The major salivary glands associated with the oral cavity, pancreas, and liver are considered to be the extramural glands of the digestive system. Each of these glands has numerous functions aiding the digestive process; their secretory products are delivered to the lumen of the alimentary tract by a system of ducts. Major Salivary Glands There are three pairs of major salivary glands: parotid, sublingual, and submandibular.…

The alimentary canal , the tubular portion of the digestive tract, extends from the oral cavity to the anus. Aliquots of food that was swallowed at the level of the oral cavity enters the alimentary canal to be churned, liquefied, and digested so that its nutritional elements and water can be absorbed and its indigestible components eliminated. The approximately 9-m-long alimentary canal is subdivided into several…

The digestive system—composed of the oral cavity, alimentary tract, and associated glands—functions in the ingestion, mastication, deglutition (swallowing), digestion, and absorption of food, as well as in the elimination of its indigestible remnants. Regions of the digestive system are modified and have specialized structures in order to be able to perform these varied tasks. This and the following two chapters detail the histology and functions of…

The lungs and the airways leading to the external environment constitute the respiratory system , whose role is to provide oxygen (O 2 ) to and eliminate carbon dioxide (CO 2 ) from the cells of the body. The following four events, collectively known as respiration , have to occur in order to accomplish this task: Air has to move in and out of the lungs…

The integument is composed of skin and its appendages: sweat glands , sebaceous glands , hair , and nails . It is the largest organ, constituting 16% of the body weight. The skin invests the entire body, becoming continuous with the mucous membranes of the digestive system at the lips and the anus, the respiratory system in the nose, and the urogenital systems where they surface.…

The endocrine system consists of ductless glands , distinct clusters of cells within certain organs of the body, and individual endocrine cells (known as the diffuse neuroendocrine system [DNES] ), situated in the epithelial lining of the respiratory system and in the digestive tract. (The latter are discussed in Chapter 15, Chapter 17 , respectively.) The endocrine glands , the subject of this chapter, are abundantly…

The lymphoid system is responsible for the immunological defense of the body. Some of its component organs— bone marrow , lymph nodes , thymus , and spleen —are surrounded by connective tissue capsules, whereas its other components, members of the diffuse lymphoid system , are not encapsulated. The cells of the lymphoid system protect the body against foreign macromolecules, viruses, bacteria, and other invasive microorganisms, and…

The circulatory system is composed of two separate but related components: the cardiovascular system and the lymphatic vascular system. The function of the cardiovascular system is to carry blood in both directions between the heart and the tissues. The function of the lymphatic vascular system is to collect lymph , the excess extracellular fluid (tissue fluid), and to deliver it back to the cardiovascular system. Thus,…

Blood, a specialized connective tissue, is a bright- to dark-red, viscous, slightly alkaline fluid (pH 7.4) that accounts for approximately 7% of the total body weight with a total volume of approximately 5 L in an average adult. It is composed of formed elements— red blood cells (RBCs; erythrocytes) , white blood cells (WBCs; leukocytes) , and platelets —suspended in a fluid component (the extracellular matrix),…

The human nervous system is composed of perhaps a trillion neurons, each with a large number of interconnections. Some of these neurons possess receptors that are specialized for receiving different types of stimuli (e.g., mechanical, chemical, thermal), which are transduced into nerve impulses that may eventually arrive at specified nerve centers. These impulses are then transmitted to other neurons for processing and are conveyed to higher…

Muscle cells are specialized for contraction that permits movement. Organisms harness the contraction of muscle cells and the arrangement of the extracellular components of muscle to permit locomotion, constriction, pumping, and other propulsive movements. Muscle cells are called striated or smooth muscle depending on the respective presence or absence of a regularly repeated arrangement of myofibrillar contractile proteins, the myofilaments. Striated muscle cells display characteristic alternations…

Cartilage and bone are both specialized connective tissues that function in supporting parts of the body. The former has a firm pliable matrix that resists mechanical stresses; the latter is one of the hardest tissues of the body, and protects vital organs such as the brain, spinal cord, bone marrow, and heart. Both cartilage and bone have cells that are specialized to secrete the matrix in…

Connective tissue is derived from mesoderm , the middle germ layer of the embryonic tissue, except in certain areas of the head and neck, where mesenchyme develops from neural crest cells of the developing embryo and is known as ectomesenchyme . Mesenchyme and ectomesenchyme give rise to multipotential cells of the embryo, known as mesenchymal cells , which migrate throughout the body, giving rise to the…

Epithelium Epithelial tissue forms two distinct structural and functional forms: sheets of contiguous cells ( epithelia ), which cover the external and internal surfaces of the body, and clusters of cells ( glands ), which originate from invaginated epithelial cells. All three germ layers give rise to epithelia. The oral and nasal mucosae, cornea, epidermis of skin, and glands of the skin and mammary glands are…

Cells of similar structure and function assemble to form structural and functional associations, known as tissues , in all multicellular organisms. Groups of these tissues are assembled in various organizational and functional arrangements into organs , which carry out functions of the body. The four basic tissue types are epithelium , connective tissue , muscle , and nervous tissue . Each of these tissues and their…

Summary The nucleus , the largest organelle of the cell, houses the nucleoplasm , nucleolus, and the chromatin . The nucleus is separated from the cytoplasm by a double membrane, known as the nuclear envelope , which is perforated by numerous openings, known as nuclear pores and their associated structures that, together with the pore, form the nuclear pore complex that functions in a bidirectional nucleocytoplasmic…

The basic functional unit of complex organisms is the cell. Cells that have or serve a common purpose congregate to form tissues, which, in animals — specifically, mammals — are placed in four categories (epithelium, connective tissue, muscle, and nervous tissue). These tissues assemble to form organs, which, in turn, are collected into the various organ systems of the body. Each organ system performs a collection…

Although histology is the microscopic study of tissues of living organisms, be they animals or plants, this textbook discusses only mammalian—more specifically, human—tissues. However, the term has evolved to have a broader concept, named microscopic anatomy , because its subject matter encompasses not only the microscopic structure of tissues but also those of the cell, organs, and organ systems. The body is composed of cells, extracellular…