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Exchange Surface & Breathing
Cells need to take in oxygen (for aerobic respiration) and nutrients (e.g. glucose). They also need to excrete waste
products like carbon dioxide and urea.
Smaller organisms have a higher surface area to volume ratio.
Volume = L x W x H Surface area = L x W of each side and add together.
Single-celled organism use direct diffusion across the plasma membrane. It is quick because of the short distance to
travel and the relativity high surface area to volume ratio.
Multicellular organisms have specialized exchange organs because some cells are deep within the body (big distance to
travel) they also have a low surface area to volume ratio (cannot supply a enough substance though a small outer
The Gaseous Exchange System
Air enters the trachea, splits into two bronchi, one bronchus branches off into bronchioles, each bronchiole ends with
alveolus. The ribcage, intercostal muscles and diaphragm work together to move air in and out.
Each alveolus is made from a single layer of thin flat alveolar epithelium. They have a network of capillaries for their own
blood supply, these are made from capillary endothelium.
O diffuses out of the alveoli across the alveolar epithelium across the capillary endothelium into haemoglobin.
CO diffuses into the alveoli across the capillary endothelium across the alveolar epithelium into alveoli space.
o Many alveoli, large surface area for diffusion.
o Alveoli epithelium and capillary endothelium is only one cell thick (short diffusion pathway).
o All alveoli have a good supply of blood, constantly taking away O and bring CO.
o Diaphragm and intercostal muscles for ventilation keep the concentration gradient high (of O and CO).
Goblet cells Secret mucus, this traps microorganisms and dust from inhaled air, stopping it reach the alveoli.
Cilia Hair like on epithelial cells. They beat mucus secreted by goblet cells, moving it (plus dust and microorganisms)
upwards away from the alveoli toward the throat where it can be swallowed (preventing lung infection).
Elastic fibres On breathing in the lungs inflate and elastic fibres stretch. Then fibres recoil to help push air out when
Smooth muscles Controls the diameter of the airways, during exercise smooth muscle relaxes making the tubes wider,
less resistance to airflow, air can flow in and out more easily.
Cartilage Provides support but is also flexible, to stop the trachea and bronchi collapsing when breathe in and when
Part of the lung Cartilage Smooth Elastic Goblet Epitheliu
Muscle Fibres Cells m
Trachea Large C-shaped pieces Ciliated
Bronchi Smaller pieces Ciliated
Larger bronchiole None Ciliated
Smaller bronchiole None None Ciliated
Smallest bronchiole None None None No Cilia
Alveoli None None None No Cilia
o The intercostal and diaphragm muscles contract.
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Causes the ribcage to move upwards and outwards, the diaphragm flattens.
o Volume of the thorax increases, lung pressure decreases so air flows into the lungs.
o Active process.
o The intercostal and diaphragm muscles relax.
o Causes the ribcage to move downwards and inwards, the diaphragm curves again.
o Volume of the thorax decreases, lung pressure increases so air is forced out of the lungs.
o Passive process.
Tidal volume (TV) The volume of air in each breath (typically 0.4dm).…read more