## Air

We all feel air.

Here is the last post of this project (see About for details). In my high-school science text book I read :  “by volume, air contains 78% nitrogen, 21% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases”. But at same time I learned that due to rise in level of carbon dioxide in our atmosphere, we are facing the problem of global warming. So, this gave rise to following two questions:

How do scientists determine the composition of our atmosphere?

Chemists such as Joseph Priestley (1733-1804), Antoine Lavoisier (1743-1794) and Henry Cavendish (1731-1810) made the first measurements of the composition of the atmosphere. To determine composition of a given mixture of gases what you will have to do, is to perform various reactions and study their stoichiometry (i.e. the amounts of substances that are involved in reactions) using appropriate gas laws. The abundance of a given gas is determined by its mole fraction (the amount of a constituent, expressed in moles, divided by the total amount of all constituents in a mixture) in given mixture of gases.  To determine the moles of each gas in given sample we will have to use the gas law written in terms of the specific volume $v$, the reciprocal of density, as : $Pv= R'T$,  where $R'$ is specific gas constant defined as the ratio $\frac{R}{M}$ (where $R$ is ‘universal gas constant’ and $M$ is ‘molar mass of gas’).  For the equations involved, see [7] and [8].

Study reactions to determine composition of air!

We should also understand the reason for the abundance of nitrogen in the atmosphere. Nitrogen is not stable as a part of a crystal lattice, so it is not incorporated into the solid Earth. This is one reason why nitrogen is so enriched in the atmosphere relative to oxygen. The other primary reason is that, unlike oxygen, nitrogen is very stable in the atmosphere and is not involved to a great extent in chemical reactions that occur there. Thus, over geological time, it has built up in the atmosphere to a much greater extent than oxygen.

You must have seen suc pie charts in your textbook (credit: Mysid [Public domain], via Wikimedia Commons)

Nowadays, region specific composition of air is determined and then it is averaged over whole of Earth (thus generating the pie chart shown above). For a sample data see [9]. The existing measurement techniques for atmospheric chemical composition measurements can be separated into three main groups: (1) passive sampling,  (2) active sampling and  (3) remote sensing techniques. Principally, active techniques draw the air sample through the detector or sampling device by a pump, whereas passive techniques use the diffusion of air to the sampling device. In remote sensing techniques, the analyzed air volume and the detector are at different locations. Total or partial column measurements are possible only with remote sensing techniques.

Why text-books (and Wikipedia) claim that there is a fixed composition of atmosphere with very low amount of carbon dioxide and on the other hand various environment organizations keep on claiming that carbon dioxide levels are too high in atmosphere?

The graph shows recent monthly mean carbon dioxide globally averaged over marine surface sites. The dashed red line with diamond symbols represents the monthly mean values, centered on the middle of each month. The black line with the square symbols represents the same, after correction for the average seasonal cycle. (source: http://www.esrl.noaa.gov/gmd/ccgg/trends/global.html)

Now, here  is a catch. Global warming is “attributed” primarily to increasing industrial $CO_2$ emissions into Earth’s atmosphere. The global annual mean concentration of $CO_2$ in the atmosphere has increased markebly since the Industrial Revolution, from 280 ppm to 400 ppm (0.04%) as of 2015.

Despite its relatively small concentration, $CO_2$ is a potent greenhouse gas and plays a vital role in regulating Earth’s surface temperature through the greenhouse effect.  Most of the light energy from the sun is emitted in wavelengths shorter than 4,000 nanometers (.000004 meters). The heat energy released from the earth, however, is released in wavelengths longer than 4,000 nanometers. Carbon dioxide doesn’t absorb the energy from the sun, but it does absorb some of the heat energy released from the earth. When a molecule of carbon dioxide absorbs heat energy, it goes into an excited unstable state. It can become stable again by releasing the energy it absorbed. Some of the released energy will go back to the earth and some will go out into space. Thus, carbon dioxide lets the light energy in, but doesn’t let all of the heat energy out, similar to a greenhouse.

