8.1 Introduction to the Human Body, Homeostasis & the Sensory Organs
Key Takeaways
- The structural hierarchy runs cell, tissue, organ, organ system, organism; the four primary tissue types are epithelial, connective, muscular and nervous.
- Homeostasis is the maintenance of a stable internal environment, operating almost entirely by negative feedback with receptor, control centre and effector.
- Positive feedback amplifies rather than corrects and is used only in processes meant to complete quickly - childbirth, blood clotting and the nerve action potential.
- The five Jnanendriyas map to the five Tanmatras and Mahabhutas: ear to sound and ether, skin to touch and air, eye to form and fire, tongue to taste and water, nose to smell and earth.
- Trataka exercises the extraocular muscles and stimulates lacrimal secretion; Jala Neti raises nasal mucociliary clearance; both are sensory-organ practices with specific physiological rationales.
8.1 Introduction to the Human Body, Homeostasis & the Sensory Organs
Unit 3 carries 20 marks and 20 questions, exactly as much as the Bhagavad Gita and Patanjali combined. Its first five items are anatomy and physiology, and this section covers three of them: 3.1 introduction to the human body, 3.3 homeostasis, and 3.4 the sensory organs. Sections 8.2–8.5 cover the systems named in item 3.2.
Structural organisation of the body
The body is organised in levels, each built from the one below:
| Level | Description | Example |
|---|---|---|
| Chemical | Atoms and molecules | Water, protein, ATP |
| Cellular | The smallest living unit | Neuron, myocyte, hepatocyte |
| Tissue | Groups of similar cells with a shared function | Skeletal muscle tissue |
| Organ | Two or more tissues working together | Heart, lung, kidney |
| Organ system | Organs cooperating in a function | Cardiovascular system |
| Organism | All systems together | The human being |
The four primary tissue types
| Tissue | Function | Examples |
|---|---|---|
| Epithelial | Covering, lining, secretion, absorption | Skin epidermis, gut lining, glandular tissue |
| Connective | Support, binding, transport, storage | Bone, cartilage, tendon, ligament, adipose, blood |
| Muscular | Contraction and movement | Skeletal (voluntary, striated), cardiac (involuntary, striated), smooth (involuntary, non-striated) |
| Nervous | Conduction and integration of signals | Neurons and neuroglia |
Blood counts as connective tissue — an examiner's favourite, because it is counter-intuitive.
Anatomical position and terms
All directional terms assume the anatomical position: standing erect, feet slightly apart, arms at the sides, palms facing forward.
| Term | Meaning |
|---|---|
| Superior / Inferior | Above / below |
| Anterior (ventral) / Posterior (dorsal) | Front / back |
| Medial / Lateral | Toward / away from the midline |
| Proximal / Distal | Nearer to / further from the point of attachment |
| Superficial / Deep | Toward the surface / away from it |
The three planes — sagittal (left/right), frontal or coronal (front/back) and transverse (upper/lower) — are covered with their asana correlates in Section 8.2.
Body cavities
Dorsal: cranial (brain) and vertebral (spinal cord). Ventral: thoracic (divided by the mediastinum into two pleural cavities and the pericardial cavity) and abdominopelvic, separated from the thoracic by the diaphragm — the muscle on which the entire practice of Pranayama depends.
Homeostasis
Homeostasis (homeo, similar + stasis, standing) is the maintenance of a relatively stable internal environment despite changing external conditions. Claude Bernard called it the constancy of the milieu intérieur; Walter Cannon named it.
Note the word relatively. Homeostasis is not a fixed value but a dynamic equilibrium oscillating within a narrow range — which is exactly the physiological correlate of Samatvam.
The components of a homeostatic loop
| Component | Role | Example (thermoregulation) |
|---|---|---|
| Stimulus | A change producing deviation from the set point | Body temperature rises to 38.5 °C |
| Receptor | Detects the change | Peripheral and hypothalamic thermoreceptors |
| Control centre | Compares to the set point, decides the response | Hypothalamus |
| Effector | Produces the response | Sweat glands, cutaneous blood vessels |
| Response | Restores the set point | Sweating, vasodilation, heat loss |
Negative feedback — the default
Negative feedback opposes the original change and returns the variable toward its set point. Almost all homeostatic control is negative feedback: temperature, blood glucose, blood pressure, blood pH, osmolarity, calcium.
| Variable | Normal range |
|---|---|
| Core body temperature | ~37 °C |
| Blood pH | 7.35–7.45 |
| Fasting blood glucose | ~70–100 mg/dL |
| Blood pressure | ~120/80 mmHg |
| Blood oxygen saturation | 95–100% |
Positive feedback — the exception
Positive feedback amplifies the original change. It is inherently unstable and is used only where a process must run rapidly to completion:
- Childbirth: oxytocin → uterine contraction → more cervical stretch → more oxytocin.
