Topic 11.3 - Kidneys

Excretion is the removal of

metabolic waste products

from the body. It is the

jobs of kidneys to clean the

bloodstream which supplies

pathways and carries waste

from tissues.

Urea is a common waste

product that must be

removed. It is a waste

product of amino acid

deamination.

11.3.1

11.3.U9 The type of nitrogenous waste in animals is correlated with evolutionary history and habitat.

http://www.bio.miami.edu/dana/dox/nitrogenouswaste.html

Mammals metabolize ammonia into a molecule called urea.

Much of metabolic waste is in the form of Nitrogen

Reptiles and birds go one step further, packaging their nitrogenous waste as uric acid. It requires more metabolic energy to make than urea, but is less toxic, and requires very little water to flush from the body.

Animals such as fish and amphibians, which have constant access to waterflush their nitrogenouswaste primarily as ammonia.

Terrestrial animals, however, because they have less access to water, have been under selective pressure to "repackage" their toxic ammonia as less toxic molecules which can be flushed with less water

http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/44_08NitrogenousWastes_L.jpg

11.3.U1 Animals are either osmoregulators or osmoconformers.

Osmoconformers maintain an internal conditions that are equal to osmolarity of their environment.

Minimizing the osmotic gradient minimizes the water movement in and out of cells. A disadvantage is that internal conditions may be sub- optimal.

Most Osmoconformers are marine invertebrates, e.g. starfish.

Osmoregulators tightly regulate their body osmolarity, which always stays constant, irrespective of their environment.

Kidneys play a large role in osmoregulation by regulating the amount of water reabsorbed. A disadvantage is that osmoregulation costs the animal ATP.

Osmoregulators are much more common in the animal kingdom, e.g. bony fish.

Osmoregulation in saltwater fish Osmoregulation in freshwater fish

https://en.wikipedia.org/wiki/Osmoregulation#Regulators_and_conformers

11.3.U2 The Malpighian tubule system in insects and the kidney carry out osmoregulation and removal of

nitrogenous wastes.

#Malpighian tubules are longer and more convoluted than shown in this simplified illustration, they extend into the body cavity, where they are surrounded by hemolymph.

http://biology-forums.com/index.php?action=gallery;sa=view;id=1281

*Hemolymph is a fluid (analogous to the blood) that circulates in the interior of the insect’s body remaining in contact with the tissues.

The removal of nitrogenous waste and osmoregulation in insects by the Malpighian tubule

*

#

11.3.1

Anatomy of a Kidney

11.3.2

• Renal artery = carries blood into each kidney

• Renal vein = drains blood from the kidneys

• Ureter = carries away urine that has collected in the

renal pelvis

• Medulla = contains

loops of Henle,

maintains salt/water

balance in body

• Cortex = contains

Bowman’s bodies

and nephrons

11.3.2

Anatomy of a Kidney

11.3.2

Kidneys are made up of over a million filtering units called

nephrons. Structures in a nephron include…

11.3.3

Nephron Structure

• Glomerulus = capillary bed that filters substances from the blood

• Bowman’s Capsule =

capsule surrounding

glomerulus

• Tubule that extends from

Bowman’s capsule that

includes:

– proximal convoluted

tubule

– loop of Henle

– distal convoluted tubule

• Capillary bed that surrounds

the previous tubule.11.3.3

Malphigian Body

efferent

arteriole

afferent

arteriole

Bowman’s Capsule

glomerulus basement

membrane

beginning of

proximal

convoluted tubule

11.3.3

Within the Malphigian body, the afferent arteriole carries

blood into glomerulus and the efferent drains it back out.

The efferent diameter is smaller than the afferent, which

creates a higher than normal blood pressure within the

glomerulus.

efferent

arteriole

afferent

arteriole

11.3.3

Ultrafiltration describes the process by which substances are

filtered through the glomerulus under the unusually high

blood pressure caused by the afferent/efferent flow.

Fenestrations (gaps) between the glomerulus cells create a path

of low resistance out into the Bowman’s capsule.

