Animal Function (Physiology and Development) - Stage 2

What is the main purpose of respiration?

Obtain oxygen for aerobic respiration and remove carbon dioxide, a metabolic waste product.

Why is breathing energetically costly but essential?

Enables oxygen uptake for ATP production and carbon dioxide removal

What is the definition of respiration?

All processes that move oxygen into and carbon dioxide out of the body

What are the 2 types of respiration?

• External respiration: Gas exchange between environment and body

• Cellular respiration: Use of oxygen in mitochondria to produce ATP

What is Dalton’s Law?

Total gas pressure equals sum of all individual pressures

What is PO₂ at sea level?

Approximately 21 kPa (from 21% of 101 kPa total pressure)

Why does oxygen availability decrease with altitude?

PO₂ drops, meaning fewer oxygen molecules per breath, even though the percentage of oxygen remains 21%

What is partial pressure (PO₂)?

Measure of the “driving force” for gas diffusion; gases move from high → low partial pressure

Compare oxygen content in air and water?

• Air: 210 ml/L

• Freshwater (4C): 10 ml/L

• Seawater (40C): 5 ml/L

Why does warm, salty water hold less oxygen?

Oxygen solubility decreases with increasing temperatures and salinity

Why is carbon dioxide easier to remove than oxygen in aquatic animals?

Carbon dioxide is 20x more soluble than oxygen

What is Fick’s law?

Q˙​=(P1​−P2​)×A×D​/X, where diffusion rate depends on pressure gradient, surface area, diffusion coefficient and barrier thickness

According to Fick’s law, how can organisms maximise oxygen uptake?

• Increase pressure gradient (P1-P2)

• Increase surface area (A)

• Decrease thickness (X)

• Use respiratory pigments (haemoglobin) to maintain low dissolved oxygen and sustain the gradient

Why can only very small organisms rely solely on diffusion?

Diffusion is very fast over micrometres but too slow over millimetres-metres

Why did animals evolve specialised respiratory systems?

Most are too large for simple diffusion and need efficient oxygen uptake

What are the 2 main evolutionary solutions for gas exchange?

• Enhance diffusion (Optimise Fick’s law)

• Use bulk transport (convection) to move gases actively

What is convection?

Bulk movement of fluids (air, water, blood) to move gases faster than diffusion alone

What are the 2 forms of convection?

• Ventilation: Moving respiratory medium over the exchange surface

• Circulation: Moving blood or fluid within the body

What are the 2 main flow types in ventilation?

• Tidal flow: medium moves in and out the same path (e.g. lungs)

• Flow-through: Medium moves one way (e.g. fish gills)

Why is water breathing more difficult than air breathing?

• Water has 1/30 the oxygen of air

• It’s dense and viscous, costly to move

What are the advantages of breathing water?

• No evaporative loss

• Buoyancy supports body weight

Why do most aquatic animals use unidirectional flow instead of tidal ventilation?

Reduces energy cost by preventing repeated acceleration of dense water

What are gills?

Evaginated, vascularised structures that increase surface area for gas exchange

What is the basic structure of gills?

Gill arches support filaments; each filament has many lamellae where water and blood exchange gases

What is countercurrent flow in gills?

Water and blood flow in opposite directions to maintain a constant oxygen gradient

Why is countercurrent exchange efficient?

Prevents equilibrium, enabling up to 90% oxygen extraction

How does concurrent flow (same directions) differ?

Equilibrium is reached quickly and gas exchange stops

What is buccal and opercular pumping?

Active movement of water from mouth over gills via coordinated muscle action

What is ram ventilation?

Water forced across gills by forward swimming motion; used by fast swimmers like tuna and sharks

Why is ram ventilation efficient at high speeds?

Uses body motion to move water, reducing need for muscular pumping

What is an obligate ram ventilator?

Shark species that must keep swimming to breathe

How do bottom dwelling sharks breath at rest?

Use buccal pumping and sometimes draw water through a spiracle behind the eye

Which aquatic species use tidal ventilation?

