The human being is a multidimensional structure and there are various sheaths that envelop his soul: the physical body, called in Sanskrit the annamayakosha, the astral body, called the linga sharira or sukshma sharira, and which includes three layers, and the causal body, or karana sharira, with its sheath of bliss called the anandamayakosha. Without an understanding of these layers it will be very difficult to understand the true influence of hatha yoga. The asanas, pranayama, and relaxation influence not only our physical body but each and every one of our different levels of existence, from the gross to the most subtle.

Yoga, as well as the majority of methods of eastern medicine, speaks of an astral body composed of prana, in which are found chakras or energetic centers, and nadis that interconnect the chakras, the most important being the ida, pingala and sushumna.

The astral and physical bodies are united by a nadi resembling a silvery thread, through which the vital energy flows. The death of the physical body occurs when this thread is cut, which constitutes their final separation.

In the linga sharira or sukshma sharira, we find three different layers, or koshas:

The pranamayakosha

The pranamayakosha is the pranic sheath, composed of 72,000 nadis which interconnect in the energetic centers, or chakras. Even though the form of this sheath is subtle, it is very similar to the form of the physical body. It is composed of the vital airs or pranas and the five senses of action, or karmendriyas, which are: speech, manual movement, locomotion, procreation and excretion. The pranic sheath is made up of 72,000 nadis, as is indicated in the Hatha yoga pradipika:

catur-asiti-pithesu
siddham eva sadabhyaset
dva-saptati-sahasranam
nadinam mala-sodhanam

“Out of the 84 asanas,
siddhasana should always be practiced,
because it cleanses the impurities
of the 72,000 nadis”. (1.41)

dva-saptati-sahasranam
nadinam mala-sodhane
kutah praksalanopayah
kundaly-abhyasanadrte

“There is no other way,
but the practice of the kundalini,
for washing away the impurities
of 72,000 nadis”. (3.123)

dva-saptati-sahasrani
nadi-dvarani pañjare
susumna sambhavi saktih
sesastveva nirarthakah

“In this body there are 72,000 openings of nadis;
of these, the sushumna, which has the samhhavi shakti in it,
is the only important one,
the rest are useless.” (4.18)

And it is also mentioned in the Upanishads:

dva-saptati-sahasrani
pratinadisu taitilam

“In each of the 72,000 nadis,
there is a material which is like oil.” Ksurikopanishad (17b)
 

The manomaya kosha

The manomaya kosha is the mental sheath, which consists of the instinctive mind, and includes as much the manas as the citta; that is to say, the seat of desires and the governance of the sensory and motor organs. It includes the five senses of knowledge, or jñanendriyas, which are: hearing, touch, vision, taste and smell.
 

The vijnanamaya kosha

The vijnanamaya kosha is the intellectual sheath, which includes the ego or ahankara, that is to say, "what we believe ourselves to be", the idea of "I" that relates what happens to itself and perceives itself as “the doer”, and the buddhi, the intellect or discriminating principle that evaluates and decides.