Inorganic compounds

There are a number of other inorganic compounds that arc also essential to the functions of the body: minerals, acids and bases. It is important to be familiar with some basic chemical definitions when considering these substances.

sitive:

When dissolved in water, the molecules of many substances break apart into charged particles, called ions. This charge may either be negative or j ions with one or more positive charges arc called cations and ions with one or more negative charges are called anions.

An electrolyte is a chemical substance that, when dissolved in water, splits into ions, and is thus capable of conducting an electric current. Scxiium chloride (NaCl) is an example of an electrolyte in the body, its ions being sodium (Na⁺) and chloride (Cl ) in solution.

Minerals

The principal cations in the body are sodium (Na⁺). potassium (K⁺). calcium (Ca ²*). and magnesium (Mg²⁺). The principal anions include chloride (Cl ) and bicar­bonate (HC()_l ). These ions are essential to the func­tions of the body and it is vital that they are present in sufficient and balanced quantities. Stxiium and chlo­ride are mainly found in the extracellular fluid, while potassium is mainly found in the intracellular fluid (i.e. inside the cells). The concentration of these ions is important in the regulation of fluid balance between the intracellular and extracellular fluid. This balance is maintained by special pumps' in the cell mem­brane. An imbalance will lead to Signilicant problems.

Calcium, phosphorus and magnesium are impor­tant minerals that are found in storage in bone tissue. Calcium is essential for many processes in the body, such as muscle contraction, nerve conduction and bkxxi clotting. Iron and copper are also essential to nor­mal bLI I General structure of an ammo acid. The 'R^, group vanes from ammo acid to amino aod

then coiled and folded to give the specific structure of a particular protein (Fig. 1.12).

Proteins generally fall into one of two groups:

1. Globular - the functional proteins. These are associated with cellular chemical reactions and

include hormones and enzymes

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Fibrous - the structural proteins. These arc

insoluble and are part of the composition of

various structures in the body. They include keratin, collagen and elastin.

By means of digestive enzymes the body breaks down the proteins acquired from the diet into their con­stituent amino acids, which can then be absorbed through the mucous membrane of the digestive sys­tem into the blood.

Chemical reactions in the body

Most of the chemical reactions that take place in the bto molecules such as oxygen, e.g. lining the blood vessels and the alveoli of the lungs.

T Simple columnar epithelium - this has tall narrow cells and is one layer thick. Generally, simple columnar epithelium Iinesorgans that have an absorptive function, e.g. the small and large intestines, or a secretory function, e.g. digestive glands.

4. Ciliated epithelium - this is a more specialised epithelium consisting of a single layer of columnar shaped cells (Fig. 2.11. The free surface of the cells has tiny hair like projections called cilia whose function is to ’waft* foreign particles along the epithelial surface and out of the body. Ciliated epithelium lines the upper respiratory trad, where it helps to trap solid particles that have been inhaled, preventing them from entering the more distal parts of the respiratory system.

5. Slriitilicil epithelium - this is composed of a number of layers of cells and is thicker and tougher than theolhcr types of epithelium. It is found in areas that are subjected Io wear and to friction and shearing forces, e.g. the epidermis of the skin (see page 1 51I.

6. Trimsitiinuil epithelium - a type of specialised Stratilicd epithelium found lining parts of the urinary system, i.e. structures and lubes that are

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Capableof considerable distension and variations in internal pressure and capacity, such as the bladder and ureters. The cells are able to change Ihcir shape according Io circumstances and thus Iheir appearance varies wilh the degree of distension of the structure.

Glands

Glandular tissue is a modification of epithelial tissue. The epithelium, in addition to its protective function, may also be a secretory membrane. Glands are either:

Unicellular qlands - these have individual secretory cells are interspersed throughout the tissue. The most common type is the ι∣oblet cell, which secretes clear sticky mucus directly onto the membrane surface. The epithelium is known as a mucous membrane. Mucustraps particles, providing extra protection, and also lubricates the epithelial surface. Mucous membranes are found covering the oral cavity, lining the vagina and the trachea and in many other parts of the body. Multicι^,llιιl(irIjlimils- these consist of many secretory cells folded to form more complex glands. They vary in shape and intricacy relating to their position and function in the body. Examples of some of the types of gland found in Ihe body are shown in Figure 2.2.

Glands may be categorised as either:

Exocrine glands - these have a system of ducts through which their secretory products are transported directly to the site where they will be used Endocrine glands - do not have a duct system (ductless glands) and their secretions, known as hormones, are carried by the blood to their target organ which may be some distance away (see Chapter 6).