer to reabsorption in DCT)

The salt is reabsorbed through the hormone Aldosterone by means of a Sodium pump mechanism.

Aldosterone is a hormone that increases the reabsorption of sodium and water and the release (secretion) of potassium in the kidneys.

How do the kidneys aid in maintaining the acid-base balance in the body?

It is the tubular secretion of H+ and NH4+ from the blood into the tubular fluid (i.e. urine - which is then excreted from the body via the ureter, bladder, and urethra) that helps to keep blood pH at its normal level.

The movement of these ions also helps to conserve sodium bicarbonate (NaHCO3).The typical pH of urine is about 6.