The human body, similar to all higher organisms, is organized into a hierarchy of different levels:
• A cell is the smallest unit of life, that in principal can survive on its own.
• A tissue consists primarily of cells from the same origin, and the extracellular matrix they form. A tissue is an ensemble of cells, organized to do a specific job.
• An organ is a structural unit composed of different tissues. Thus, it combines the functions of the various tissue components.
• An organ system is made up of organs that function together to perform a specific function. For example, the digestive organs make up the digestive system. For the most part, the individual organs are related to each other morphologically.
• An organism is composed of several organ systems.
A Overview of the internal organs of the human body
Anterior view of the human body, displaying the internal organs. For clarity, the nervous system and most of the small intestine and endocrine organs are not shown.
Since, by definition, every structural unit composed of different tissues is referred to as an organ (according to this definition, every muscle is an organ), the term is commonly used for structures in the skull, neck, and body cavities. The organs situated inside the body cavities are referred to as internal organs or viscera. This atlas is a study aid for learning gross anatomy. Thus, the individual organs are discussed with respect to their topography. However, since groups of individual organs form morphological and functional systems, which due to evolutionary processes don’t conform to topographical anatomy, those organ systems along with their embryology will be discussed first. This overview will aid in understanding the location, shape and function of the internal organs in the developing organism.
Note: Peripheral nerves, bone marrow, and blood are usually not referred to as “organs.” For the sake of completeness, they will also be discussed since they are part of whole organ systems.
Oral cavity with teeth and salivary glands, pharynx, esophagus, stomach, small intestine, large intestine, rectum, pancreas, liver, and gallbladder
Nasal cavity and paranasal sinuses, larynx, trachea, lungs
Kidneys, ureters, bladder, urethra
♀ Uterus, uterine tubes, ovary, vagina, Bartholin’s glands
♂ Testicles, epididymis, ductus deferens, seminal vesicles, prostate, Cowper’s gland
Heart, vessels, blood, and bone marrow
Bone marrow, tonsils, thymus, spleen, lymph nodes, thoracic duct
Thyroid, parathyroid glands, suprarenal (adrenal) glands, paraganglia, pancreas (islet cells), ovaries, testicles, pituitary gland, hypothalamus
Brain, spinal cord, peripheral nervous system (with somatic and autonomic components)
C Evolution of body cavities
While in fish (a) all internal organs are situated in a single common body cavity, in mammals (b), the diaphragm separates the thoracic cavity from the abdominal cavity. Due to shared evolutionary history, the structures of these two body cavities are basically identical. The different anatomical terms used for similar structures (e.g., pleura – peritoneum) are functionally meaningless. In mammals, there is no physical structure that separates the abdominal cavity from the pelvic cavity. They form a continuous space that in terms of its topographical anatomy is divided only by the superior border of the bony pelvis. The anatomical unit of the abdominal and pelvic cavities is of clinical significance as there are no anatomical barriers to restrict the spread of inflammation or tumors between these two compartments. The diaphragm acts as a barrier to stop tumors or inflammation from spreading from the abdominal to the thoracic cavity and vice versa.
A Differentiation of the germ layers (after Christ and Wachtler)
After the formation of the trilaminar embryonic disc at the end of the third week (see B) the primordia (precursor cells destined to become a specific tissue or organ) of the different tissues and organs are arranged according to the body plan. In the subsequent embryonic period (weeks 4 to 8), the three germ layers (ectoderm, mesoderm, and endoderm) give rise to all major external and internal organs (organogenesis). At the same time, the trilaminar embryonic disc begins to fold, resulting in major changes in body form and internal structure. By the end of the embryonic stage, the major features of the body are recognizable and the organs have moved into their eventual position within and outside of the body cavities.
B Neurulation and Somite Formation (after Sadler)
a, c, and e Dorsal views of the embryonic disc after removal of the amnion;
b, d, and f Schematic cross-sections of the corresponding stages at the planes of section as marked in a, c, and e; Age is in postovulatory days.
During neurulation (formation of the neural tube from the neural plate), the neuroectoderm differentiates from the surface ectoderm, due to inductive influences from the notochord, and the neural tube and neural crest cells move inside the embryo.
a and b Embryonic disc at 19 days: The neural groove is developing in the area of the neural plate.
c and d Embryonic disc at 20 days: In the paraxial mesoderm, flanking both sides of the neural groove and notochord, the first somites have formed (they contain cellular material assigned to form the spinal column, muscles, and subcutaneous tissue). Immediately lateral to the paraxial mesoderm is the intermediate mesoderm, and lateral to that is the lateral plate mesoderm. The neural groove is beginning to close to form the neural tube and the embryo begins to fold.
e and f Embryonic disc at 22 days: Eight pairs of somites are seen flanking the partially closed neural tube which is sinking below the ectoderm. In the lateral plate mesoderm, the intraembryonic coelom, or future body cavity, arises. It will later develop both a parietal and a visceral layer (somatopleure and splanchnopleure). On the side facing the coelom, a mesothelial lining develops from the somato- and splanchnopleure. It later forms the serous membranes lining the pericardial, pleural, and peritoneal cavities. The neural tube migrates deeper into the mesoderm, and the somites differentiate into sclerotome, myotome, and dermatome.