Summary
Highlights
Blood is categorized into 8 types (A+, A-, B+, B-, AB+, AB-, O+, O-) based on the presence, absence, or combination of specific antigens (A, B, and RhD) on red blood cells. The immune system produces antibodies against antigens not present in one's own blood. Therefore, mismatched blood transfusions can lead to a fatal clotting reaction when antibodies attack unknown antigens.
Blood pressure is the force exerted by blood on the walls of blood vessels as the heart pumps it through the body. An ideal reading is around 120/80 mmHg. The higher number, systolic pressure, represents the pressure when the heart contracts to pump blood. The lower number, diastolic pressure, is the pressure when the heart relaxes between beats.
Blood rain refers to red-colored water droplets. This phenomenon can occur when brownish dust and sand (e.g., from the Sahara Desert) mix with water droplets in clouds, or when red spores of algae like Trentepohlia Annulata or Haematococcus Pluvialis get carried into rainclouds, with the latter producing a red pigment called astaxanthin when stressed.
The red color of human blood is primarily due to red blood cells, which contain hemoglobin. Hemoglobin is an iron-rich protein that, when bound with oxygen, gives blood its characteristic red hue. However, not all animals have red blood; some, like octopuses and spiders, have blue blood due to hemocyanin (a copper-containing protein) which turns blue when oxygenated, while some lizards have green blood due to biliverdin.