REFERENCES:

[1] Mackenzie, F.T. and J.A. Mackenzie (1995) Our changing planet. Prentice-Hall, Upper Saddle River, NJ, p 288-307.
(After Warneck, 1988; Anderson, 1989; Wayne, 1991.), http://eesc.columbia.edu/courses/ees/slides/climate/table_1.html

[2] World Meteorological Organization, Measurement of atmospheric composition, https://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/Provis2014Ed/Provisional2014Ed_P-I_Ch-16.pdf

[3] Chemicool, Composition of air, http://www.chemicool.com/elements/composition-of-air.html

[4] How scientists determine the composition of each planet, http://usatoday30.usatoday.com/tech/columnist/aprilholladay/2006-09-25-measuring-planets_x.htm

[5] Measures of Atmospheric Composition, http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap1.html

[6] Abundance of Nitrogen in Earth’s Atmosphere, https://www.soest.hawaii.edu/GG/ASK/atmo-nitrogen.html

[7] The composition of air, http://kiwi.atmos.colostate.edu/group/dave/pdf/Composition_of_Air.pdf

[8] Concept of mixtures. Mixtures of ideal gases, http://twt.mpei.ac.ru/TTHB/2/KiSyShe/eng/Chapter1/1-5-Concept-of-mixtures-Mixtures-of-ideal-gases.html

[9] NOAA/ESRL. “Annual Mean Carbon Dioxide Data”, ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt

[10] How does carbon dioxide cause global warming? , http://www.pa.msu.edu/sciencet/ask_st/083194.html

-GK

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Posted in Aerology, Chemistry |

## Antenna

Have you ever noticed that there are lots of antennas (or antennae) around you? In this article we will discuss the two type of antennas.

Natural antenna

Types of natural antennas (credit: L. Shyamal (Own work) [CC BY-SA 2.5], via Wikimedia Commons)

These are one of one or more pairs of appendages i.e. a projecting part with a distinct appearance used for sensing in arthropods i.e. invertebrates with exoskeleton and segmented body. This word originated in mid-17th century as latin alteration of the Greek keraioi ‘horns (of insects)’ used by Aristotle (384 BC – 322 BC).

A green stink or shield bug (credit: https://goo.gl/gUXLGR)

They are sensory organs, although the exact nature of what they sense and how they sense is not the same in all groups, or always clear. The functions may variously include sensing touch, air motion, heat, vibration (sound), and especially olfaction (smell) or gustation (taste). Mosquitoes are able to detect sounds as well as odors with their antennae and some flies are able to gauge air speed while they are in flight. Ants and honey bees are able to communicate with nest mates by touching antennae, which allows them to pass on information about food sources and danger [see: http://goo.gl/9ojaCq ]. For more photos of insect antennae visit: The Dragonfly Woman.

Structure of natural antenna (credit: StudyBlue Inc.)

The three basic segments of the typical insect antenna are
(1) scape or scapus (base),
(2) pedicel or pedicellus (stem), and
(3) flagellum, which often comprises many units known as flagellomeres.

The scape is mounted in a socket in a ring-shaped region of insect’s body hardened by conversion into a protein called sclerotin, often a raised portion of the insect’s head capsule. That projection on which the antenna pivots is called the antennifer. The whole structure enables the insect to move the antenna as a whole by applying internal muscles connected to the scape.

The pedicel (the second segment) contains the Johnston’s organ which is a collection of sensory cells. Also, it is flexibly connected to the distal end of the scape and its movements in turn can be controlled by muscular connections between the scape and pedicel.

The number of flagellomeres can vary greatly between insect species. True flagellomeres are connected by membranous linkage that permits movement. Though the flagellum of “true” insects does not have any intrinsic muscles, some other Arthropods do however have intrinsic muscles throughout the flagellum.

Now, working of antennae varies from one insect to another, and like many other biological processes, it’s a complicated process to be explained in simple words. In a nutshell, there are hair like structures on antennae, their main purpose is sensory and for this reason they are usually called sensilla (sensory receptors or sensors, singular sensillum) rather than hairs. For detailed structure and working of antenna of the rove beetle Aleochara bilineata, visit: http://cronodon.com/BioTech/Insects_antenna.html

Artificial Antenna

Some common types of artificial antennas (credit: http://www.sharetechnote.com/html/Handbook_LTE_AntennaPerformance.html)

It is a device that converts electric signal into radio waves and vice versa. The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi (1874-1937). Because of his prominence, Marconi’s use of the word antenna (Italian for pole) spread among wireless researchers, and later to the general public.

Dipole Antenna (credit: http://www.ventenna.com/HFp-D.html)

The first antennas were built in 1888 by Heinrich Hertz to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas (see photo above) at the focal point of parabolic reflectors for both transmitting and receiving. Electromagnetic waves have both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave. Radio signals are a form of electromagnetic wave, and they are the way in which radio signals travel. [see: http://goo.gl/kq2eUm ]

Omnidirectional vs Unidirectional Antennas (credit: Erik Hersman, https://goo.gl/o6k0qb)

According to their applications and technology available, antennas generally fall in one of two categories:
1. Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions.
2. Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern.

