Respiration and circulation
-earlier than about 2.4 billion years ago, there was little oxygen in Earth’s atmosphere.
-anaerobic cyanobacteria emitted plums of oygen as a by-product of photosynthesis.
-but most of it was trapped by reaction with iron and minerals.
-however, the atmosphere started to change, becoming enriched with oxygen, bc the reactants of the
surface became satured with oxygen.
-so thanks to the amount of oxygen, new oxygen-consuming evolved, creating stystems.
-not all circulatory systems are the same, but all have to deliver oxygen to animal tissues.
-for example humans have a heart and blood vessels, while insects don’t, bc air enters in their body
through spiracle, small openings connected to their organs; that works for them bc they are small.
-another example is leeches’ circulatory system: they have 2 hearths that squish blood in one
direction.
-or a whale’s hearth weighs 600 kg and it pushes a lot of blood through all his giant body.
-so, all systems have the same job: delivering oxygen to tissues and collecting carbon dioxide.
-also, all systems begin and end at tissue surfaces where gas exchange occurs by the molecular
process of diffusion.
-for example gills are surfaces for diffusive exchange in water.
-humans have lungs, that are covered in the interior with large diffusive surfaces, that allow
diffusive gas exchange.
-surfaces are so large bc molecular traffic occurs by diffusion.
-the system consists in taking up the oxygen molecules from the athmosphere and transport them
where they are needed in the body through blood.
-in some animals as salamanders, the exchange surface is the skin itself.
-so a trade happens: one molecule of carbon dioxide is produced for every molecule of oxygen
consumed in the metabolic reaction.
-a human inhales about 11 000 liters of air each day.
-considering that air is 21% oxygen, it means that we inhale 2 235 liter of oxygen.
-the air we exhale has only 16% O2 with the missing 5% replaced by CO2.
-our body requires 550 liters of oxygen per day for metabolism.
-air enters the body through the nose and mouth when the diaphragm (a muscle below the lungs)
contracts and flattens out, increasimg the volume of the chest.
-the air goes into the pharynx and then into the trachea (if it’s food and not air it goes into the
esophagus).
-in the trachea there is mucus and cilia that clean it.
-then there are bronchi and bronchioles that are connected to lungs.
-they continue branching until they meet alveoli, microscopic air sacs, surrounded by capillaries.
-in the alveoli gas exchage happens.
-the oxygen arrives in the alveoli as a cloud of molecules.
-the cloud of oxygen reaches the capillaries.
