Physiology

• The nasal cavity extends from the external opening to the pharynx, where it joins the rest of the respiratory system
• The nasal cavity is separated in the middle by the nasal septum, which is a cartilage that shapes and separates the nostrils
• Each nostril is divided into roof, floor, and walls
• The nasal cavity is divided into the vestibule, respiratory sections, and the olfactory sections
• The nasal vestibule is the enlarged area at the nostril opening
• The respiratory section of the nasal cavity is the passages for air to travel into the respiratory system
• In each nostril, the respiratory section contains four conchae (bumps) called turbinate bones or lobes that are covered by the nasal mucosa
• Underlying these conchae, there are passages to interior body structures called meatuses
• The olfactory receptors of smell sensation are found in the olfactory region of the nasal cavity
• The main function of the paranasal sinuses is to adjust speech and produce mucus for the nasal passageway
• The nasolacrimal ducts drain tears from the lacrimal (tear) ducts of the eyes to the nasal mucosa
• The nasal cavity is surrounded by a ring of paranasal sinuses and meatuses in the nasal cavity
• The nasolacrimal ducts connect the lacrimal (tear) ducts in the eye to the nasal cavity
• The nasal (respiratory) mucosa lines the nasal cavity from the nostrils to the pharynx
• The initial region of the nasal cavity is lined by several cell layers of stratified squamous epithelium (flat cells)
• The posterior part of the nasal cavity is lined with pseudostratified columnar ciliated epithelium and contains goblet cells that cover a basement membrane
• The sub-mucosa underlies the basement membrane and is made up of glands that secrete mucus, nerves, blood vessels, and blood plasma
• The surface layer of the nasal mucosa is made up of epithelial cells that provide a physical barrier against invading microbes and allergens
• Endothelial cells line the walls of arteries in the nasal mucosa and are involved in allergic responses
• Mucus glands produce sticky mucus that moistens air and traps foreign particles entering the respiratory airways
• The cilia project from epithelial cells and line the nasal mucosa, draining mucus from the nasal passage to the throat and stomach during their movement
• The olfactory system processes smell sensory information, with Bowman’s glands secreting most of the mucus lining the nerves of the olfactory system
• The conchae, also known as turbinate bones, increase the total surface of the mucosa and create instability in air entering the respiratory passage
• The main function of the nasal cavity is to allow air to enter the respiratory system upon respiration and regulate the air flow and particles it contains
• The secreted mucus, which lines the mucosa, provides a physical barrier against invasion by pathogens and traps pathogens when they enter the nasal cavity
• The nasal mucosa plays an important role in mediating immune responses to allergens and pathogens that enter through the nose and preventing them from invading the nasal cavity and spreading to other structures
• The veins in the nasal cavity warm air entering the respiratory airways, and the nasal nerves regulate the congestion response and innervation of the mucosa is associated with trigeminal and maxillary nerves
• The eye allows us to see things with different shapes and colors, converting light into electrical signals that are sent to the brain for interpretation
• The human eye acts like a camera that collects, focuses, and transmits light through the eye lens to create an image on the retina
• Anterior chamber:

  • Space in the front of the eye, between the cornea and the iris
  • Full of a fluid
• Aqueous humour or intraocular fluid:

  • Clear, watery liquid produced by the ciliary body
  • Flows through the pupil into the anterior chamber
  • Consists of 99.9% water and 0.1% nutrients
  • Helps maintain eye pressure and provide nutrients to the internal part of the eye
  • Drains through the trabecular meshwork to keep eye pressure at a normal level
• Choroid:

  • Layer of tissue rich with blood vessels
  • Located between the sclera and the retina
  • Keeps the retina and back of the eye well-nourished
• Ciliary body:

  • Located between the iris and the choroid
  • Helps the lens in its position, changing its shape for focusing or accommodating
  • Produces the aqueous humour
• Conjunctiva:

  • Clear, thin membrane that lines the inside of the eyelids and the white part of the eye
  • Contains tiny blood vessels that supply the eye and eyelids with oxygen and nutrients
  • Helps keep the eye moist and protected from rubbles and infections
• Cornea:

  • Curved, clear layer that forms the front of the eye
  • Acts as the eye’s ‘windscreen’
  • Refracts light entering the eye before it passes further through the eye
• Iris:

  • Coloured part of the eye, behind the cornea
  • Controls the amount of light that enters the eye by adjusting the pupil size
• Lens:

  • Transparent flexible disc-like tissue in the eye
  • Located directly behind the iris and the pupil
  • Helps to focus light and images on the retina
• Drainage angle:

  • Part of the eye at the junction of the iris and cornea
• Macula:

  • Small and highly sensitive area in the centre of the retina
  • Responsible for detecting very fine details and colours
• Fovea:

  • Small bump in the centre of the retina
  • Contains only cone light receptor cells
  • Provides the sharpest vision