Electromagnetic Wave (credit: SuperManu (Self, based on Image:Onde electromagnetique.png) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons)

The electric field results from the voltage changes occurring in the antenna which is radiating the signal, and the magnetic changes result from the current flow. The lines of force in the electric field run along the same axis as the simple antenna, but spreading out as they move away from it. This electric field is measured in terms of the change of potential over a given distance, e.g. volts per metre, and this is known as the field strength. Similarly when a simple antenna receives a signal the magnetic changes cause a current flow, and the electric field changes cause the voltage changes on the antenna. If you are comfortable with electrical terminologies, for more details you may refer: http://www.summit-antenna.com/EN/TechView.aspx?id=22&AspxAutoDetectCookieSupport=1

Posted in Biology, Information Technology, Physics | Tagged , , , , , ,

## Shadow

Hand Shadows II, From “Le Magasin Pittoresque”, 1861 (source: https://www.flickr.com/photos/seriykotik/7356719426/in/photostream/)

In childhood one would have enjoyed making butterfly, bird, dog etc. using hand-shadow. Some movies with horror/thriller/suspense genre use “shadow” as a creepy concept in from of “Shadow Person”. Due to this “shadow person” I myself faced difficulty in sleeping alone at night. But today, as a matured person, I will discuss the scientific aspects of what we call “shadow”.

As you may already know from your high-school science course that “shadow” is produced on a screen behind the object when light from a source is blocked (either completely or only partially) by the object. Note that, it is appropriate to construct the shadow using simple straight-line rays only if the optical waves (light has two characters: wave and particle!) can be treated as simple straight-line rays (called geometrical optics). If you look at your own shadow on the ground; the shadow of your feet is sharp, the shadow of your head is not. Similarly, the shadow of the bottom part of a tree-trunk or post is sharp, whilst the shadow of the higher parts becomes more and more hazy towards the top. These peculiarities are likewise a consequence of the sun’s not being a mere point. Moreover, the shadow of a butterfly, of a bird (ever noticed?) looks like a round spot. For some interesting “shadow on snow” photos see [11].

Shadow of a person (credit: Jill Flynn, http://goo.gl/nSjQM2)

A clear unambiguous shadow (which I call “perfect shadow”) requires a source of light that either is very small compared to the dimensions of the object or else is very far away (called “point source”). In real life, we generally encounter perfect shadows during day time, because Sun (our star!) being very far away from us, acts “like” a point source.

False (perfect) shadows (most of times along with the lengthening shadows) are used to make a flat image appear “3-dimensional”. This works because our eyes are accustomed to the idea that a shadow is produced when an object is in front of a screen [2]. Also, as an object moves away from source of light, the shadow lengthens faster and faster. This follows from simple geometrical properties of rays (can be explained using “elementary-trigonometry”).

Shadow of hand using a battery-powered electric lamp. (credit: Hiren Shah, https://youtu.be/1-w_g1W87QU)

But at night, we seldom get perfect shadows. This is because the general source of light at night is street-light (no celestial body at night is bright enough to cast shadow on earth, see [3]). Now street-light (generally sodium-lamp in India and LED-lamp in developed countries) may be too near/large to be regarded as a “point source”, we call such sources of light “extended sources”. For extended sources, the shadow is constructed using the “principle of superposition”, where the extended source is modeled as a large number of point sources, and the shadow produced by each of these point sources is constructed independently, then the light striking a screen behind the object is the sum of the contributions from all of the light sources taken individually. Also, the shadow produced by two or more point sources can be understood by the principle of superposition in the same way. This superposition leads to two regions of shadow,

1.  umbra: the portion of the shadow which is totally dark because light from all sources is absent;
2. penumbra: the portion of the shadow which is illuminated by some of the sources and is therefore not completely dark.

This ambiguity in shadow boundaries caused due to superposition, leads to what we perceive as creepy effects of shadow at night. But, this ambiguity in boundaries of shadow lead to some very interesting physical effects. Let us analyse some of these effects:

Shadow of a tree without leaves on snow (credit: Jeff Filipiak, http://goo.gl/ajyJYz)

Double Shadow: When the trees have lost their leaves, we may see the shadows of two parallel branches superposed upon each other. A branch quite near to us gives a sharp and dark shadow, one more distant gives a broader and more greyish shadow. If they accidentally fall one upon the other, we see a bright line in the middle of the sharpest shadow, so that this looks double. This phenomenon is called “double shadow” [1] and will apparently be visible whenever both branches subtend an angle smaller than the sun’s disc.

