Protection against external environment
The skin provides protection against microbes, chemicals and dehydration by forming a physical barrier. The skin also plays an important role in regulation of body temperature.
The skin consists of two layers:
Epidermis: The epidermis is composed of the outermost layers of cells in the skin. The cells of the epidermis grow towards the surface from the deepest layer. The surface of the skin consists of dead cells that are worn off constantly. The epidermis acts as a barrier between the dry environment and moist internal tissues of the body.
Dermis: The dermis is the layer of the skin located between the epidermis and subcutaneous tissues. The dermis contains blood vessels, lymph vessels, hair follicles and sweat glands. The sweat glands have ducts leading to the surface. Hairs grow from the follicles. The dermis is also rich in sensory nerve endings which provide sensations of touch, pain and heat.
Defence against infection
The body has many mechanisms to protect itself from infectious agents. These mechanisms are of two types:
- Non-specific defence mechanisms
- Specific defence mechanisms
Non-specific defence mechanisms
Non-specific defence mechanisms work against all invaders. They do not act against any specific invader. An example of non-specific defence mechanism is the skin. The skin provides protection from microbes and chemical agents by forming a physical barrier. Other examples include:
- Mucus: Mucus secreted by mucus membranes traps microbes and other invaders
- Antimicrobial substances: Antimicrobial substances such as hydrochloric acid present in gastric juices kill ingested microbes
- Inflammatory response
Specific defence mechanisms
Specific defence mechanisms provide protection against specific foreign bodies. When a foreign body invades past the non-specific defence mechanisms the body produces a specific response against it. Such bodies are called antigens. Antigens include:
- Bacteria and other microbes
- Pollen grains
- Cancerous cells
- Transplanted tissues or organs
Once the body is exposed to an antigen, it usually develops immunity against it. Over its lifetime the body develops immunity against millions of antigens.
Sometimes the body produces very powerful responses to antigens that are harmless. This is known as an allergic reaction.
Body movement is essential for the survival of an organism. Movement is necessary for obtaining food, preventing injury and reproduction. Most body movements are under voluntary control. However, reflex actions are involuntary.
The skeleton provides the framework of the body and enables movements. Movements take place at joints between the bones. Skeletal muscles maintain posture and balance.
Survival of the species
Reproduction is the process by which individual organisms i.e. offspring are produced from their parents. Reproduction ensures the survival of the species. It is a fundamental feature of all known life. Every individual organism exists as a result of reproduction. Individuals with better genes are most likely to survive and pass their genes to the next generation. This forms the basis of natural selection (survival of the fittest).
Ova (eggs) are produced in ovaries in females. During the reproductive years, one ovum is produced during the monthly cycle. It travels through the uterine tube towards the uterus. In males, spermatozoa are produced in the testes which are located in the scrotum. The spermatozoa pass through the vas deferens to the urethra. During secual intercourse, the spermatozoa are deposited in the vagina.
The spermatozoa move upwards through the uterus and fertilize the ovum in the uterine tube. The female egg cell fuses with the sperm cell and forms the zygote. The zygote passes into the uterus and embeds itself in the uterine wall. It grows to maturity during gestation which lasts about 40 weeks.
When the egg is not fertilized, it is expelled from the uterus along with the uterine lining. This is known as menstruation. The reproductive cycle in females is associated with hormonal changes controlled by the endocrine system. It generally takes 28 days and takes place continuously between puberty and menopause. It doesn’t occur during pregnancy.
There is no reproductive cycle in males but hormones are involved in production and maturation of spermatozoa.