11.3.4

The basement membrane of the Bowman’s capsule acts a

filtration barrier and prevents blood cells and large plasma

proteins from passing through.

Podocytes form the inner

membrane of the Bowman’s

capsule. The interdigitation

of their extensions create

gaps for filtrates to pass

through, but not large

molecules.

Cells, proteins and other

molecules remain in blood

and leave in efferent arteriole.11.3.4

11.3.4

11.3.4

Some of the filtrate that leaves the glomerulus/Bowman’s

capsule are necessary for the body and so need to be

reabsorbed. This occurs in the proximal convoluted tubule.

Substances that leave the lumen

of the tubule are returned to the

blood by way of the peritubular

capillary bed.

The wall of the tubule are one

cell thick and contain microvilli

that increase the surface area for

reabsorption. They contain

both active and passive

transport channels.

11.3.6

Salt ions move into the tubule cells via active transport and

water follows as a result of osmosis.

All glucose molecules are reabsorbed in a fully functioning

nephron, which can only occur via active transport.

11.3.6

11.3.6

Osmoregulation is the control is the control of water balance

of the blood, tissue or cytoplasm of an organism.

The amount of water eliminated each day depends on factors

including:

- Amount of water ingested - Perspiration rate

-Ventilation rate (water exhaled)11.3.5

After the proximal convoluted tube, the remaining water and

solutes enter the descending portion of the loop of Henle.

The first section is

relatively permeable to

water but not ions, so

solute conc. rises.

The ascending portion

is relatively impermeable

to water and permeable

to ions, which are

pumped into the

intercellular fluid.11.3.7

The loop of Henle extends into the medulla, so it has a higher

ion concentration (hypertonic) as compared to fluids in the

tubules and collecting ducts.

This decreases the

overall volume of

the filtrate, but also

results in

reabsorption of

water in the

collecting ducts.

11.3.7

Next, the filtrate enters the distal convoluted tube where the

solute concentration is fine-tuned before entering the

collecting duct. At this point, the water concentration is high.

11.3.7

The water permeability of the

collecting duct depends on the

presence/absence of

antidiuretic hormone (ADH),

which is secreted by the

posterior lobe of the pituitary

gland.

Osmoreceptors detect blood

osmolarity (solute

concentration) and trigger

ADH release when osmolarity

increases (less water).

11.3.7

High osmolarity / low water levels induce ADH secretion,

which increases the water permeability of the collecting duct.

The water is then enters the peritubular capillaries and returns

to the bloodstream.

Low osmolarity prevents ADH secretion, which maintains the

collecting ducts water impermeability.11.3.7

11.3.7

11.3.7

Throughout the excretion process, the concentration of

various molecules changes (proteins, glucose and urea).

Molecule

Amount blood

plasma

(mg / 100 mL)

Amount in

glomerular

filtrate

(mg / 100 mL)

Amount in

urine

(mg / 100 mL)

Proteins >700 0 0

Glucose >90 >90 0

Urea 30 30 >1800

11.3.8

• Proteins are too large to cross through the fenestrations and

basement membrane in the glomerulus / Bowman’s capsule

and so remain in the blood plasma through the efferent

arteriole.

• Glucose is able to pass through the basement membrane

and become part of the glomerular filtrate. When it reaches

the proximal convoluted tube, it is reabsorbed into the

bloodstream through the peritubular capillaries.

• Urea passes through the glomerular fenestrations/basement

membrane and is not reabsorbed in the loop of Henle or

proximal/distal convoluted tubes. Its concentration is

magnified in urine due to water reabsorption in the

collecting duct.

11.3.8

When an individual has diabetes, blood-glucose levels are not

properly regulated by the hormones insulin/glucagon. High

levels of glucose in the blood (hyperglycaemia).

Glucose becomes part of the glomerular filtrate, but due to

high levels they can not be actively transported out fast

enough. This results in glucose being present in urine.11.3.9

11.3.7