Geometry of a Total Solar Eclipse (credit: Sagredo (Own work) [Public domain], via Wikimedia Commons)

Solar Eclipse: A solar eclipse can occur when the Moon passes between Earth and Sun. Thus, a solar eclipse can occur only at New Moon (day). The type of solar eclipse (Partial, Hybrid, Annular & Total) depends upon the region of Moon’s shadow through which a region of Earth passes. The partial solar eclipses are dangerous to look at because the un-eclipsed part of sun is still very bright. The Sun’s distance from Earth is about 400 times the Moon’s distance, and the Sun’s diameter is about 400 times the Moon’s diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size: about 0.5 degree of arc in angular measure. To see a total solar eclipse, you have to be in just the right spot on the earth, the point where umbra of Moon’s shadow hits Earth. For amazing photos see [5]. In partial solar eclipse the Moon moves in front of the solar disk until only a thin crescent is left. Apart from the awestruck moment of darkness in daytime, the shape of shadows formed by crescent-sun’s light corresponds to the crescent shape.  For instance, the shadows of our fingers take on an extraordinary claw-like shape. Similarly, each small dark object throws a crescent-like shadow; the shadow of a small rod consists of a number of such crescents, while
a curvature appears at the ends.[1]

Geometry of a Lunar Eclipse (credit: Sagredo [Public domain], via Wikimedia Commons)

Lunar Eclipse: A lunar eclipse occurs only when the sun, Earth and moon are aligned exactly, or very closely so, in a straight line with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon. The type of lunar eclipse (Penumbral, Partial & Total) depends upon the region of Earth’s shadow through which the Moon passes. For a total lunar eclipse to occur, the direct sunlight should be completely blocked by the earth’s shadow, so the Moon must pass directly behind the Earth into its umbra (shadow). The only light seen on moon’s surface on total lunar eclipse is some light from the Sun refracted through the earth’s atmosphere. This light looks red due to Rayleigh scattering of the more blue light (for the same reason that the sunset looks red). Unlike a solar eclipse, which can be viewed only from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place, due to the smaller size of the Moon’s shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full moon. For amazing photos see [4]

Why don’t we have each kind of eclipse once every month (during Full Moon and New moon)? Since the Moon’s orbit around Earth is tipped by about 5 degrees to Earth’s orbit around the Sun, the Moon spends most of the time either above or below the plane of Earth’s orbit.

Earth’s shadow and Belt of Venus at sunrise, seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Note: the lowest blue-grey area is not the sky but the surface of the Pacific Ocean (credit: Brocken Inaglory [CC BY-SA 3.0 or GFDL], via Wikimedia Commons)

Earth Shadow: The earth shadow (also sometimes known as the dark segment) is visible from the surface of the Earth, as a low, flat, dark band which stretches for nearly 180° and is bounded below by the horizon and above by the pinkish anti-twilight arch (called “Belt of Venus” ,though it has nothing to do with Venus!). It is the shadow that the Earth itself casts on its atmosphere. It is visible twice a day: in the eastern sky as the sun sets and in the western sky as the sun rises.

Cloud Shadow (credit: NASA Earth Observatory image created by Jesse Allen and Robert Simmon, using Advanced Land Imager data from the NASA EO-1 team, http://goo.gl/AglCqx)

Cloud Shadow: Cloud shadows, in effect the  inverse of “Sun rays”, . Sun rays (also called “crepuscular rays”) are rays of sunlight which appear to diverge from the point in the sky where sun is located, because of perspective effect (just like the appearance of parallel railway lines converging at a point). These rays, which stream through gaps in clouds, are columns of sunlit air separated by darker cloud-shadowed regions.

Mountain shadow (source: http://goo.gl/aVRaBy)

Mountain Shadow: Mountain shadows look triangular regardless of the mountain’s shape when seen from its summit. This is a perspective effect just like “Sun rays”. The finite size of the sun also causes the umbra region of the shadow to converge and eventually taper away and this tapering sets limits to the length of umbra region of the shadows. The limit of the length of umbra region of a shadow for the Earth is over a million miles and for a high mountain it can be two to three hundred miles. Thus, triangular shadows are not seen from objects much smaller than mountains because their shadows are not long enough.

