‘Identify the role of the kidney in the excretory system of fish and mammals’

Recall Active and passive transport

Active transport uses energy to transport substances across a membrane it would normally not be able to cross due to a diffusion gradient or its own properties

Passive transport is the movement of substances across a membrane without energy expenditure.

Role of the kidneys

The primary role of the kidneys is osmoregulation.

Osmoregulation is the regulation of salt and water levels in the body.

The kidneys ensure that the concentration of blood and interstitial fluid is constant

Waste

Unicellular organisms can get rid of waste using passive transport( diffusion and osmosis)

Multicellular organisms are too large to rely solely on diffusion and osmosis and need to utilise active transport (use of energy)

3 forms of Nitrogenous wasteNitrogenous waste occurs in 3 forms:

AmmoniaUreaUric Acid

Paste in table 5.4.2

In Aquatic Organisms

Fish do not excrete nitrogenous wastes through the kidneys; they use their gills

Ammonia dissolves in water. Fish do not need to conserve water and therefore excrete ammonia as soon as it is made, directly into the water

Fish still have kidneys, however they are solely responsible for waste removal and water concentration control. It is salts that are excreted through the gills.

Freshwater fish are Hypertonic. Water moves by osmosis into the fish and salt diffuses out. Tubules (loop of henle) will reabsorb ions (active transport) and keep them in the body producing a lot of dilute urine.

Saltwater fish are hypotonic. Salt diffuses into the fish and water moves out (osmosis). Therefore saltwater fish need to drink all the time and excrete ions through their gills and kidneys. Marine fish need to conserve water and therefore produce concentrated urine.

In Aquatic Organisms

In Terrestrial Organisms

Terrestrial organisms are not surrounded by water and therefore must try and excrete waste while conserving water.

They do this by converting it to urea which is safe to store

They urinate less frequently and make concentrated urine (saves water)

In most birds, reptiles and some insects, ammonia is converted into Uric Acid

(more of a paste than a liquid).

Very little water content

Less toxic than ammonia or urea so can be stored in the body for a very long time

In Terrestrial Organisms

Paste in table 5.4.3

‘Distinguish between active and passive transport and relate these to processes occurring in the mammalian

kidney’

Paste in fig 5.4.2

The Structure of a Kidney

The Structure of a KidneyA kidney is made up of around a million nephrons.

The nephron is a regulatory unit; it absorbs or secretes substances in order to maintain homeostasis.

This regulation maintains the constant composition of body fluids.

The Structure of a Kidney

A nephron is made up of a Glomerulus (a ball of capillaries)Within a Bowman’s capsule, connected to a proximal tubule, leading to the loop of Henle, which connects to the distal tubule.

This all connects to the collecting duct which leads to the bladder.

The Structure of a Kidney

The nephrons are densely surrounded by capillaries

Paste in figure 5.4.3 + 5.4.4

The role of nephronsThree processes occur in the nephrons

(kidneys):

Filtration

Reabsorbtion

Secretion

FiltrationFiltration:

Within the Bowman’s capsule is the glomerulus, a dense clump of capillaries.

Filtration uses passive transportThe blood pressure here is so high that fluid

and substances from the blood are forced into the Bowman’s capsule, and form a fluid called the glomerular filtrate. It contains:Substances the body can reuse: Glucose,

water, amino acids, etcWastes: Urea and poisons.

ReabsorptionReabsorption:

The substances the body can reuse are reabsorbed into the capillaries surrounding the nephron. Eg, vitamins and hormones.

This is active transport and requires energy. Some other substances passively re-enter the blood. Eg, water and salts. This occurs in the proximal and distal tubules and in the loop of Henle.

Secretion

Secretion: This is the process where substances are transported from the kidney, into the collecting duct. This is active transport of substances such as potassium, hydrogen and ammonia.

A main function is to balance the pH of the blood

Urine then travels to the ureter and on to the bladder.

Kidney Function

Flow ChartDraw a flow chart to show the movement of

blood through the kidneys. Include the structures that the blood is travelling through and what is being taken out and put in. Include the words filtration, re-absorbtion and secretion.