• Lamprey: Pumps water in/out of branchial pouches while attached to a host; inefficient but adequate

• Sea cucumber: Moves water through a ‘Respiratory tree’ via the anus; sufficient for low activity levels

What is the key challenge for water breathers vs air breathers?

• Water: Moving dense, viscous water across gills

• Air: Preventing water loss through evaporation

What primarily drives ventilation in terrestrial animals?

Carbon dioxide removal and acid-base balance

What triggers the urge to breathe when holding your breath?

Rising carbon dioxide levels, not falling oxygen

What is the tracheal system in insects?

Network of air-filled tubes (tracheae) that deliver oxygen directly to tissues; no blood transport of gases

What are spiracles?

Small valved openings on the thorax and abdomen that regulate airflow and minimise water loss

What are tracheae and tracheoles?

• Tracheae: Larger, cuticle-lined air tubes (gas impermeable)

• Tracheoles: Fine, gas-permeable tubes ending near cells for direct diffusion

How do spiracles help conserve water?

Close for long periods to prevent evaporative loss

What is discontinuous ventilation in insects?

Periods of closed spiracles interrupted by brief openings to release carbon dioxide

What are advantages of discontinuous ventilation?

• Minimise water loss

• Enhanced carbon dioxide release efficiency

• Reduces oxygen buildup that could form reactive oxygen species

How do active insects ventilate their tracheal system?

Abdominal pumping (active ventilation)

Why is insect gas exchange unique among animals?

Provides direct cellular oxygenation without lungs or circulatory transport

What are air breathing fish?

Fish capable of using atmospheric oxygen in addition to gills, often in low oxygen waters

When do air breathing fish switch to air breathing?

When water PO₂ drops below critical levels

What structures are used for air breathing in fish?

Highly vascularised regions of the alimentary tract (stomach, intestine or buccal cavity)

What is the difference between facultative and obligate air breathers?

• Facultative: Use air only when necessary

• Obligate: Must breathe air to survive

Why are the gills of the electric eel reduced?

Prevent loss of oxygen from blood back into water

What respiratory modes do amphibians use?

Combination of lungs, skin and gills (in larvae)

What is cutaneous respiration?

Gas exchange across moist skin, crucial in frogs and salamanders

Which amphibians rely entirely on cutaneous respiration?

Lungless salamander

How does gas exchange change during frog development?

• Tadpole: 50% skin and 50% gills

• Adult: 90% lungs and 10% skin

How do frogs primarily remove Carbon dioxide?

Through skin

What is buccopharyngeal (positive pressure) breathing?

Using buccal cavity as a pump to push air into the lungs

What are the steps of frogs breathing?

1. Nostrils open → air enters buccal cavity

2. Glottis opens → old air expelled

3. Nostrils closed → buccal floor rises → fresh air pushed into lungs

What is the function of frogs vocal sacs?

Sound resonance for calls

Why can’t reptiles use cutaneous respiration effectively?

Their thick, keratinised skin is impermeable to gases

What is aspiration breathing?

Negative pressure breathing where thoracic expansion draws air in by suction

Describe mechanics of reptile breathing

1. Thoracic muscles expand → pressure drops → air drawn in

2. Muscles relax → pressure increases → air expelled

How do lizards ventilate their lungs?

Intercostal muscles expanding the rib cage

How do turtles and tortoises ventilate despite rigid shells?

Use muscle sheets attached to limbs to alter internal volume

What is hepatic piston mechanism in crocodilians?

Muscle pulls the liver backward to expand the thoracic cavity, drawing air in

What evidence supports hepatic piston breathing?

X-ray studies show the liver sliding like a syringe plunger during respiration

What is the structure of the mammalian respiratory system?

Trachea → bronchi → bronchioles → alveoli

What are alveoli?

Tiny (500 million) air sacs for gas exchange; 130 m² SA, 0.5 µm barrier

How do mammals ventilate their lungs?

Negative pressure, diaphragm and intercostals contract to draw air in

What happens during exhalation in mammals?