REFERENCES:

[1] Marcel Minnaert. De natuurkunde van ‘t vrije veld. L Licht en k1eur in het landschap , B. V. W.J. Thieme & Cie, Zutphen (1937)
I referred to English translation of original German work. Two different English translations are available:

[1(i)] First German edition (1937) translated by “H. M. Kremer-Priest” and revised by “K. E. Brian Jay” under title “Light and Colour In the Open Air” originally published by G. Bell & Sons, Ltd. and republished by Dover Publications Inc. (1954)

[1(ii)] Fifth German edition (1974) translated and revised (added colour photos/ poems to chapters) by “Len Seymour” under title “Light and Color in the Outdoors” and published by Springer-Verlag New York. Inc. (1993)

[2] Judah Levine. Light and Color (Shadows and geometrical optics), http://www.colorado.edu/physics/phys1230/phys1230_fa01/topic8.html (12 September, 2001)

[3] James Foster. Objects that case shadow on earth, Science Question of the Week, Goddard Space Flight Center, http://web.archive.org/web/20070627044109/ http://www.gsfc.nasa.gov/scienceques2005/20060406.htm (7 April, 2006)

[4] Fred Espenak. Lunar Eclipses for Beginners, http://www.mreclipse.com/Special/LEprimer.html (2009)

[5] Fred Espenak. Solar Eclipses for Beginners, http://www.mreclipse.com/Special/SEprimer.html (2009)

[6] Ron Hipschman. Why Eclipses Happen, http://www.exploratorium.edu/eclipse/why.html

[7] Dept. Physics & Astronomy University of Tennessee. Solar Eclipses, Astronomy 161 (The Solar System), http://csep10.phys.utk.edu/astr161/lect/-time/eclipses.html

[8] Taryn Biggs, Susan McPhail, Kurt Nassau, Hemant Patankar, Margaret Stenerson, Firman Maulana, Michael Douma & Sally E. Smith (Causes of Color). What causes layers in the sunrise and sunset?, http://www.webexhibits.org/causesofcolor/14E.html

[9] Les Cowley. Cloud Shadows, http://www.atoptics.co.uk/atoptics/clshad.htm

[10] Les Cowley. Mountain Shadow , http://www.atoptics.co.uk/atoptics/mtshad.htm

[11] Jeff Filipiak. Watching the snow: images of beauty I find in shadow patterns, http://milwaukeesnow.com/2014/02/13/watching-the-snow-images-of-beauty-i-find-in-patterns/

Posted in Optics, Physics | Tagged , , ,

## Walking on Wall

We all fancy some superhero in our childhood. I myself wanted to be a superhero, a person who is able to fly in air and walk vertically on walls. It is pretty much clear that why we can’t fly (compare ourselves with bats) and we can’t “walk” on wall (yes, we can train ourselves to climb a certain kind of surfaces, but we can’t walk on them as we walk on floor).

Trisha Brown’s Walking on the Wall, re-created at the Barbican. (Photograph by Martin Godwin for the Guardian)

But, there are a number of animals who can actually walk on wall (i.e. they use same locomotion for climbing as for walking on floor). Let me give some examples of them:

Ants walking on wall

What fascinate me is the fact that ants can climb almost every kind of surface (some ants are not able to climb Teflon surfaces). Also, spider’s legs are studded with microscopic hairs which allows them to stick to walls via electrostatic attraction (van der Waals forces). Spiders and most insects also possess tiny tarsal claws that can grip the minute texture of surfaces that, to our eyes, appear smooth.

Snail walking up (Credit: Michal Maňas, CC-BY-3.0)

Lubricated with a mucus layer secreted by a gland near the mouth, snails are able to glide, albeit slowly, on a layer of slime. This terrestrial gastropod mollusk’s flat underside undulates in a wave-like motion to propel it forward. Its slimy excretions, combined with a smooth, flat base, creates a powerful suction, allowing snails to climb walls, trees, etc. Snails can even walk on vertical Teflon surfaces, for discussion on this topic see a blog post by Michal Maňas (https://gastropods.wordpress.com/2013/06/01/climbing-competition-gecko-vs-snail/)

Phelsuma v-nigra comoraegrandensis on the plastic. (Photo by Petr Bogush, CC-BY-3.0)

Lizards have been the subject of endless scientific study due to their  ability to calmly stroll up even the smoothest surfaces – glass, for example. They achieve this due to their hairy feet. Geckos use superfine hairs called setae to adhere via van der Waals forces (which attract molecules to each other) to pretty much any surface.

