glands
Figure 1-15 Female reproductive system.
The exterior genitalia (collectively called the vulva) are the introitus, the labia minora and majora, the clitoris, and Bartholin’s glands.
The major female sex hormones associated with the reproductive system are estrogen and progesterone.
These respective hormones allow for the development of proper secondary sexual characteristics (to be discussed in greater detail later) for the female and male.
Organs and functions of the male and female reproductive systems will be discussed in further detail in Chapter 15: The Reproductive System.
Respiratory system
The respiratory system is an anatomical assembly of structures, commonly separated into an upper and lower system that are responsible for breathing.
The structures included are the nose, pharynx, larynx, trachea, bronchi, lungs, and diaphragm. Breathing, more accurately termed respiration and ventilation, is the introduction and release of gases, oxygen and carbon dioxide, respectively, into and out of the body.
The human body requires oxygen for the survival of all living cells, and the byproduct of each cell’s use of oxygen is carbon dioxide, which must be released as it becomes toxic at certain levels.
Oxygen is exchanged for carbon dioxide in the respiratory system, hence is given the name – “gas exchange”.
Gas exchange is responsible for regulating an appropriate acid-base balance in the body, as part of homeostasis.
Respiration is under the direct control of the autonomic nervous system and is physiologically functional in the medulla oblongata and the pons, parts of the brain stem. Housed in this part of the brain stem is a series of interconnected neurons that make up the body’s respiration regulatory center. Inspiration, also called inhalation, is an active process initiated by the body’s diaphragm.
Figure 1-16 The respiratory system.
The organs and processes involving respiration will be discussed in more detail in Chapter 9: The Respiratory System. For now, a brief description provides a foundation for the basic functions of the breathing process.
When the diaphragm contracts, it forces contents of the abdominal cavity downward, allowing the ribcage to be expanded, which creates room for lung expansion. This generates increased “thoracic pressure” (remember that term for future discussion) and allows air to flow into the lungs from the atmosphere. Oxygen is then passed down through to tiny sacs called alveoli, which allow for the exchange of oxygen with the uptake of carbon dioxide from the surrounding pulmonary capillaries.
Carbon dioxide is then released from the body via exhalation. Exhalation, also called expiration, is generally a passive process, meaning no muscular contraction is needed in order to it to occur. The lungs have natural elasticity, similar to a rubber band. As the lungs are filled, air stretches the lungs until their natural recoil threshold is met, causing the remaining gases to be exhaled until the lungs reach a state of equilibrium with the atmospheric pressure. Disorders of the respiratory system are studied by a branch of medicine called pulmonology.
Skeletal system
The skeletal system is an organ system composed primarily of bones responsible for the structural framework and support of the human body. An infant’s structure is not identical to an adult skeleton.13 At birth, an infant has more bones than an adult but over time, some of these bones mature and fuse together.
An adult human body has 206 bones that make up approximately 50% of the body’s total weight.14
The human skeleton is classified into two units: the axial skeleton and the appendicular skeleton.
The axial skeleton, named so because they refer to the bones in the vertical axis, is composed of 80 bones that include the vertebral (spinal) column, the rib cage, and the skull.15 The appendicular skeleton, named after the term appendage, is made up of the remaining 126 bones, including the bones of the shoulder girdle, the pelvic girdle, and the arms, legs, hands, and feet.16
Figure 1-17 X-rays of skeletal bones and joints.
Along with some auxiliary structures such as ligaments, tendons, joints, and cartilage – the skeletal system functions as a platform permitting movement, protection of organs, providing support and stability, storage of minerals such as calcium and iron, and production of red blood cells or hematopoiesis. These processes will be discussed in much greater detail in Chapter 4: The Skeletal System.
Urinary system
The urinary system functions as a whole to remove excess, unnecessary, and sometimes harmful substances, including toxins, fluids, and metabolic byproducts from the body through the production and excretion of urine.
Figure 1-18 Urinary system components.
The urinary system, also called the excretory system, is the organ system responsible for the formation, storage, and excretion of urine. In the human body, the macroscopic structures include the kidneys, ureters, bladder, and urethra. The kidneys are “bean-shaped” organs that float just beneath the ribcage in the abdominal cavity. They are retro-peritoneal, meaning they are located behind the peritoneum.
Although a relatively small structure, (each kidney is approximately four to six inches long and three inches wide), the kidneys receive anywhere from 20–25% (approximately 1200 ml of the total blood pumped from the heart each and every minute!17
The kidney’s prime function is to filter excess fluid, toxins, and byproducts from the blood, and expel it form the body. Once urine leaves the kidneys, it is carried to the bladder by two hollow tube-like projections, called ureters. From the bladder, urine is expelled from the body via the urethra.
Other functions of the kidneys include salt and water regulation, pH balance, medication clearance, and erythropoietin production. Several other structures play a significant role within the urinary system and will be discussed in greater detail later. Some of these structures include the glomerulus, the renal corpuscle, Bowman’s capsule, the nephron, and the renal tubules.
The urinary system will be discussed in greater detail in Chapter 12: The Urinary System.
■Anatomical Terminology
In order to avoid ambiguity and confusion, standard anatomical terminology is used universally when studying the science of any organism, and with specific regards to its anatomy. Because an organism
