Only $35.99/year

General Biology Notes 1

Terms in this set (358)

- Occurring as sheets of cells, epithelial tissues cover the outside of the body and line organs and cavities within the body
- Two forms occur in the human body:
(1) Covering and lining and epithelium - Forms the outer layer of the skin; lines open cavities of the digestive and respiratory systems; covers the walls of organs of the closed ventral body cavity.
(2) Glandular epithelium - Surrounds glands within the body. All glands are made up of epithelial cells.
- Because epithelial cells are closely packed, often with tight junctions, they function as a barrier against mechanical injury, pathogens, and fluid loss.
- Epithelium has two names (the first name indicates the number of cell layers, the second describes the shape of its cell)
(1) Simple epithelia - Consist of a single cell layer (found where absorption, secretion, and filtration occur).
(2) Stratified epithelia -Are composed of two or more cell layers stacked on top of each other (typically found in high abrasion areas where protection is needed).
- All epithelial cells have six sides but they vary in height.
- Epithelial tissues have five main characteristics:
(1) Polarity
(2) Specialized contacts
(3) Supported by connective tissue
(4) Avascular and innervated
(5) Regeneration

- Occurring as sheets of cells, epithelial tissues cover the outside of the body and line organs and cavities within the body
- Two forms occur in the human body:
(1) Covering and lining and epithelium - Forms the outer layer of the skin; lines open cavities of the digestive and respiratory systems; covers the walls of organs of the closed ventral body cavity.
(2) Glandular epithelium - Surrounds glands within the body.
- Because epithelial cells are closely packed, often with tight junctions, they function as a barrier against mechanical injury, pathogens, and fluid loss
- Epithelium has two names (the first name indicates the number of cell layers, the second describes the shape of its cell)
(1) Simple epithelia - Consist of a single cell layer (found where absorption, secretion, and filtration occur).
(2) Stratified epithelia -Are composed of two or more cell layers stacked on top of each other (typically found in high abrasion areas where protection is needed).
- All epithelial cells have six sides but they vary in height.
- Epithelial tissues have five main characteristics:
(1) Polarity
(2) Specialized contacts
(3) Supported by connective tissue
(4) Avascular and innervated
(5) Regeneration
- Functions of epithelial cells include:
(1) Secretion: Secretion is the movement of material from one point to another chemicals, or a secreted chemical substance from a cell or gland. The classical mechanism of cell secretion is via secretory portals at the cell plasma membrane called porosomes.
(2) Selective absorption: The process of absorbing or assimilating substances into cells or across the tissues and organs through diffusion or osmosis, as in absorption of nutrients by the digestive system, or absorption of drugs into the bloodstream.
(3) Protection: This involves the skin (i.e. protection from wear and tear, drying, or excess moisture).
(4) Transcellular transport: Transcellular transport involves the transportation of solutes by a cell through a cell. Epithelial cells use primary and secondary active transport, often in conjunction with passive diffusion through ion channels, to produce transcellular transport across epithelial tissues.
(5) Sensing: Relates to the extracellular environment.
- Aid in circulation by ensuring that oxygenated blood is supplied throughout the body by acting as a bridge between veins and arteries
- Critical to the exchange of substances between the blood and the interstitial fluid (takes place across the walls of capillaries)
o Some substances are carried across the capillary wall in vesicles that form on one side by endocytosis and release their contents on the opposite side by exocytosis
o Small molecules, such as O2 and CO2, diffuse across the endothelial cells or, in some tissues, through microscopic pores in the capillary walls
o Porous openings in fenestrated capillaries provide the route for the transport of small solutes such as sugars, salts, and urea, as well as bulk flow of fluid into tissues driven by blood pressure within the capillary
- Smallest (about one cell layer thick) and most numerous type (there are networks of capillaries throughout the body called capillary beds/capillaries have the largest total cross-sectional area) type of main blood vessel
- Very thin (with sometimes porous) walls constructed from an inner layer of endothelium and the basal lamina
- Very low blood pressure is needed with capillaries and this can be regulated by rings of smooth muscle located at the entrance to capillary beds called Precapillary Sphincters or by contraction/dilation of the smooth muscle in an arteriole
- Movement of fluid between the capillaries and surrounding tissues is controlled by blood pressure (which tends to drive fluid out of the capillaries, this is the stronger force so there is a net loss of fluid) and the presence of blood proteins (which tends to pull fluid back, osmotic pressure)
- Reabsorption in the proximal tubule is critical for the recapture of ions, water, and valuable nutrients from the huge volume of filtrate
- NaCl in the filtrate diffuses into the cells of the transport epithelium, where Na+ is actively transported into the interstitial fluid
- This transfer of positive charge out of the tubule drives the passive transport of Cl-, as well as the movement of more Na+ from the lumen into the cells of the tubule wall by facilitated diffusion and cotransport mechanisms
- As salt moves from the filtrate to the interstitial fluid, water follows by osmosis (the salt and water then diffuse from the interstitial fluid into the peritubular capillaries)
- Glucose, amino acids, potassium ions, and other essential substances are also actively or passively transported from the filtrate to the interstitial fluid and then into the peritubular capillaries
- Processing of filtrate in the proximal tubule helps maintain a relatively constant pH in body fluids
- Cells of the transport epithelium secrete H+ into the lumen of the tubule but also synthesize and secrete ammonia, which acts a a buffer to trap H+ in the form of ammonium ions
- The more acidic the filtrate, the more ammonia the cells produce and secrete (a mammal's urine usually contains some ammonia from this source)
- The proximal tubules also reabsorb about 90% of the buffer bircarbonate from the filtrate, contributing further to pH balance in body fluids
- Material to be excreted become concentrated (many wastes leave the body fluids during the nonselective filtration process and remain in the filtrate while water and salts are reabsorbed)