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