Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

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 constituents of the adaptive immune system include specific leukocytes called lymphocytes (B cells and T cells), as well as antiantibodies, also called immunoglobulins. For example, vaccines induce adaptive immunity.

      While an exceptionally complex and highly specialized system, the human body’s immune system is not without fault; disorders of the immune system are the reason for allergies, hypersensitivity reactions, autoimmune disorders, tumors, and other devastating diseases.

      Muscular system

      The muscular system is the organ system that allows the body to move, keep balance, control posture, and provide heat to keep the body warm.

      Three distinct types of muscle tissue – skeletal muscle, cardiac muscle, and smooth muscle are common in the human body.

      Each muscle type has a different form, and therefore provides a different, yet essential function(s). Skeletal muscle and cardiac muscle tissue have striations, or a series of linear markings, visible under a microscope. Smooth muscle, as its name suggests, does not. Smooth muscles are employed to automatically regulate the secretion and release of substances, such as acid, enzymes, and bile from the lumen of hollow organs such as the stomach, intestines, and gallbladder.

      Skeletal muscle, also called voluntary muscle, is under conscience control. Muscles are innervated by nerves and in direct communication with the peripheral nervous system to receive electrical impulses from the brain, telling the muscles to contract.

      Skeletal muscles are connected to bones, organized in opposing groups centered on a joint, which allows for a variety of movements by the human body.

      Figure 1-10 Types of muscle tissue.

      Cardiac muscle, like smooth muscle, is not under conscious control and is only found in the heart. Its main function is to contract the chambers of the heart to circulate blood throughout the body. A unique feature of cardiac muscle are intercalated discs, observable under a microscope, that allow for the synchronized contraction of cardiac tissue necessary for normal blood flow to occur.

      Figure 1-11 Skeletal and muscle structure.

      Nervous system

      The nervous system is an organ system which, through a network of highly specialized cells called neurons and unique chemicals called neurotransmitters. Neurotransmitters transmit nerve impulses. Their function is to completely manage and direct the actions of the human body.

      The form and function of the nervous system is divided into two main arrangements:

      •Central nervous system (CNS)

      •Peripheral nervous system (PNS)

      The CNS is formed by the brain and spinal cord which receive and process information from the body. The PNS is formed by the cranial nerves, spinal nerves, and ganglia (groups of cell bodies) that function to transmit information between the body and the CNS.

      Figure 1-12 Neurons.

      The PNS is further divided into the autonomic nervous system (ANS), which regulates and controls the body below the level of consciousness (think “automatic”