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” nervous system) and the somatic nervous system (SNS) which is under conscious or voluntary control. The ANS is also further subdivided into the sympathetic nervous system and the parasympathetic nervous system. A simpler way of appreciating this is through the visual aid of a flow chart:
Figure 1-13 Structures of the nervous system.
The CNS and PNS both function to transmit and receive electronic impulses between the central and peripheral divisions of the body. This is accomplished through the use of cells and nerves. Neurons and glial cells comprise the two main types of cells in the nervous system. Neurons, or nerve cells, are responsible for “communicating” with each other through connections known as synapses, or cell-to-cell junctions that rapidly transmit and receive chemical or electrical signals. Types of neurons and respective structures will be discussed in detail in Chapter 14: The Nervous System.
Glial cells (named from their Greek origin meaning “glue”) are supporting cells in the nervous system. Glial cells maintain nerve cells by gluing them in place, supplying them with nutrients, and removing pathogens and damaged or dead neurons.
One very important function of a particular glial cell, called an oligodendrocyte in the CNS and a Schwann cell in the PNS, is the production of myelin – a fatty sheath wrapped around the axons of neurons, electrically insulating them and allowing for more rapid and efficient transmission of impulses. The axons, long and slender projections of a neuron, are arranged and travel in bundles, making up the bulk of a nerve. Nerves as well as the individual parts of a nerve cell (the axon, cell body, and dendrites) will also be discussed in greater detail later.
Figure 1-14 Male reproductive organs.
Reproductive system
The reproductive system or genital system is a coordination of structures, glands, and hormones that function to together for the sole purpose of reproduction.
Human reproduction is a form of sexual reproduction conventionally by way of sexual intercourse, also called copulation or coitus, between a male and female.3
During sexual intercourse, a man’s erect penis is inserted into a female’s vagina until ejaculation, or the release or semen, takes place. Sperm from the semen travels up the vagina where it reaches the cervix and enters the uterus and eventually the fallopian tubes, where internal fertilization of the female egg cell, or ovum, occurs.
The process of internal fertilization and embryogenesis will be discussed in greater detail later. Nevertheless, unique from other organ systems, the reproductive systems allow for significant disparity between the two sexes.
All male sex organs are considered to be external genitalia and include the:
•testes
•vas deferens
•epididymis
•seminal vesicles
•prostate
•penis
The main male sex hormone associated is testosterone.
The female sex organs can be classified as either interior or exterior. The interior organs are the:
•ovaries
•fallopian tubes
•uterus
•cervix
•vagina
