Tissues, Cells and Organs

The human body consists of many millions of cells. The cell is the basic structural unit of the body. A typical body cell has the following features;

A cell membrane – this acts as a boundary for the cell contents. It also acts as a barrier, allowing certain substances into the cell, while keeping other substances out. The cell membrane is said to be ‘selectively permeable’.

A nucleus – this determines what type of cell it will be, e.g. liver cell, nerve cell, skin cell,…. The ‘information’ that determines this is located on long, threadlike structures called chromosomes, which are only found in the nucleus.

The cell cytoplasm – this is a ‘jelly-like’ substance found outside of the nucleus. This is where most of the chemical reactions performed by the cell take place. These chemical reactions are controlled by specialized proteins called enzymes.

Mitochondria – these are small oval shaped structures located in the cell cytoplasm. This is where the energy releasing chemical reactions take place. Cells that are active, have a lot of mitochondria; e.g. muscle and liver cells. While cells that are less active, only have a few mitochondria; e.g. skin cells and fat cells.

A collection of similar cells that work together to perform a particular function, is known as a tissue. There are 4 basic tissue types found in the body; -- epithelial, -- muscle, -- nervous and – connective.

Epithelial tissue is the type of tissue found in any covering or lining in the body; e.g the outer layer of the skin (epidermis) or the inner lining of the digestive tract and other body ‘tubes’. There are two basic types of epithelial tissue; -- simple (a single layer of cells) and – compound or stratified (multilayered).
Simple epithelial tissue is found in areas where a very thin membrane is required. This allows the exchange of substances across the thin lining and processes such as filtration.
Simple epithelial tissue is found in the tiny air sacs of the lungs and in the filtration membrane of the kidneys. This enables the exchange of oxygen into the bloodstream and the release of carbon dioxide from the blood into the air. In the kidneys, the blood is filtered to remove urea and other waste products. This thin lining, called the capsular membrane, allows this process to occur.

Muscle tissue is normally the most common of the 4 tissue types. The main characteristic of muscle tissue is its ability to contract (shorten) when stimulated by a weal electrical signal – a nervous impulse. Muscle tissue also generates a considerable amount of heat. There are 3 basic types of muscle tissue; -- skeletal, -- visceral or ‘smooth’ and – cardiac.
Skeletal muscle is the most common of the 3 types. These are the muscles that enable the skeleton to move. Skeletal muscle is attached to bone, so when it contracts it pulls on the bone causing movement.

Nervous tissue – Its main characteristic is its ability to respond to a stimulus and generate a nervous impulse which is a tiny electric signal. It is also capable of carrying this impulse around the body. Nervous tissue enables us to be aware of changes in our surroundings and in our own bodies. It also allows us to coordinate movement and behaviour. We have specialised sense cells which respond to a particular stimulus. A stimulus is a change in light, temperature, pressure, sound, etc… which produces a response or reaction in a living organism. These specialised cells or tissues that detect stimuli are called receptors. Some of these receptors are scattered throughout the skin, while others are concentrated into special organs such as the eye and ear.
These are some of the receptors found in the human body;
-- receptors in the eye which are sensitive to light,
-- receptors in the ears which are sensitive to sound,
-- receptors in the ears which are sensitive to changes in position and enable us to keep our balance.
-- receptors on the tongue and in the nose which are sensitive to chemicals and enable us to taste and smell.
-- receptors in the skin that are sensitive to pressure, touch and temperature changes.

The special property of receptors is that they are able to convert the stimulus into a weak electrical signal -- a nerve impulse. This impulse then passes along specialised cells called sensory neurones. This carries the signal, usually to the brain. The brain interprets the signal and makes us aware of the sensation.
The brain and/or spinal cord acts as the coordinator. This is where the signal is processed and a response is initiated. The brain and spinal cord together are the central nervous system (CNS). The response is usually movement, which involves the contraction/relaxation of muscles, or it maybe a secretion from a gland. The nerves that carry the signal from the CNS to the muscle/gland are called motor neurones. The muscle(s) or gland(s) are known as the effectors. Finally, the movement or secretion is the response.

Stimulus à Receptor à Coordinator à Effector à Response

The neurones that branch off the CNS and carry a nervous impulse throughout the body are known as the peripheral nervous system (PNS). Some of these peripheral nerves connect directly to the brain. These are the cranial nerves (12 pairs); examples include the optic nerve, auditory nerve, oculomotor nerve and the nerves that stimulate the facial muscles. There are other neurons that connect directly to the spinal cor. These are known as spinal nerves (31 pairs); examples include the sciatic and peroneal nerves and the phrenic nerve.

Connective tissue is a very diverse tissue type. The main categories of this type of tissue are;
-- loose connective or aerolar tissue. This is ‘jelly-like or semisolid’ and can be found in the dermis of the skin, between muscles and supporting blood vessels, nerves and fibres.
-- adipose tissue consists of fat cells. This acts as an energy store and also as a heat insulator. Adipose tissue can be found under the skin and surrounding the major organs.
-- dense connective tissue consists of tightly packed bundles. Examples include collagen, ligaments and tendons. Some dense connective can be elastic which allows the tissue to extend and recoil. For example, the walls of the larger arteries and deep lung tissue, the elastin fibres of the skin.
-- Cartilage is firm tissue that provides some structural support but is not as strong as bone. It is also smooth and slippery. Examples include the cartilage at the ends of bones in movable joints, the pads of cartilage between successive vertebrae, tip of the nose, pinna of the ear,..
-- blood which consists of cells (RBC’s and WBC’s) and cell fragments (platelets) suspended in a watery fluid called blood plasma.
-- bone has rigidity and considerable strength. Bone is strengthened by salts such as calcium and phosphate. The two types of bone tissue are compact bone which is the familiar strong weight bearing bone on the outside. There is also the softer cancellous bone which has a spongy/honeycomb appearance, the ‘bone marrow’. This tends to be located in the longer, larger bones. Bone marrow has an important role in the production of red and white blood cells.

Organs and organ systems;
When 2 or more of the 4 tissue types work together to perform a particular function, this is known as an organ; e.g. heart, lungs, stomach, liver,…… Arteries, veins and the walls of the digestive and respiratory tracts are also regarded as organs – because they contain more than 1 tissue type.
A number of organs that work together to perform a particular function is known as an organ ‘organ system’; e.g. digestive, nervous, circulatory, endocrine, reproductive, skeletal-muscular systems,…….

Basic organization of the Human Body;

Cells  Tissues  Organs  Organ systems  Human body