Complete Notes on Human Respiratory System

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Two external nares (nostrils)- open into two nasal chambers. Each nasal chamber is differentiated into vestibular, respiratory and olfactory regions. Three bony ridges, the nasal conchae arise from the wall of each nasal chamber which increase the surface area of the nasal chamber. The nasal chambers open into the pharynx through internal nares. The pharynx provides passage to both air and food.

The pharynx leads into the larynx. In man it is more prominent; therefore, it is called 'Adam's apple' in man. Human larynx consits of the following structures.

(i) Glottis-slit-like aperture

(ii) Cartileges-nine pieces (three are single and three are paired): (a) Epiglottis made up of elastic cartilage; (b) Thyroid certilage-largest cartilage of the larynx made up of hyaline cartilage; (c) Cricoid cartilage like signet ring, made up of hyaline cartilage; (d) Arytenoid cartilages-mainly consist of hyaline cartilage but their apical parts are made up of elastic cartilage; (e) Cartilages of santorini (corniculate cartilages)-made up of elastic cartilage; (f) Cuneiform cartilages-consist of elastic cartilage

(iii) Hyoid bone-embryo logically it is considered a bone of skull.

(iv) Thyrohoid membrane-connects hyoid bone and thyroid certilage.

(v) Vocal cords-two pairs; one pair false vocal cords and one pair true vocal cords. The rima glottidis is a gap between the vocal cords that communicate with the larynx pharynx above. Sound is produced by true vocal cords.

(vi) Laryngeal ligaments and Muscles-connect different certilages of the larynx.

The larnyx leads into the trachea. Its rings are made up of hyaline cartilage. The trachea leads into the primary bronchi which enter the right and left lungs.

Each lung is enclosed by two membranes: outer parietal pleuron and inner visceral pleuron with a pleural cavity in between two pleura. The cavity is filled with pleural fluid secreted by the pleura.

The left lung is smaller than the right lung. The left lung has a concavity, the cardiac notch, where the hearts lies. The left lung has two lobes however right lung has three lobes.

As soon as the primary bronchus enters each lung it divides to form secondary and tertiary bronchi.

The tertiary bronchi sub-divide into smaller branches, the bronchioles which are without cartilaginous rings. The bronchioles decrease in diameter and their epithelium also becomes thinner are they go deeper into the lungs. After repeated branching, one of the smaller bronchioles enters a lung lobule called lobular bronchiole. The latter gives rise to about six terminal bronchioles which further subdivide into respiratory bronchioles.

The respiratory bronchioles open into two or three alveolar ducts which terminate in expanded passage, the atria, (sing atrium) which in turn lead into alveolar sacs or pair sacs. The latter open into several alveoli (also called acini). The alveoli have very thin wall consisting of squamous epithelum. The wall of the alveoli has an extensive network of blood capillaries interspersed with elastic and reticular connective tissue fibres. Due to very intimate contact of blood capillaries with the alveoli, the exchange of gases takes place easily. Thus each alveolus may be called a miniature lung where exchange of gases takes place.


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