Anatomy and Physiology Study Guide for the HESI Exam

Integumentary System

The integumentary system is composed of the skin, hair, nails, and glands. It serves as the body’s first line of defense against environmental hazards such as pathogens, harmful chemicals, and dehydration. Additionally, it plays a role in temperature regulation, sensory reception, and vitamin D synthesis.

The Skin

The skin is the largest organ of the body and has multiple layers that work together to provide protection and support:

• epidermis—This is the outermost layer, composed of stratified squamous epithelial tissue. It is avascular (lacking blood vessels) and primarily consists of keratinized cells, which provide waterproofing and protection. The epidermis is the body’s earliest defense against invading bacteria, as it forms a protective barrier that prevents pathogens from entering.

• dermis—The dermis is the thick middle layer made of connective tissue, containing blood vessels, nerve endings, hair follicles, and glands. This layer supports the epidermis and provides elasticity and strength.

• hypodermis (subcutaneous layer)—This is the deepest layer, composed mostly of adipose tissue (fat). It provides insulation and cushioning for the body.

Beyond serving as a protective barrier, the skin also supports accessory structures that contribute to its functions. Hair and nails, both made from the fibrous protein keratin, play an essential role in protection. Hair helps regulate body temperature and provides sensory input, while nails reinforce and protect the tips of fingers and toes.

Another important function of the skin is vitamin D production. When exposed to sunlight, the skin synthesizes vitamin D, which is essential for calcium absorption and bone health.

Glands in This System

The integumentary system contains several types of glands, each serving a distinct function:

• sebaceous (oil) glands—These secrete sebum, an oily substance that moisturizes the skin and hair, preventing dryness and cracking. Sebum also has antimicrobial properties that help protect the skin from infections.

• sudoriferous (sweat) glands—These glands regulate body temperature through sweat production:

  • eccrine glands—Found all over the body, these glands primarily function in thermoregulation by releasing sweat that evaporates and cools the body.

  • apocrine glands—Located in areas like the armpits and groin, these glands become active during puberty and secrete a thicker sweat that, when broken down by bacteria, produces body odor.

Endocrine System

The endocrine system is made up of glands that secrete hormones directly into the bloodstream. These hormones travel through the circulatory system to regulate growth and development, metabolism, reproduction, and stress response. Unlike the nervous system, which sends rapid electrical signals, the endocrine system works more slowly, often taking hours or even days to produce an effect.

Endocrine vs. Exocrine Glands

The endocrine system should not be confused with the exocrine system, which consists of glands that release substances through ducts instead of directly into the bloodstream.

Endocrine glands secrete hormones that regulate body functions. Examples include the pituitary gland, thyroid gland, and adrenal glands.

Exocrine glands secrete substances such as digestive enzymes, saliva, and milk through ducts. Examples include the salivary glands, sweat glands, and mammary glands.

Since adrenaline (epinephrine) is secreted directly into the bloodstream, it is considered an endocrine hormone, whereas milk, digestive enzymes, and saliva are exocrine products because they are released through ducts.

Primary Endocrine Glands and Hormones

The major endocrine glands produce hormones that regulate a variety of body functions. Below is a summary of the key glands, the hormones they produce, and their primary functions:

Gland	Hormones Produced	Function
Hypothalamus	dopamine, thyrotropin-releasing hormone (TRH)	This gland regulates pituitary gland activity and controls hormone release.
Pituitary Gland (Master Gland)	Anterior lobe: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, follicle-stimulating hormone (FSH), luteinizing hormone (LH)	These control growth, metabolism, stress response, and reproductive processes.
	Posterior lobe: oxytocin, vasopressin (ADH)	Oxytocin stimulates uterine contractions during labor and plays a role in milk ejection, while ADH helps regulate water balance by controlling kidney function.
Pineal Gland	melatonin	This regulates sleep-wake cycles.
Thyroid Gland	triiodothyronine (T3), thyroxine (T4), calcitonin	These control metabolism, energy levels, and calcium balance.
Parathyroid Glands	parathyroid hormone (PTH)	PTH regulates blood calcium levels, ensuring proper nerve and muscle function.
Adrenal Glands	Medulla: adrenaline (epinephrine), noradrenaline (norepinephrine), dopamine	These hormones prepare the body for “fight-or-flight” by increasing heart rate, blood pressure, and energy.
	Cortex: androgens, aldosterone, cortisol	Androgens (such as DHEA) contribute to the development of secondary sex characteristics and serve as precursors to sex hormones like testosterone and estrogen. Aldosterone regulates sodium and water balance, while cortisol helps the body manage stress and inflammation.
Pancreas	insulin, glucagon	Insulin lowers blood sugar by allowing cells to absorb glucose, while glucagon raises blood sugar by releasing stored glucose from the liver.
Testes (Males)	testosterone	Testosterone develops male reproductive tissues and secondary sex characteristics.
Ovaries (Females)	estrogen, progesterone	These regulate the menstrual cycle, support pregnancy, and maintain secondary sex characteristics in females.

The pituitary gland, often referred to as the “master gland,” is crucial in controlling other endocrine glands. It secretes hormones that influence growth, metabolism, and reproductive processes. The posterior lobe of the pituitary gland produces oxytocin, which stimulates uterine contractions during labor and helps with milk ejection in breastfeeding mothers.

Located in the abdomen, the pancreas plays a key role in regulating blood sugar. It produces insulin, which allows cells to absorb glucose and lower blood sugar levels, and glucagon, which has the opposite effect, raising blood sugar when levels are too low. This balance is essential for maintaining energy and metabolic stability.

