Exchange Surfaces and Breathing

Gas Exchange

• Cells need to take in oxygen and nutrients and excrete waste products like carbon dioxide and urea

Multicellular organisms need exchange organs

• In single-celled organisms, substances can diffuse directly into or out of cell across cell membrane. Diffusion rate is quick because of small distances substances have to travel - large surface area to volume ratio
• In multicellular animals, diffusion across outer membrane is too slow:
  - Some cells are deep within body - big distance between them and outside environment
  - Larger animals have low surface area to volume ratio - difficult to exchange enough substances to supply large volume of animal through relatively small outer surface

Gas Exchange

Lungs are exchange organs in mammals

• As you breathe in, air enters trachea
• Trachea splits into two bronchi - one bronchus leading to each lung
• Each bronchus branches off into smaller tubes calles bronchioles
• Bronchioles end in small air sacs called alveoli
• Ribcage, intercostal muscles and diaphragm work together to move air in and out

Gas exchange happens in alveoli

• Alveolus made from single layer of thin, flat cells called alveolar epithelium
• Alveoli arranged in bunches at end of bronchioles
• Surrounded by network of capilleries, giving each alveolus own blood supply
• Oxygen diffuses out alveoli, across alveolar epithelium and capillery endothelium and into haemoglobin in blood
• Carbon dioxide diffuses into alveoli from blood, crossing capillery endothelium and alveolar epithelium. After entering alveolar space, it's breathed out

Gaseous Exchange System

Lungs adapted for efficient gaseous exchange

• Large surface area - many alveoli, vast capillery network - increased rate of diffusion
• Thin exchange surface - alveolar epithelium and capillery endothelium each only one cell thick, capilleries close to alveoli walls - short diffusion pathway across exchange surface - increased rate of diffusion
• Organism maintains steep concentration gradient of gases across exchange surface - good blood supply from capilleries, constantly take away oxygen and bring carbon dioxide, maintaining concentration gradient - breathing in and out refreshes air in alveoli and removes waste products, keeping concentration gradients high - increased rate of diffusion

Gaseous Exchange System

Gaseous exchange system has different parts with different functions

• Goblet cells - secrete mucus - mucus traps microorganisms and dust particles in inhaled air, stopping them from reaching alveoli
• Cilia - found on surface of cells - beats mucus upwards away from alveoli towards throat, where it's swallowed - prevents lung infection
• Elastic fibres - found in walls of trachea, bronchi, bronchioles and alveoli - helps breathing out - on breathing in, lungs inflate and elastic fibres are stretched - fibres recoil and push air out when exhaling
• Smooth muscle - in walls of trachea, bronchi and bronchioles - allows diameter to be controlled - during exercise, smooth muscle relaxes, making tubes wider - less resistance to airflow and air can move in and out of the lungs easily
• Rings of cartilage - in walls of trachea and bronchi - provide support - strong but flexible - stops trachea and bronchi collapsing when you breathe in and pressure drops

Breathing

Inspiration

• Intercostal and diaphragm muscles contract
• Ribcage moves up and out. Diaghragm flattens but remains curved upwards, increasing volume of thorax. Diaghragm contracts
• As the volume of the thorax increases the lung pressure decreases. Creates low pressure in chest that acts to pull air through mouth and airways to lungs
• Air flows into lungs
• Inspiration is active process - requires energy

Breathing

Expiration

• Intercostal and diaphragm muscles relax
• Ribcage moves down and diaphragm curves again and expands
• Thorax volume decreases causing air pressure to increase
• Air forced out of lungs
• Expiration is passive - doesn't require energy

Tidal volume is volume of air in normal breath

• Tidal volume - volume of air in each breath
• Vital capacity - maximum volume of air that can be breathed in or out
• Breathing rate - how many breaths are taken - usually per minute
• Oxygen uptake - rate at which a person uses up oxygen
• A spirometer investigates breathing