Squirrel ‘walking’ on wall? (Source: https://goo.gl/qFZqGp)

Squirrels’ sharp, hook-like claws, coupled with a highly mobile ankle that allows them to rotate their rear feet  around backwards, lets them hang from and climb a variety of surfaces.

A herd of alpine ibexes climbed up a dam in Gran Paradiso National Park in Northern Italy (Photograph by Paolo Seimandi)

Many goats also have feet custom made for vertical exploits. Watching sheep go bounding up a vertical wall, the writer Ellen Meloy (her book on “Colorado mountain sheep” is called Eating Stone: Imagination and the Loss of the Wild) once observed, they get their power from their backsides: “The rump carried all the muscle … with nothing beneath their hooves but air and a foothold barely larger than my lower lip.”

All examples of animals discussed above are quadrupeds (walk on four legs). A question arises,

Why can’t animals walking on two legs (biped), walk on vertical surfaces like wall?

Though there are very few bipeds as compared to quadrupeds (see: A comparison of spinal ligaments–differences between bipeds and quadrupeds), but still I am curious to know answer of this question because I myself am a biped! A simple answer to this question would be that we are not designed to walk on walls.

Since, humans were not bipedal from starting (referring to our genus, “Homo”), we evolved to become bipedal (see: Human skeletal changes due to bipedalism). So, I have a question to ask,

How can we evolve in such a way that we are able to walk on walls?

Akira Nishi had published an article in 1988 : “Bipedal walking robot capable of moving on a vertical wall“, which makes me even more curious to know answer of above question.

-GK

Posted in Biology, Evolution, Physics, Science |

## Acid Reflux

I suffered from acid reflux (commonly known as “acidity”) for a long time in my childhood. My father (being a doctor) advised me to consume “curd” or “cold milk” but not “water”, and this cure worked. But, I had two questions in my mind (which many teachers and doctors failed to answer), which I will try to answer in this blog post:

Q1: We know that curd and milk are acidic in nature, since both have pH value approximately 5 and 6 respectively.  But still Doctors advise us to consume curd or milk in acidity. How can this help our stomach buffer to tackle acidity?

Image source: http://bit.ly/1JorKmb

Since milk is a weak acid, it can act like a buffer solution (not exactly a buffer). Hydrogen ions (H+) make a solution acidic; the more H+, the more acidic a solution is. As a result, when we mix milk and stomach acid, the resulting solution will be less acidic than stomach acid. And also due to the presence of calcium salts in milk,  it acts much like antacids. Calcium carbonate, a popular antacid, acts as a buffer as well, absorbing H+ ions to resist changes in pH. Also cold milk is suggested as it is feebly dissociated as an acid.

In case of curd, we have probiotics, useful bacteria that help digestion. Due to conversion of lactose to lactic acid, while formation of curd from milk, the pH level decreases. But the bacteria count also increases, thus mitigating the effect of fall in pH and helping in maintaining stomach pH.

Q2: Our stomach contains HCl of pH 2 (approximately). Then why doesn’t we feel the burning sensation if we drink water to “kill” hunger when we are very hungry?(since addition of water in acid is highly exothermic).

The credit for this goes to the special structure of our stomach. The average adult stomach holds about three liters of fluid. Our stomach is made up of a variety of layers, like: the serosa (the outer layer that acts as a covering for the other layers); two muscle layers (the middle layers that propel food from the stomach into the small intestine); the mucosa (the inner layer made up of specialized cells, including parietal cells, g-cells and epithelial cells).

Parietal cells produce HCl (hydrochloric acid), a strong acid that helps to break down food. ­The g-cells produce gastrin, a hormone that facilitates the production of HCl by the parietal cells. The stomach is protected by the epithelial cells, which produce and secrete a bicarbonate-rich solution that coats the mucosa. Bicarbonate is alkaline, a base, and neutralizes the acid secreted by the parietal cells, producing water in the process. [This continuous supply of bicarbonate is the main way that our stomach protects itself from autodigestion (the stomach digesting itself) and the overall acidic environment.] In general, this mechanism is stable enough to protect stomach from highly exothermic reaction involved in addition of water to acid.

Hence, we should not drink water in case of acidity, since our stomach’s acid-defense mechanism is already in poor state.

-GK

Posted in Health and Medicine, Science | Tagged , , , , , ,