Other Organs with Endocrine Functions

Although some organs are not classified as primary endocrine glands, they also produce hormones that regulate bodily functions:

• kidneys—secrete erythropoietin (EPO), which stimulates red blood cell production

• liver—produces insulin-like growth factors that support cell growth and metabolism

• heart—releases atrial natriuretic peptide (ANP), which lowers blood pressure by reducing sodium levels in the blood

Common Endocrine System Disorders

The endocrine system functions through a gland-circulatory system-target tissue loop. If any part of this system is disrupted, hormonal imbalances can occur:

• The gland may overproduce or underproduce hormones, causing excessive or diminished responses.

• Circulatory problems may prevent hormones from reaching target organs effectively.

• If the receptors on target organs are not functional, hormones may not be recognized or properly utilized.

These imbalances can lead to various endocrine disorders, including:

• diabetes mellitus—a condition where insulin production is insufficient or ineffective, leading to high blood sugar

• Graves’ disease—a form of hyperthyroidism where the thyroid overproduces hormones, resulting in weight loss, rapid heartbeat, and nervousness

• Addison’s disease—deficiency of cortisol and aldosterone, causing fatigue, low blood pressure, and electrolyte imbalances

• osteoporosis—weakening of bones, often due to low estrogen levels affecting calcium absorption

• obesity—can be influenced by hormonal imbalances, particularly involving insulin and thyroid function

Skeletal System

The skeletal system consists of the bones of the body and the supportive tissues that connect them. Bones are living organs composed of minerals, cells, and protein fibers, working together to support movement, protection, blood cell production, and mineral storage.

Bone Functions

At birth, humans have approximately 300 bones, but some fuse during growth, leaving adults with 206 bones. The skeletal system accounts for about 30 to 40% of total body mass and serves several essential functions:

• structural support—provides a framework for the body

• protection—shields vital organs (e.g., the skull protects the brain, ribs protect the heart and lungs)

• movement—serves as attachment points for muscles

• blood cell production—occurs in bone marrow, where red blood cells, white blood cells, and platelets are made

• storage—stores minerals like calcium and iron, as well as fat in yellow marrow

Because they contain multiple tissue types, including bone tissue, blood vessels, nerves, and marrow, bones are considered organs rather than just structural components.

Bone Structure

Bones have several layers, each with a specific function:

• compact bone—the hard, dense outer layer that provides strength

• spongy bone—a porous, flexible inner layer that helps absorb impact

• bone marrow—found in the deepest layer of bones, where blood cells are produced in adults

Yellow bone marrow stores fat, while red bone marrow is responsible for producing blood cells. In adults, all blood cells are produced in red bone marrow, primarily found in flat bones like the pelvis, sternum, and vertebrae.

Divisions of the Skeleton

The human skeleton is divided into two main parts, each having different purposes.

Axial Skeleton (80 Bones)

The axial skeleton provides structural support and protects the brain, spinal cord, and thoracic organs. It consists of the following parts:

• skull—composed of 22 bones that protect the brain and form the structure of the face

• vertebral column—made up of 33 vertebrae, divided into five sections:

  • cervical vertebrae (7)—located in the neck
  • thoracic vertebrae (12)—located in the upper and mid-back
  • lumbar vertebrae (5)—located in the lower back
  • sacral vertebrae (5, fused)—form the back of the pelvis
  • coccygeal vertebrae (4, fused)—commonly known as the tailbone

• rib cage—composed of 12 pairs of ribs and the sternum, providing protection for the heart and lungs

Appendicular Skeleton (126 Bones)

The appendicular skeleton enables movement and interaction with the environment. It includes:

• upper limbs—consist of the humerus, radius, ulna, carpals, metacarpals, and phalanges (the radius and ulna are the two bones in the lower arm)

• lower limbs—consist of the femur, patella (kneecap), tibia, fibula, tarsals, metatarsals, and phalanges (the patella is located between the femur and the tibia and fibula)

• shoulder girdle—includes the clavicle and scapula, allowing arm movement

• pelvic girdle—includes the hip bones, which provide support for the lower limbs and protect pelvic organs

Types of Bones

There are five types of bones found in the body, and they are classified by shape:

• long―Long bones make up most of the limbs. As their name suggests, they are longer than they are wide. Examples of long bones include the tibia and femur of the legs and radius and ulna of the arms.

• flat―Flat bones are thin bones used by the body for protection or muscular attachment. Examples include the bones of the cranium and pelvis, as well as the sternum and ribs.

• short―Short bones are wider than they are long. They provide stability but little movement. Examples include the carpal bones in the wrist and the tarsals found in the foot.

• sesamoid―Sesamoid bones are found inside tendon or muscle. An example is the patella of the knee.

• irregular―Irregular bones have a unique shape that doesn’t allow them to be classified in any of the other four bone types. Examples include the hyoid bone of the neck and the vertebrae of the spine.

Supportive Tissues of the Skeletal System

In addition to bones, the skeletal system relies on connective tissues that support and stabilize joints:

• tendons—Tendons connect muscle to bone and aid in movement.

• ligaments—Ligaments connect bone to bone, providing joint stability.

• cartilage—Cartilage is a firm but flexible connective tissue found between bones. It cushions joints and reduces friction. Cartilage is composed of collagen and elastin fibers and is found in areas such as the nose, ears, rib cage, and bronchial tubes.

Joints and Movement

A joint is where two or more bones meet. Joints are classified by movement type:

• synovial joints—The most common and moveable joints, these contain a fluid-filled capsule for smooth movement (e.g., knee, elbow, shoulder).

• cartilaginous joints—Connected by cartilage, these joints allow limited movement (e.g., intervertebral discs).

• fibrous joints—Connected by dense connective tissue, these allow little to no movement (e.g., skull sutures).