Recent comments in /f/askscience

Revenge_of_the_User t1_jc5u19t wrote

Reply to Why can we make grafts from one plant to another slightly different plant and these are not rejected, but the human does reject grafts from a different human? by Juan_D_2314

Youre comparing apples to oranges.

Animals and plants have completely different systems evolved for dealing with things like infection and pests or parasites. Right down to cell structure.

Many plants have evolved such that a broken branch can heal to become its own entire plant (a genetic clone of the original) and a lot of that is down to cell structure and how relatively simple a plant is in operation.

Compared to how varied animals are; we span every environment to some degree, we consume a huge variety of food (which puts us at risk of as many different infections and parasites as you can imagine) we exist in various states of health for extended periods of time, and we have differing aspects of health (i.e. a tree needs sun, air, and water. We have dietary needs far more complex, we have mental health that can and does have physical implications) so for humans/animals, we evolved an immune system specifically for the purpose of handling many of these issues. And the issues are so varied, the immune system must make a distinction between the body its trying to protect and foreign material - else it cant do what needs to be done (the person dies due to a lack of effective immune system) or the opposite extreme where the immune system doesnt properly make the distinction between friend and foe; known as an autoimmune disease where it attacks tissues its supposed to protect. This is why it attacks another humans tissue. That tissue could carry disease or parasites, it isnt the tissue its programmed to protect. And the immune system has evolved successfully by attacking these things.

Animals and plants are very, very different.

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Lynxesandlarynxes t1_jc5tzqh wrote

Reply to Why is death by respiratory failure in cases when the problem is only muscular (and not a problem with the lungs themselves) so prevalent? why is it so hard to assist respiration long term? Why are ventilators so unsophisticated and brutal on the lungs? by Eschatologists

  • Facilitating mechanical ventilation (MV) typically requires an artificial airway e.g. endotracheal tube, tracheostomy. Inserting these is not without risk, both short- and long-term. A first hurdle.
  • The respiratory tree from nose - to - bronchi is crucial immune defence, helps humidify and warm inspired air and helps clear mucous from your lungs. With MV the presence of said artificial airways bypasses these beneficial mechanisms.
  • MV is essentially backwards. When you inspire naturally your diaphragm contracts to (in short) create negative pressure in your chest, drawing air into your lungs. When you're done inspiring the elastic recoil of your chest etc. pushes air out. This is called 'negative pressure' ventilation. MV is 'positive pressure' ventilation - it blows the lungs up like a balloon with each mechanical inspiration, then they deflate elastically during expiration. This creates issues whereby the alveoli can be subjected to an unnaturally high pressure (barotrauma) and sometimes also volumes of air (volutrauma). Neither are good and have a host of possible short-term (pneumothorax, ALI) and long-term (CLI) sequelae.
  • Ventilator-associated pneumonia. Normally you swallow or spit out your saliva and other upper respiratory tract secretions. When you've an artificial airway in place, these secretions accumulate at the point of the airway's cuff (a small balloon that helps stop it moving). These accumulated secretions contains normally harmless oral bacteria, but over time they'll develop a biofilm, bypass the cuff and enter the lungs causing a pneumonia.

There are other reasons, though I have to go to work now!

Source: Anaesthetist/intensivist

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