Why does the place where one is slapped look red?

SALMAN SAEED, Kadapa, Andhra Pradesh

Lewis in 1927 described triple response to skin injury. The triple response is a result of a firm strong stroke across the skin like a slap or pencil point slash. It contains a series of responses: 1. red reaction; 2. flare; 3. wheal.

The red reaction: It is due to filling of the smallest vessels of the skin, called the capillaries. It is as a result of release of histamine from the damaged skin which dilates these capillaries.

The red reaction outlines the stroke and is sometimes called as red line. It is not mediated by nerves. Histamine is a polypeptide (complex protein), present in skin, intestines and lungs, i.e., at the surfaces in contact with the outside world. When it is released it causes allergic skin reaction, urticaria, bronchial asthma and ulcers in stomach.

Flare: Flare is the phase after red reaction and is due to filling of smaller arteries of skin called arterioles. Arterioles are larger than capillaries.

Flare is an irregular red area around the red line. The skin temperature in the flare region is raised because of increased blood flow. It is mediated by nerves. As flare is due to nerves, the arterial filling is long lasting, may spread around and may remain for a few hours.

Wheal: Wheal means, “a suddenly formed elevation of the skin surface”. If the skin stroke is strong enough a raised area appears extending from red line but within the flare area.

Eventually, the raised area may develop into a blister. This is due to further increase in blood filling of both capillaries and arterioles. This response is again due to histamine.

Some individuals are unusually prone to develop a striking triple response. This response is common to wide variety of skin injuries like a slap, slash, firm pressure and also to chemicals like creams and ointments.

DR. R. RAM, Associate Professor Nephrology, NIMSHyderabad

This week's question

Why does water appear white when it falls from a height?

Akshay Pola, Bidar, Karnataka

Why is coldness not felt when we cover our ears with a cloth?

S. Shanthi, Ranipet, Tamil Nadu

In a railway track at a curve, the inner track is comparatively less in height then the outer. Why?

V.G. Sathiyanarayanan, Chennai

Biscuits kept in room temperature lose their crispness after some time, more so if the weather is chill. But the same kept in the much cooler temperature of a fridge remain crisp even after a few weeks. Why?

A. Shanmugasundaram, Bengaluru

What technology is used for achieving air suspension in buses?

N. Govindarajulu, Anantagiri, Tamil Nadu

Correction

The answer refers to histamine as a polypeptide (complex protein) present in skin, intestines and lungs. However histamine is not a polypeptide and is formed upon decarboxylation of the amino acid histidine.

More In: Science | Sci-Tech