00:00:08 Every day, we take around 25,000 breaths of air.
00:00:13 And each breath contains a gas essential for life
00:00:17 —oxygen.
00:00:20 We need a steady supply of oxygen
00:00:22 because our body cells use it to produce energy from the food we eat.
00:00:27 But how does oxygen get from your lungs
00:00:30 to the trillions of individual cells throughout your body?
00:00:35 When you take a breath,
00:00:38 oxygen passes from your lungs
00:00:40 into your bloodstream,
00:00:42 where it’s transported
00:00:43 by the workhorses of oxygen transport
00:00:46 —red blood cells.
00:00:50 Their unique shape and composition
00:00:52 make these cells perfectly suited
00:00:54 to deliver oxygen precisely where and when it’s needed.
00:00:59 Each one is a tiny disk
00:01:01 with its center curved inward on both sides.
00:01:04 This shape lets the cells bend and fold themselves
00:01:08 so that they can squeeze through even the narrowest blood vessels.
00:01:12 Red blood cells are unusual in other ways.
00:01:15 They don’t have a nucleus
00:01:17 or the specialized structures that other cells need to survive.
00:01:21 For this reason,
00:01:23 red blood cells don’t use up any of the oxygen they transport.
00:01:27 And their unique makeup
00:01:29 leaves more room for their most important component
00:01:33 —hemoglobin.
00:01:35 Each red blood cell contains
00:01:37 over 250 million hemoglobin molecules.
00:01:41 Hemoglobin molecules work like tiny machines
00:01:44 absorbing as much oxygen as possible
00:01:46 when red blood cells pass through the lungs.
00:01:49 Each hemoglobin molecule has four locations
00:01:53 where an oxygen molecule can attach.
00:01:56 As soon as an oxygen molecule binds to one of those locations,
00:02:00 something remarkable happens:
00:02:03 The tightly folded structure of the hemoglobin changes,
00:02:07 making it possible for as many as three more oxygen molecules
00:02:11 to be absorbed.
00:02:14 But how does the hemoglobin
00:02:16 know when and where to deliver the oxygen?
00:02:20 When a part of the body is exerted,
00:02:22 it uses more oxygen.
00:02:25 So as blood travels through your body,
00:02:28 it passes by tissues that need oxygen
00:02:31 and that release carbon dioxide
00:02:33 as waste into the blood.
00:02:35 In the presence of carbon dioxide,
00:02:37 hemoglobin’s bond to oxygen changes.
00:02:41 The oxygen molecules that have been riding along
00:02:44 can then break free to be absorbed by other body cells.
00:02:47 As for the carbon dioxide that needs to be removed,
00:02:51 most of it is chemically changed
00:02:53 and carried by the blood’s plasma.
00:02:55 But some also attaches to the hemoglobin molecules
00:02:58 and returns to the lungs,
00:03:00 where, along with the carbon dioxide unloaded from plasma,
00:03:05 it can be exhaled
00:03:07 and where the very next breath
00:03:09 brings in a fresh supply of oxygen
00:03:11 ready for transport.
00:03:14 Constantly zipping through your body’s thousands of miles of blood vessels,
00:03:18 red blood cells wear out after only about four months.
00:03:23 But healthy bone marrow continuously produces
00:03:26 up to two million new red blood cells every second as replacement cells.
00:03:32 And the liver breaks down the worn-out cells
00:03:35 and recycles the iron found in the hemoglobin molecules.
00:03:41 It’s an amazing system
00:03:44 of delivery, pickup, and disposal.
00:03:49 What do you think?
00:03:50 Did the oxygen transport system in our bodies,
00:03:53 with its trillions of red blood cells, simply evolve?
00:03:57 Or was it designed?
