The primary bronchioles arise from the tertiary bronchi

Tertiary bronchus

The tertiary bronchi arise from the secondary bronchi. The respiratory epithelium lining their lumen is surrounded by a layer of smooth muscle. This layer is composed of two ribbons of smooth muscle that spiral in opposite directions...

. They are histologically distinct from the tertiary bronchi in that their walls do not have hyaline cartilage

Hyaline cartilage

Hyaline cartilage consists of a slimy mass, pearly bluish in colour with firm consistency and considerable collagen. It contains no nerves or blood vessels, and its structure is relatively simple....

 and they have Clara cell

Clara cell

Clara cells are dome-shaped cells with short microvilli found in the small airways of the lungs.Clara cells are found in the ciliated simple epithelium...

s in their epithelial lining. The epithelium

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 starts as a simple ciliated columnar epithelium

Simple columnar epithelium

A simple columnar epithelium is a columnar epithelium that is uni-layered. In humans, a simple columnar epithelium lines most organs of the digestive tract including the stomach, small intestine, and large intestine...

 and changes to simple ciliated cuboidal epithelium

Simple cuboidal epithelium

Simple cuboidal epithelia are epithelial cells in a single layer of cubelike cells with large, spherical central nuclei. Simple cuboidal epithelia are found on the surface of ovaries, the lining of nephrons, the walls of the renal tubules, and parts of the eye and thyroid.On these surfaces, the...

 as the primary bronchiole decreases in size. The diameter of the primary bronchioles is often said to be less than 1 mm, though this value can actually range from 5 mm to 0.3 mm. As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely on elastic fibers attached to the surrounding lung

Human lung

The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately of airways and 300 to 500 million alveoli, having a total surface area of about in...

 tissue for support. The inner lining (lamina propria

Lamina propria

The lamina propria is a constituent of the moist linings known as mucous membranes or mucosa, which line various tubes in the body ....

) of these bronchioles is thin with no glands present, and is surrounded by a layer of smooth muscle

Smooth muscle

Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

. As the primary bronchioles get smaller they divide into terminal bronchiole

Terminal bronchiole

A terminal bronchiole is a bronchiole at the end of the conducting zone. At the transition into the respiratory zone, alveoli become present.The terminal bronchiole is the most distal segment of the conducting zone. Each of the terminal bronchioles divides to form respiratory bronchioles which...

s.