Most cells contain cytoplasm and are bound by a plasma membrane. Within them are various structures, called organelles (see Table 1.1), and a specialized part of the cell, called the nucleus (Fig. 1.2). The fluid surrounding the organelles is called cytosol.

Epithelial cells line the internal and external surfaces of body organs (Fig. 1.3), forming the outer layer of the skin, the mucous membranes, the lining of the lungs, gut, reproductive and urinary tracts, and also the endocrine and exocrine glands. Epithelial cells are often ‘polarized’ and have different characteristics on their apical (top) surface and their basal surface (which is in contact with the basement membrane). Epithelial cells are relatively undifferentiated and tend to undergo frequent mitotic divisions (see Chapter 7). This is because they are often exposed to wear and tear and so replacement epithelial cells are generated from a basal layer where cell division takes place. Epithelial cells form a barrier, which allows secretion and absorption of substances from one compartment to another. The skin is a specialized epithelial layer. The basal layer produces cells that are enriched with the protein keratin. The outer layers of skin cells are dead and so lack cytoplasm; it is these keratinized dead cells that provide the barrier function of the skin. Epithelial cells are classified by shape (cuboidal or columnar) and the number of layers. If there is a single layer of cells, the epithelium is described as simple; if there is more than one layer of cells (such as skin), it is stratified. Pseudostratified cells are a single layer of cells that appear to consist of more than one layer. Glands are derived from epithelial tissue.

Muscle cells contain contractile elements, so the cells can generate the mechanical force required for movement of the body or substances within the body (Fig. 1.4) or change shape and size. Muscle tissue is formed from the mesodermal layer of the embryo (see Chapter 9). There are three types of muscle tissue: skeletal, cardiac and smooth muscle. Skeletal muscle may be attached to bones and controls movement of the skeleton. Skeletal muscle can also be attached to the skin, for instance the muscles of the face involved with expression. Contraction of skeletal muscle is usually under voluntary or conscious control. Skeletal muscle is often described as ‘striated’ because of the striped appearance of the sarcomeres of the muscle observed under the light microscope. Skeletal muscle fibres can be subdivided into slow and fast twitch fibres. Fast twitch fibres contract more strongly but they tire easily, whereas slow twitch fibres can contract for prolonged periods.

Cardiac muscle is only found in the heart; it has some structural similarity with skeletal muscle. Smooth muscle and cardiac muscle are usually under involuntary control (meaning there is no conscious awareness of the control). Smooth muscle surrounds many of the ‘tubes’ in the body, maintaining the function of several body systems. Smooth muscle cells are linked by gap junctions, and muscle contraction is relatively slow. Blood pressure is maintained by the contraction of a smooth muscle layer in the walls of the blood vessels. If the smooth muscle constricts, described as ‘vasoconstriction’, the internal lumen of the vessel will decrease and blood pressure will increase. ‘Vasodilatation’ is the opposite condition: the smooth muscle relaxes and the lumen diameter increases, so blood pressure falls. Organized synchronized waves of smooth muscle contraction, for instance in the gut, renal system and uterine tubes, generate peristaltic waves; these produce unidirectional movement of the contents within the lumen of the tube (Fig. 1.5).

Connective tissue functions to connect, anchor and support body structures (Fig. 1.6). Connective tissue cells often produce an extracellular matrix composed of proteins in a ground substance of sugars, proteins and minerals. Bone is a type of connective tissue, whereas collagen is an example of an extracellular matrix. Adipose tissue is composed of specialized cells that store fat for future energy requirements and have an endocrine role. Adipose tissue also acts as an insulating layer to conserve body heat loss and so contributes to the maintenance of the homeothermic status of the organism. Fibrous tissue is an example of dense connective tissue. It is a tough tissue that forms ligaments, tendons and protective membranes.

Neurons are cells that are specialized to initiate and conduct electrical signals (Fig. 1.7). Neurons require the presence of glial cells for nourishment and support; glial cells are also involved in the propagation of the electrical impulses in the neurons. As neurons are so highly specialized, they do not usually undergo further mitotic divisions once developed. Therefore, in the fetal and early neonatal period, the number of neurons produced far exceeds the level required for normal neurological function. To survive and function, neurons need regular stimulation. Throughout life, millions of neurons become dysfunctional and die.

Homeostasis is the term used to describe the processes of the various physiological systems that maintain the constancy of the internal environment. Multicellular animals are able to maintain an internal stability that is essential for the optimal functioning of all body systems, whereas simple unicellular organisms tend to inhabit stable environments or have adapted to overcome fluctuations in the environment, for instance by forming spores during dry periods. Unicellular organisms rely on basic nutrients being present in the environment to allow cell growth and reproduction.

The evolution of multicellular organisms and the development of motility meant that these animals were able to move within the environment to seek out the conditions that suited them best and so optimize their ability to reproduce. Mammals have developed homeostasis to a high degree. Motility, together with the homeostatic challenge of counteracting fluctuations in the external environment, places a huge energy burden upon these individuals. This increased energy requirement is above the basal metabolic rate, which is the rate of energy required to maintain essential functioning only.

Homeostasis can be considered to have three main components:

Homeostasis is regulated by the nervous system, the endocrine system and behavioural factors that are dependent on conscious or subconscious action by the organism. A homeostatic control system requires monitoring a variable, detecting changes and generating responses which will restore the composition of the internal environment (Fig. 1.9). (This type of system, involving a process called negative feedback, is dealt with in more detail under Hormonal regulation in Chapter 3, p. 64.)

Temperature regulation is one example of homeostasis (Fig. 1.10). Enzymes regulating biochemical changes, and physiological and metabolic functions, have optimal activity within a narrow temperature range. Outside this physiological temperature range, the protein structure of the enzyme begins to denature, so the configuration (shape) of the enzyme distorts, which affects its functional activity. A warm-blooded (homeothermic) animal is well prepared to react quickly and efficiently to changes within the environment, unlike a cold-blooded (poikilothermic) animal, which depends upon the ambient temperature of the environment.

The nervous system coordinates body functions. It monitors physiological processes by processing input from the senses, integrating them and initiating responses or motor output. The nervous system is an organization of millions of neurons, or nerve cells, and glial cells, which support and regulate the composition of the nervous system. It is composed of the brain, the spinal cord (in the centre of the vertebral column) and the neurons throughout the body. The skull and the vertebral column protect the brain and the spinal cord. The brain and spinal cord form the central nervous system (CNS) and the remainder is the peripheral nervous system (Fig. 1.11). Neurons usually consist of a cell body and dendrites (extensions) and an axon or nerve fibre, which carries information from the cell body to or from the CNS. They are of different sizes; some neurons have axon projections over 1 m in length. A nerve is a collection of axons running alongside each other over the same distance. A ganglion is a collection of cell bodies of neurons within the peripheral nervous system. Ganglions are located in dorsal (back) or ventral (front) branches of the spinal cord. The spinal cord and spinal nerves are organized on a segmental basis; this corresponds to the embryonic origin of the dermatomes (see Chapter 9). Cranial nerves carry information between the brain and regions of the head. Neurons that carry information towards the brain, entering the dorsal roots of the spinal cord, are sensory or afferent neurons. Neurons carrying information from the CNS to the skeletal muscles, and leaving the spinal cord at the ventral roots, are motor or efferent neurons (Fig. 1.12). Neurons that carry information between a sensory neuron and the CNS (or between the CNS and a motor neuron) are known as interneurons.