How it arises?
Anger arises in him who thinks of his enemy. Even if you have forgotten the feeling of annoyance, it lurks in the mind in a dormant form. The effect is there for some time. If you renew a number of times the same kind of thought of jealousy, envy or hatred about the same person, the effect lasts longer. Repetition of angry feeling intensifies hatred. Mere ill-feeling develops into intense malice by repetition of anger.
On days when you have many troubles, vexations, worries from the morning to evening, a trifling causes much irritation in the mind. The balance of mind is upset by a paltry affair. A single harsh word throws you out of the balance, whereas when you are peaceful throughout the day, even a strong abuse and severe censure do not produce any effect whatsoever. Anger resides in the astral body. But, it percolates into the physical body just as water percolates through the pores into the outer surface of an earthen pot.
What are the effects?
Just as heat melts lead, just as heat and borax melt gold, even so, Lust and Anger, the heating factors of the mind, melt it. When you are angry, the mind becomes disturbed. Similarly, when the mind is disturbed, the body also becomes disturbed. The whole nervous system is agitated. You become enervated.
Anger spoils the brain, nervous system and blood. When a wave of anger arises in the mind, Prana(breath) begins to vibrate rapidly. You are agitated and excited. Blood becomes hot. Many poisonous ingredients are formed in the blood. When the blood is agitated, the semen also is affected.
Once a child sucked the breast of his mother when she was in a fit of violent fury or rage and died immediately on account of poisoning by virulent chemical products that were thrown into the blood of the mother when she was in great excitement. Many such cases have been recorded. Such are the disastrous effects of anger. Even three minutes of violent hot temper may produce such deleterious effects in the nervous system that it will take weeks or months for the repair of injury.
Anger clouds understanding. When the mind is violently agitated, you cannot understand a passage of a book clearly. You cannot think properly and clearly. You cannot write a letter with a cool mind. When the lamp is flickering through wind, you do not see the objects clearly. Even so, when the Intellect(mind) is flickering or agitated by anger, chaos arises in the Intellect and you are not able to see and understand things properly. Intellect is all light.
A man who is a slave to anger, may have washed himself well, anointed himself well, dressed his hair and put on white garments; yet he is ugly, being overcome by anger. There are symptoms on the face to indicate the presence of anger in the mind. If you get anger, you will lose the battle of life. If you have an easily irritable mind, you will not be able to do your daily duties and business in an efficient manner.
Mind – Its Mysteries and Control by Sri Swami Sivananda