Basic Anatomy and Physiology

 

Anatomy and physiology is the study of the body's systems and structures and how they interact. Anatomy focuses on the physical arrangement of parts in the body, while physiology studies the inner functioning of cells, tissues, and organs. This section will review the body's major systems: the musculoskeletal system, the circulatory system, the nervous system, the digestive system, the respiratory system, and the integumentary system.

 

The Musculoskeletal System

The musculoskeletal system is a system of bones and muscles working together. The musculoskeletal system provides structure to the body, allows for movement, and physically protects the body's other systems.

The skull consists of multiple flat bones that interlock and form a protective space for the brain. They also create the structure of the face and mouth with many attachments for the muscles that allow for all head movement. 

The spine is made of multiple interlocking vertebrae with a central channel for the spinal cord and exit points for the nerves. Like the skull, it protects the spinal cord and provides attachment points for both muscles and ribs.

The thoracic cage or "rib cage" provides the rigidity of the chest, which is vital to the expansion and contraction of the lungs, making the rib cage vital to the respiratory system. It also serves to protect the vital organs within the chest. 

The pelvic girdle is one of the most complex anatomical structures in the body. It transfers the upper body's weight from the spine to the legs and has a massive number of attachment points for various large muscle groups of both the trunk and the legs. 

Like the pelvis, the limbs are complex and have many different joints and attachment points to allow for precise and varied movement. 

The muscles consist of bundles of smaller fibers (myofibrils) that are anchored to a bone via a tendon and have one or more nerves from the peripheral nervous system that allows for voluntary and involuntary contraction. All bodily movements stem from the muscles pulling against the bones across the joints. This type of muscle is known as "Skeletal muscle" or "Striated" muscle due to the arrangement of the muscle fibers. There is another type of muscle in the body known as "smooth muscle, " a component of many bodily systems. This form of muscle is loosely arranged and does not have the characteristic striations of the previously mentioned skeletal muscle.

 

The Circulatory System

The role of the circulatory system is to deliver oxygen and glucose to the cells of the body and then remove waste. It is comprised of the heart, blood vessels, and the blood itself. The anatomy and physiology of the circulatory system are extremely complex, but its essential elements can be broken down into a relatively simple framework.

The anatomy of the circulatory system is simple at a superficial level, it consists of a pump, pipes, and the fluid they carry. 

The heart is a four-chamber pump that fills with blood when it relaxes and propels it through the body when it squeezes. The chambers are separated by valves that prevent the backflow of blood. The coronary arteries run across the surface of the heart and provide oxygen to the muscle. Within the heart's muscle is a network of modified heart muscle cells that act almost like neurons, transferring electrical signals through the heart in a precise and structured manner. 

The blood vessels carry blood and regulate its flow to different areas of the body. The vessels are smooth muscle tubes that can expand and contract based on signals from hormones and the nervous system. Vessels are present in varying sizes, with the largest ones being near the heart and the smallest within the body's various tissues. There are different types of vessels; arteries, arterioles, veins, venules, and capillaries all have unique functions, which will be further reviewed in later sections.

The blood is not traditionally considered to have anatomy, but know that it has many parts in the form of different cells, red blood cells, white blood cells, platelets, and a variety of proteins/hormones/chemicals all have different roles.

The Physiology of the circulatory system is complicated by the many types of cells in the heart and blood.

The heart's muscle cells (cardiac myocytes) are unique in that they are electrically connected and do not require a nerve signal to contract. This allows them to beat in a rhythmic manner that allows for the effective pumping of blood. A collection of specially modified myocytes known as the SA node act as the pacemaker for a healthy heart, creating the electrical signals that spread through the myocytes and lead to a heartbeat. Other specialized myocytes act as fast pathways for these electrical signals, ensuring that the spread of electricity through the heart results in a coordinated and effective contraction.

As mentioned above, the blood is a complex mix of cells and other compounds. The most relevant of these are red blood cells; these cells have a protein known as hemoglobin that allows them to carry large amounts of oxygen from the lungs to the tissue in the body. White blood cells combat infection, and platelets help to block off any holes that form in the system.

 

The Nervous System

The nervous system controls the entire body. It has fibers that run across every inch of the body, controlling muscles, organs, and glands; while returning information to the spinal cord and brain to allow them to make decisions. Neurons have several parts, dendrites that receive signals, axons that transmit them, and the cell body, which maintains the nerve cell. 

The anatomy of the nervous system is divided into the central and peripheral systems, with the central nervous system acting as the control system for the body and the peripheral as communication lines that relay information to and from the central system. 

The central nervous system (CNS) is made up of the brain and spinal cord; both of these structures are made up of a large number of neurons and support cells, with both large blood vessels and capillaries supplying the large amount of energy the neurons require. 

The peripheral nervous system (PNS) is extensive and covers all areas of the body. These nerves have a myriad of functions controlling movement in the body, controlling the function of the organs, and returning sensory information from all across the body to the spinal cord and brain. The nerves of the PNS branch off of the spinal cord. The somatic nervous system (SNS), a branch of the PNS, is responsible for controlling voluntary muscle movements.

The autonomic nervous system (ANS) is a division of the peripheral nervous system (PNS) that controls and regulates the involuntary functions of the body. It plays a crucial role in maintaining internal homeostasis by regulating various physiological processes, such as heart rate, blood pressure, digestion, respiratory rate, and body temperature, among others. The ANS is responsible for coordinating responses to changes in the external and internal environments to ensure the body's proper functioning. It operates independently and continuously without conscious effort or control.

The ANS is further divided into two main branches:

1. Sympathetic Nervous System: The sympathetic nervous system is often referred to as the "fight or flight" system because it prepares the body to respond to stressful or threatening situations. When activated, it increases physiological arousal and mobilizes energy reserves to help the body deal with perceived threats.  

