Exchange surfaces

The need for specialized exchange surfaces

The need for specialized exchange surfaces

• SA:V ratio = Surface Area ÷ Volume

• As organisms get larger, SA:V decreases.

• Single-celled organisms: high SA:V, diffusion is sufficient.

• Multicellular organisms: low SA:V and higher metabolic activity, so diffusion alone is insufficient.

• Multicellular organisms need specialised exchange surfaces to efficiently absorb substances and remove wastes.

Features of efficient exchange system

• Large surface area → increases rate of exchange (e.g. root hair cells).

• Thin surface → short diffusion distance (e.g. alveoli - one cell thick).

• Good blood supply and/or ventilation → maintains a steep concentration gradient (e.g. alveoli, gills).

Mammalian gaseous exchange system

Nose/Mouth → Trachea → Bronchi → Bronchioles → Alveoli

Cartilage: C-shaped rings in the trachea (plates in bronchi); keep airways open and prevent collapse.

Smooth muscle: Surrounds airways; contracts to narrow them, relaxes to widen them (constriction and dialation).

Ciliated epithelium: Cilia beat mucus up towards the throat to be swallowed or removed.

Goblet cells: Produce mucus to trap dust and pathogens.

Elastic fibres: Stretch during inhalation and recoil during exhalation.

Trachea

• C-shaped cartilage rings keep airway open and prevent collapse.

• Goblet cells secrete mucus to trap dust and pathogens.

• Ciliated epithelium moves mucus towards the throat.

• Smooth muscle allows flexibility.

• Elastic fibres stretch and recoil during breathing.

Bronchi

• Cartilage plates support airways and prevent collapse.

• Goblet cells produce mucus.

• Ciliated epithelium wafts mucus to the throat.

• Smooth muscle regulates airflow.

• Elastic fibres aid recoil.

Bronchioles

• No cartilage as support is unnecessary in small airways.

• Little/no goblet cells to prevent mucus blockage.

• Some ciliated epithelium present.

• Thick smooth muscle layer controls airway diameter (bronchodilation/bronchoconstriction).

• Elastic fibres help air movement.

Alveoli

• One-cell-thick squamous epithelium gives a short diffusion distance.

• Very large surface area from millions of alveoli.

• Dense capillary network maintains steep concentration gradients.

• Elastic fibres allow expansion and recoil.

• No cartilage, cilia, goblet cells, or smooth muscle as they would hinder gas exchange.

Inhalation mechanisms in mammals

• External intercostal muscles contract; internal intercostals relax.

• Rib cage moves up and out.

• Diaphragm contracts and flattens.

• Thoracic volume increases → lung pressure decreases below atmospheric pressure.

• Air moves into the lungs.

Exhalation mechanisms in mammals

• External intercostal muscles relax; internal intercostals contract (forced exhalation).

• Rib cage moves down and in.

• Diaphragm relaxes and domes upwards.

• Thoracic volume decreases → lung pressure increases above atmospheric pressure.

• Air moves out of the lungs.

Functions of respitory structure

• Rib cage: Protects lungs and changes thoracic volume.

• External intercostal muscles: Raise ribs during inhalation.

• Internal intercostal muscles: Lower ribs during forced exhalation.

• Diaphragm: Flattens to increase thoracic volume and domes to decrease it.

Vital Capacity, Tidal Volume, Breathing Rate & Oxygen Uptake

• Tidal volume (TV): Volume of air inhaled or exhaled in one normal breath.

• Breathing rate (BR): Number of breaths per minute.

• Vital capacity (VC): Maximum volume of air that can be exhaled after a maximum inhalation.

• Oxygen uptake: Amount of oxygen absorbed by the body per unit time.
  

• Minute ventilation = Tidal volume × Breathing rate

• Exercise → ↑ oxygen demand → ↑ oxygen uptake → ↑ breathing rate + ↑ tidal volume.

Spirometer

• A device that measures lung volumes and capacities by recording the volume of air inhaled and exhaled (exhaled breath CO2 often absorbed by soda lime to reduce toxicity)

• Produces a spirogram (graph of lung volume against time)

• Height of waves = tidal volume.

• Number of waves per minute = breathing rate.

• Maximum breath volume = vital capacity.

• During exercise: tidal volume and breathing rate increase.

Data Logger

• An electronic device that records physiological data continuously using sensors (e.g. oxygen uptake, breathing rate, heart rate).

• Higher readings = greater value of the variable being measured.

• Steeper increase = faster change.

• During exercise:

  • Oxygen uptake increases.

  • Breathing rate increases.

  • Often tidal volume increases.

• Look for trends, peaks, and comparisons between rest and exercise

Bony fish structures

• Buccal cavity: Chamber in the mouth that pumps water over the gills.

• Operculum: Bony flap covering the gills; helps create pressure changes to move water.

• Gill filaments: Long structures providing a large surface area.

• Gill lamellae (gill plates): Thin folds one cell thick on filaments; site of gas exchange with a rich blood supply; many lamellae on many gill filaments providing large SA:V.

Ventilation mechanism bony fish

Water enters

• Mouth opens, operculum closes.

• Floor of buccal cavity lowers.

• Buccal cavity volume increases.

• Pressure decreases.

• Water flows into the mouth.

Water forced over gills

• Mouth closes, operculum opens.

• Floor of buccal cavity rises.

• Buccal cavity volume decreases.

• Pressure increases.

• Water is forced over gill lamellae and exits through the operculum.

Bony fish gas exchange

• Water passing over lamellae contains a higher O₂ concentration than the blood.

• Oxygen diffuses from water → blood.

• Carbon dioxide diffuses from blood → water.

• Thin lamellae one cell thick and rich capillary network make diffusion rapid (increases concentration difference).
  

• Water flows in one direction, blood flows in the opposite direction.

• At every point along the lamella, water has a higher O₂ concentration than the blood.

• This maintains a steep concentration gradient (parallel) across the entire gill.

Histology mammalian ventilation structures

Bronchus has cartilage and much larger than bronchiole