Osmoregulation and Excretion

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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.

Slide 23

Diverse excretory systems are variations on a tubular theme

Excretory systems regulate solute movement between internal fluids and the external environment. Most excretory systems produce urine by refining a filtrate derived from body fluids.

Key functions of most excretory systems:

Filtration: pressure-filtering of body fluids

Reabsorption: reclaiming valuable solutes

Secretion: adding toxins and other solutes from the body fluids to the filtrate

Excretion: removing the filtrate from the system.

Slide 24

Key functions of excretory systems: an overview

Capillary

Excretion

Secretion

Reabsorption

Tubule --> blood

Excretory

tubule

Filtration

Blood --> tubule

Filtrate

Urine

Slide 25

Survey of Excretory Systems

Systems that perform basic excretory functions vary widely among animal groups. They usually involve a complex network of tubules.

Protonephridia flame cells / planaria

Metanephridia earthworm / similar to nephrons

Malpighian Tubules insects

Nephrons = the function unit of the kidneys / humans.

Slide 26

Protonephridia

A protonephridium is a network of dead-end tubules connected to external openings.

The smallest branches of the network are capped by a cellular unit called a flame bulb.

These tubules excrete a dilute fluid and function in osmoregulation.

Slide 27

Protonephridia: the flame bulb system of a planarian

Tubule

Tubules of

protonephridia

Cilia

Interstitial

fluid flow

Opening in

body wall

Nucleus

of cap cell

Flame

bulb

Tubule cell

Slide 28

Metanephridia

Each segment of an earthworm has a pair of open-ended metanephridia.

Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion.

Slide 29

Metanephridia of an earthworm

Capillary network

Components of

a metanephridium:

External opening

Coelom

Collecting tubule

Internal opening

Bladder

Slide 30

Malpighian Tubules