2. Parasympathetic Nervous System: The parasympathetic nervous system is often referred to as the "rest and digest" system because it promotes relaxation and conserves energy. It is most active during restful states, such as when we are sleeping or after a meal, and works to restore balance after the sympathetic system's stress response.

 

The physiology of the nervous system surrounds the ability of nerves to transfer signals. They do so via "action potentials," which allow signals to transfer down the axon of the nerve and to receptors at their end.

The action potentials that neurons send are created by the opening and closing of voltage-sensitive ion channels on the neuron's surface. This results in a "wave" of electrical energy, which travels down the neuron and eventually releases neurotransmitters from the end of the neuron.

The variety of receptors present on neurons and muscles allows neurotransmitters to be released due to an action potential to affect other neurons by stimulating other action potentials; or causing the release of calcium which causes muscles to contract. 

 

The Digestive System

The digestive system exists to break down and absorb ingested material, allowing it to be used for energy and create new cells within the body. 

You can divide the anatomy of the digestive system into the hollow and solid organs. The hollow organs convey food matter and process it, while the solid organs act as support systems, ensuring the process of digestion can proceed smoothly. 

The hollow organs are the esophagus, stomach, and intestines: The esophagus is the physical tube that connects the mouth to the stomach. The stomach both physically grinds up food and chemically digests it with acid. The intestines then absorb the nutrients and water from ground up food with help from liver bile and pancreatic enzymes.

The solid organs are the liver and the pancreas: The liver serves the dual purpose of producing bile, which helps with the absorption of fats by the intestines and detoxifies the blood. The pancreas, like the liver, has a dual role. It produces enzymes that break down protein and hormones, balancing blood glucose. 

The physiology of the digestive system is heavily dependent upon the organ in question, and many have multiple roles. The hollow organs tend to be specialized in the mechanical breakdown and absorption of food. In contrast, the solid organs create and secrete substances that assist with the chemical breakdown of food.

The stomach and intestines have a variety of special cells and receptors that work to detect their contents and absorb them. 

The liver cells, known as hepatocytes, produce bile from the body's waste, which helps absorb fat in the intestines. These same hepatocytes are filled with complex enzymes that break down countless toxins the body produces. 

The pancreas has several types of cells. Some secrete enzymes to break down proteins, while others are known as "islets," which secrete the hormones insulin and glucagon, which regulate the balance of glucose within the blood.

 

The Respiratory System

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The respiratory system is a close counterpart to the circulatory system. Its role is to bring oxygen from the air in contact with the blood inside microscopic capillaries. It interacts closely with the cardiovascular and musculoskeletal systems. Some of the largest blood vessels in the body are associated with the lungs, and the chest wall is vital in the inspiration and expiration of air.

The anatomy of the respiratory system is divided into the upper and lower respiratory tract. The division occurs at the level of the larynx. The upper respiratory tract consists of the nasopharynx and oropharynx. In comparison, the lower respiratory tract is made up of the trachea, bronchi, bronchioles, and lungs, with the movement of air through the system provided by the diaphragm.

The upper respiratory tract is responsible for the initial cleaning and warming of air before it is transmitted to the lower airways. The upper respiratory tract also carries food and fluids to the esophagus and is instrumental in producing speech.

The larynx is a cartilage "box" that divides the GI and respiratory systems. It has a physical flap, "the epiglottis," that protects the airway from food and fluids. The rest of the larynx is specialized to allow for speech production; the vocal cords and various cartilages can change shape to allow air to pass over them to create speech.

The lower respiratory tract transfers air through a branching inverted tree made up of the trachea, bronchi, and bronchioles until it reaches the alveoli. These microscopic sacks have thin walls that are covered in thin-walled capillaries. These allow for blood to come in close contact with air. 

The diaphragm is a sheet of muscle at the base of the lungs that pulls air into the airways by creating negative pressure in the chest. Remember that when the diaphragm contracts, air is drawn into the chest, which is known as inspiration.

The physiology of the respiratory system is best divided into the airways and the lungs.

The airways have physiologic mechanisms that protect them from the countless viruses and bacteria in the environment. Countless mucus-secreting cells coat the inner nose/mouth, trachea, and bronchi/bronchioles in a protective layer that inhibits bacterial growth and traps inhaled contaminants. These musocal cells are paired with cilial cells in the lower airway (trachea, bronchi, etc.) They are mobile and work to push mucus and contaminants up and out of the lower airways.

The lung's chief physiologic function is the exchange of gases between the blood and the air. They do so through the incredibly thin walls of the alveoli, which allow diffusion to naturally move gases from areas of high concentration to those of low concentration

 

The Integumentary System

The integumentary system provides the physical barrier between the inner systems of the body and the outside world. It is vital to regulate the body's internal environment, holding in fluids, keeping out bacteria, and providing a regenerating layer that prevents permanent damage to the more fragile cells of the body.

The anatomy of the integumentary system is more complex than it would first appear. It has three main layers, the epidermis, dermis, and subcutaneous layers.

The epidermis is a thick layer of dead cells that acts as a "sacrificial layer" for the body. This layer of cells gradually rubs off and protects the more fragile layers below. The dermis is the living skin layer with cells that continuously multiply and divide; it holds nerves, blood vessels, sweat glands, and oil glands. The subcutaneous layer is one of the main areas of fat storage, also acting as a significant insulating layer for the body. 

The physiology of the integumentary system is based on the continuously dividing stem cells in the dermis that create the thick epidermis. The dermis also contains countless capillaries, nerves, and glands that act to regulate the temperature through the mechanisms of vasoconstriction/vasodilation and diaphoresis (sweating).