LU 3: EXCRETION
by (51494 & 54020)

Objective

A) Maintain Homeostasis

B) Maintain:
i) concentration of ions
ii) body volume
iii) osmotic concentration

C) Remove:
i) metabolic end products (urea, uric acids)
ii) foreign substances

Excretory Organs

Coelenterates & Echinoderm
- DO NOT have excretory organ

  • excretes through cell membrane -> body cavity -> environment

Types of excretory organs

A) Epithelial excretory surfaces

i) Fish - Chloride cells

ii) Brine shrimp - salt pumps

B) Tubular excretory organs

i) Vetebrates - nephrons in kidney

ii) Insects - malpighian tubules

iii) Flatworms - flame cells

iv) Earthworms - nephridia

Excretory Products

  • Amino groups are excreted from body in three forms:
    i) Ammonia
    ii) Urea
    iii) Uric acid

A) Ammonotelic
-excrete Ammonia

i) most toxic
ii) needs large amount of water for removal
iii) highly soluble in water
iv) released through body surface / gill membranes
e.g.: Aquatic invertebrates & bony fishes

B) Ureotelic
- excrete Urea

C) Uricotelic
- excrete Uric acid

i) Largely insoluble
ii) Secreted as a paste + little water loss
e.g.: birds, reptiles, land snails & terrestial insects

Excretory System

Process

i) Filtration

ii) Reabsorption

iii) Secretion

iv) Excretion

Types of Animals

A) Flatworm

i) Flame-bulb system

ii) Protonephridium

Opening: Nephridiopore

Dead Ends: flame cells (have cilia)

Cilia allow* entry of water & solutes from interstitial fluid

Beating of cilia

Filtrate flows down the tubule

towards nephridiopore

Water & solutes reabsorbed as filtrate flows

Expelled through excretory pores

B) Earthworm

i) Metanephridia

ii) Pressure from closed circulatory system

**water & solutes are pushed from blood

into coelomic fluid

excreted as dilute urine

iii) NaCl reabsorbed by active transport

Urine - more dilute than body fluids

C) Insects & terrestrial arthropods

i) Malpighian tubules

Reabsorption of salt, water & nitrogeneous wastes

ii) Hindgut

Diuretic hormones - urine production

iii) Chloride cells

In gills (mayfly) / rectum (dragonfly) for salts retrieval

D) Fish

i) Freshwater fish

ii) Saltwater / marine fish

Chloride cells - Absorb Cl

Chloride cells - Extrude Cl

Little fresh water enters

Gills - Active NaCl absorption + Osmotic water enters

Kidney - Active NaCl reabsorption

Diluted urine excretion

Plenty of sea water enters

Gills - Active NaCl secretion

Kidney - Active MgSO4 secretion

Concentrated urine excretion

E) Amphibians

Frog kidney - Urea is eliminated by glomerular filtration & tubular secretion

Mangrove Frog

  • lives in salt water
  • carries high urea in blood - achieve osmotic equilibrium - retain low [salt] in tissues
  • urea loss is decreased (very low urine flow rate & cessation of urea secretion)

F) Reptiles

i) Freshwater reptiles

Excrete dilute urine

ii) Marine reptiles

Excess salt is eliminated by salt glands, not via kidneys

iii) Dry land reptiles

  • Waste form: Semisolid uric acid - requires very little water
  • Na & K salts of uric acid - eliminated without additional expense of water

G) Mammals

Main excretory organ = Kidney (elimination of metabolic waste products)

H) Birds

lack urinary bladder

Wastes are eliminated as uric acid (allows water retention)

Marine birds

Nasal salt glands - Excrete excess salt (sneeze / shake off salt droplets)

Kidney

Steps

Step 1: Filtration
-glomerular capsule

Step 2: Tubular reabsorption of filtrate
- Proximal tubule
- Loop of Henle
- Distal tubule

Step 3: Tubular secretion
- Distal tubule

Principal site of:

  • water balance & salt regulation (osmoregulation)

Functional Unit:
- Nephron

  • Blood flows through
    - glomerular capillaries
    - plasma (water, nutrients, waste, salts)
    & enters Bowman's capsule via filtration pressure

Na, glucose & amino acids are reabsorbed

Na-K ATPase pump supplies energy

K+ & NaCl concentrations of body fluids are regulated

Collecting duct
- Urea & water are reabsorbed

i) Ammonia is converted into urea
ii) less toxic
iii) water soluble
iv) excreted in urine form
e.g.: mammals & amphibians

Rectum - Presence of papillae

Reabsorption of ions & water (Antidiuretic hormones)

Kidneys: - Major role for Excretion of Mg & SO4 of marine fish

Water gained via skin

Water loss via skin


  • Process of removing metabolic waste products from the body