As food is passed into the esophagus and into the stomach, hydrochloric acid and enzymes continue the chemical digestion process of food breaking it down into a thick liquid known as chyme. Chyme will eventually make its way to the small intestine, where a majority of the nutrients are absorbed, then pass through the large intestine, and finally be excreted as waste material from the rectum and anus by defecation.
Endocrine system
The endocrine system is an organization of glands which regulate various functions in the human body through the use of chemical compounds.9 These chemical compounds or messengers are called hormones. They act as an information signaling system similar to the body’s nervous system.
In contrast to the nervous system, the endocrine system’s effects are initiated in a much slower and gradual manner though their effects are longer lasting, sometimes prolonged over a period of weeks.
The endocrine system, which is ductless, must be distinguished from the exocrine system, which secretes its chemicals through ducts. Exocrine glands secrete onto the skin or into a body cavity, such as sweat.
Hormones are released from endocrine glands directly into the bloodstream and travel to target tissues to generate a specific response. They regulate metabolism, growth and development, tissue function, sexual development, and mood.
Figure 1-7 Hormone (endocrine) system.
The physiological levels of hormones and the functions they perform are age-dependent and change over the course of an individual’s lifespan.
Figure 1-8 Skin anatomy.
Specific glands and secretions will be further detailed in Chapter 13: The Endocrine System. Some of the glands involved in the endocrine system include the:
•Pituitary gland
•Pineal gland
•Thyroid gland
•Parathyroid glands
•Adrenal glands
•Hypothalamus
In addition to these glands, the body contains many other organs that have secondary endocrine functions, such as the heart, kidneys, reproductive organs, stomach, pancreas, liver, and intestines.
Integumentary system
The integumentary system is composed of the skin, hair, and nails. The name is derived from its Latin origin integumentum, which meant “to cover”. The skin, hair, and nails do in fact cover the body, as one of its major functions is to serve as a protective barrier against germs, heat or cold, and help cushion internal organs against injury. The integumentary system also functions to:
•regulate temperature
•receive external stimuli such as pressure, pain, and vibration
•aid in the synthesis of vitamin D from sunlight exposure
The skin is known to be the largest organ of the human body, as it comprises approximately fifteen percent of total body weight.10 The skin is organized into two layers, called the epidermis and the dermis.
The outer layer, the epidermis, is a major abneural and avascular layer of stratified squamous epithelial cells, which are further organized into five minor layers, or strata11:
•Stratum corneum
•Stratum lucidum
•Stratum granulosum
•Stratum spinosum
•Stratum basale
It receives nourishment from the lower major layer, the dermis.
The dermis is composed of what anatomists refer to dense irregular connective tissue, called collagen and elastin.
These tissues allow for both the integrity and the flexibility of the skin. The dermis also has a nerve and vascular supply, and is the base for the other structures in the integumentary system, such as the hair and nails.
It should be noted here that some texts refer to the skin as having “three” layers. These texts are referring to a layer deep to the dermis, called the hypodermis. Technically speaking, however, the hypodermis should not be included as a segment or layer of the skin, nor part of the integumentary system.
The hypodermis, also called the subcutaneous layer, is primarily composed of adipose tissue or body fat. Its primary function is simply insulation and storage of energy.
Immune system
The immune system is a multifaceted biological system of structures and processes that serve to protect the human body from disease.12 The primary structures that form the immune system are the bone marrow and the thymus gland. Organs and functions of the immune system will be discussed in further detail in Chapter 8: The Lymphatic and Immune Systems. The secondary structures are:
•Lymph nodes
•Tonsils
•Adenoids
•Spleen
•Leukocytes (white blood cells)
Figure 1-9 Immune system cells.
These structures function together to produce a wide array of defensive mechanisms that serve to protect the human body from a diverse population of pathogens, or infectious agents. The human body’s immune system can be classified into two components: natural immunity and acquired immunity.
Natural immunity, also termed the innate immune system is the dominant defense component of the human body’s immune system.
It is a general, non-specific defense system that does not target specific pathogens, but rather responds to invading pathogens in an immediate and universal “attack-all” approach.
Individual elements of the innate immune system include the body’s inflammatory response, the “complement system”, and leukocytes, or white blood cells. Acquired immunity, also called the adaptive or specific immune system, is activated by the innate immune system to mount a highly specialized immune response, capable of recognizing, targeting, and remembering specific pathogens. This system is dynamic and adaptable, and has the ability to mount a stronger, more specific immune response due to its immunological memory.
Cells of the adaptive immune system “remember” specific pathogens. If such pathogens enter the body more than once, these “memory cells” quickly recognize and target the respective pathogens and eliminate it. Individual