- Blood clotting: activated platelets recruit further platelets.
- The nerve action potential: sodium influx opens further sodium channels.
An examiner asking for "an example of positive feedback" wants one of those three.
Where yoga enters
Yogic practice does not create homeostasis; it widens the range over which homeostasis can be maintained and improves the speed of return after a perturbation. Slow breathing and Kumbhaka train chemoreceptor tolerance to raised CO₂; Shatkarmas act directly on fluid and mucus balance; asana and relaxation raise vagal tone and heart-rate variability, which is a direct measure of homeostatic flexibility. The technical name for that capacity is allostatic load reduction, and it is the most defensible physiological claim yoga makes.
The sensory organs
The physiological account
| Organ | Sense | Receptor type | Key structures |
|---|---|---|---|
| Eye (Chakshu) | Vision | Photoreceptors | Cornea, iris, lens, retina with rods (dim light, no colour) and cones (colour, acuity); optic nerve (CN II) |
| Ear (Shrotra) | Hearing and balance | Mechanoreceptors | External, middle (three ossicles: malleus, incus, stapes) and inner ear; cochlea for hearing, semicircular canals and vestibule for balance; vestibulocochlear nerve (CN VIII) |
| Nose (Ghrana) | Smell | Chemoreceptors | Olfactory epithelium in the roof of the nasal cavity; olfactory nerve (CN I) — the only sensory pathway that does not relay through the thalamus |
| Tongue (Rasana) | Taste | Chemoreceptors | Taste buds on papillae; five modalities — sweet, sour, salty, bitter, umami; CN VII, IX and X |
| Skin (Twak) | Touch, pressure, temperature, pain | Mechano-, thermo- and nociceptors | Meissner (light touch), Pacinian (deep pressure and vibration), Merkel (sustained touch), Ruffini (stretch), free nerve endings (pain and temperature) |
The Samkhya account
The same five appear as the Jnanendriyas, the organs of knowledge, each linked to a Tanmatra (subtle element) and a Mahabhuta (gross element):
| Jnanendriya | Tanmatra | Mahabhuta |
|---|---|---|
| Ear (Shrotra) | Shabda — sound | Akasha — ether |
| Skin (Twak) | Sparsha — touch | Vayu — air |
| Eye (Chakshu) | Rupa — form | Agni — fire |
| Tongue (Rasana) | Rasa — taste | Apas — water |
| Nose (Ghrana) | Gandha — smell | Prithvi — earth |
The ordering runs from subtlest to grossest, and the pairing is fixed. Alongside these are the five Karmendriyas, the organs of action: speech (Vak), hands (Pani), feet (Pada), excretion (Payu) and generation (Upastha).
The relevance is Pratyahara. PYS 2.54 defines sensory withdrawal as the senses disengaging from their objects — and the objects in question are precisely the five Tanmatras, in the order given above.
Yogic practices acting directly on the sensory organs
| Practice | Organ | Physiological rationale |
|---|---|---|
| Trataka | Eye | Exercises the extraocular muscles; stimulates lacrimal secretion, which washes the cornea and clears the tear ducts; the after-image work trains sustained attention |
| Jala Neti / Sutra Neti | Nose | Removes stagnant mucus and particulate matter; raises mucociliary clearance; stimulates trigeminal afferents |
| Bhramari | Ear | Closing the ears removes external auditory input and directs attention to internally generated sound; the humming raises nasal nitric oxide |
| Jihva Dhauti / Danta Dhauti | Tongue, teeth, ears | Clears the tongue coating that dulls taste and harbours bacteria |
| Shavasana / Yoga Nidra | Skin | Systematic rotation of consciousness across the somatosensory map, reducing cortical arousal |
| Kapalarandhra Dhauti | Sinuses / frontal region | Massage of the forehead and sinus region after cleansing |
Summary of Exam-Key Points
- Hierarchy: chemical → cellular → tissue → organ → system → organism. Blood is connective tissue.
- Homeostatic loop: stimulus → receptor → control centre → effector → response.
- Negative feedback is the norm; positive feedback occurs in childbirth, clotting and the action potential.
- Blood pH 7.35–7.45; core temperature ~37 °C.
- Olfaction is the only sense that does not relay through the thalamus.
- Jnanendriya–Tanmatra–Mahabhuta: ear/sound/ether, skin/touch/air, eye/form/fire, tongue/taste/water, nose/smell/earth.
Which of the following is an example of positive rather than negative feedback?
In the Samkhya scheme, which Mahabhuta corresponds to the sense of touch?
Which sensory pathway is unique in NOT relaying through the thalamus before reaching the cortex?
To which of the four primary tissue types does blood belong?
What is the most defensible physiological claim about yoga's effect on homeostasis?