Muscles relax, thoracic cavity contracts and air is expelled by elastic recoil

What is dead space in mammalian lungs?

Air remaining in airways that doesn’t participate in gas exchange

Why are mammals less efficient at oxygen extraction than birds?

Tidal flow mixes fresh and stale air, reducing the PO₂ gradient

What is vital capacity?

Maximum volume of air exchanged: VC = TV + IRV + ERV ~ 4.8L

What makes bird lungs unique?

Small rigid, connected to air sacs that move air but don’t exchange gases

How many air sacs do birds have and what is their function?

~9, they act as bellows to move air through the lungs

What are parabronchi and air capillaries?

• Parabronchi: Tubes allowing one-way airflow through the lung

• Air capillaries: Microscopic gas exchange structures branching from parabronchi

What is the bird ventilation cycle?

1. Inhalation: Fresh air → posterior sacs, stale air → anterior sacs

2. Exhalation: Fresh air moves through lungs, stale air expelled from anterior sacs. → continuous one way flow

What type of gas exchange occurs in bird lungs?

Crosscurrent exchange, blood flows perpendicular to air flow

How does crosscurrent exchange compare to other systems?

More efficient than mammalian tidal flow, slightly less than fish countercurrent

Why do birds breathe less frequently but more deeply than mammals?

Their system provides efficient oxygen extraction per breath

What animal besides birds show unidirectional airflow?

Monitor lizards, crocodilians and likely dinosaurs

What does the oxygen cascade illustrate?

Progressive drop in PO₂ from air → alveoli → arterial blood → tissues → mitochondria

Where does oxygen diffusion occur in the body?

1. Lungs ( alveolar gas ←→ blood)

2. Systemic tissues (blood ←→ cells)

Why is maintaining the oxygen pressure gradient important?

Ensures continuous diffusion of oxygen into tissues

What is haemoglobin?

Respiratory pigment and metalloprotein containing ferrous iron (Fe²+) in a protoporphyrin ring that binds oxygen reversibly

What is the structure of haemoglobin?

Tetramer of 4 subunits: 22 α and 2 β globins, each with one haem group

What is the role of the globin component?

Determines oxygen affinity and can vary by species or developmental stage

What is the role of the haem component?

Contains ferrous iron, where oxygen binds reversibly

What are the 2 main functions of haemoglobin?

1. Bulk oxygen transport via red blood cells

2. Maintains pressure gradient by binding dissolved oxygen, keeping plasma PO₂ low

What are the main types of respiratory pigments and there metals?

• Haemoglobin (Hb): Fe²⁺, found in vertebrates and some invertebrates

• Myoglobin (Mb): Fe²⁺, found in muscle

• Haemocyanin: Cu²⁺, found in arthropods and molluscs

• Chlorocruorin: Fe²⁺, found in annelids

• Haemerythrin: Fe²⁺ (no haem), Found in sipunculids and brachiopods

What is myoglobins role in muscle?

Stores oxygen near mitochondria and releases it during activity

Why do mammalian RBCs lack a nucleus?

Maximise space for haemoglobin storage

How do RBCs differ in other vertebrates?

Birds, fish and reptiles retain nuclei in their RBCs

Approximately how many RBCs are in the human body?

About 25 trillion

What percentage of blood volume do RBCs occupy?

45-50%

How many haem groups are in 100ml of blood?

Approximately 5.4 × 10²⁰ haem groups

What does the oxygen dissociation curve show?

Percentage of Hb saturation versus PO₂

What is Hb saturation in arterial blood?

Nearly 100% at ~13 kPa

What is Hb saturation in venous blood at rest?

~75% (25% oxygen released)

How does the curve change during exercise?

Venous PO₂ drops to ~2-3 kPa → more oxygen released

Why is the Hb–O₂ curve sigmoid (S-shaped)?

Due to cooperative binding as each oxygen molecule increases Hb’s affinity or the next

How does myoglobin’s curve differ from haemoglobin?

Myoglobin’s curve is hyperbolic, no cooperativity and higher oxygen affinity