Human Body Systems: Anatomy, Physiology, and Health

Posted on Jul 31, 2025 in Biology

Sense Organs: Detection & Perception

Stimuli and Sensory Receptors

Stimuli: Changes that occur in our internal and external environment.

Sensory Receptors

Specialized structures that detect stimuli and generate nerve impulses.

Depending on their location (internal or external) in the body, they can be:

Interoceptors

Distributed throughout the inside of the body, detecting changes such as variations in blood oxygen levels or blood pressure.

Exteroceptors

Near the surface of the body, detecting changes in the external environment, e.g., temperature or light.

The Ear: Structure and Function

Outer Ear

Auricle (Pinna)

Cartilaginous structure covered in skin, directs sound to the ear canal.

Ear Canal

Contains small hairs and glands that secrete earwax (cerumen).

Middle Ear

Chain of Ossicles

Three tiny bones: malleus, incus, and stapes.

Eardrum (Tympanic Membrane)

Membrane that vibrates when sound waves reach it.

Eustachian Tube

Keeps equal pressure on both sides of the eardrum.

Inner Ear

Vestibular System

With the utricle and saccule, responsible for balance.

Cochlea

Spiral tube that contains sensory cells that detect sound and transform it into nerve impulses.

The Process of Hearing

• Sound waves reach the auricle and are transmitted through the ear canal to the eardrum, which vibrates.
• The eardrum transmits the sound waves to the chain of ossicles.
• The stapes sends the sound waves to the inner ear, causing the movement of the fluid (endolymph) in the cochlea. This stimulates the cells sensitive to vibrations, which generate nerve impulses that are sent to the brain via the auditory nerve.

Balance: Dynamic and Static

Dynamic Balance

Walking, running, cycling, etc.

• Controlled by the semicircular canals, which are full of endolymph. When we move, the endolymph moves. The sensory cells detect this movement and generate nerve impulses that are sent to the brain.

Static Balance

Keeping our body in the same position when we are not moving.

• The utricle and saccule detect if we are sitting down, standing up, lying face up (supine), etc.

The Eye: Structure and Vision

The Eyeball: Structure and Layers

Three Layers of the Eyeball

• Retina

  Internal layer, contains photoreceptor cells:
  • Cones: Detect colors.
  • Rods: Detect black and white, essential for night vision.
  The fovea is an area with high density of cones, responsible for sharp vision.

• Choroid

  Middle layer, contains many blood vessels that provide the eye cells with nutrients.

• Sclera

  Opaque, white outer layer; the front part (cornea) is transparent.

Internal Structures of the Eye

• Aqueous Humor

  Transparent fluid between the cornea and crystalline lens.

• Crystalline Lens

  Elastic, transparent, biconvex structure behind the iris.

• Anatomical Blind Spot

  Area where the optic nerve is connected; no cones or rods are present here.

• Vitreous Humor

  Transparent, gelatinous fluid that fills the chamber between the crystalline lens and retina.

• Iris and Pupil

  Circular structure. Color varies from person to person. The pupil is the hole in its center. The diameter of the pupil changes, regulating the amount of light that enters.

The Process of Sight

Depending on where the eye focuses, the pupil and crystalline lens adapt.

• The pupil dilates or contracts.
• The crystalline lens undergoes accommodation, changing its thickness.

The Skin: Layers and Touch Receptors

Largest organ in our body, responsible for most of our sense of touch.

Layers of the Skin

• Epidermis

  Outer layer, epithelial tissue. Outermost cells are dead.

• Dermis

  Inner layer, connective tissue. Contains:
  • Capillaries
  • Hair follicles
  • Sweat glands
  • Sensory receptors

• Hypodermis

  Deepest layer, mainly adipose cells, with protective and thermoregulatory functions.

Sensory Receptors in the Dermis

• Free Nerve Endings (Nociceptors): Detect pain.
• Groups that form Corpuscles:
  • Krause’s Corpuscle: Detects cold.
  • Pacinian Corpuscle: Detects pressure and vibrations.
  • Meissner’s Corpuscle: Detects textures.
  • Merkel’s Discs: Detect pressure.
  • Ruffini Corpuscle: Detects heat.

The Nose: Olfaction and Smell Perception

The olfactory cells, which are located in the upper lining of the nasal cavity, form the olfactory epithelium.

It contains glands that secrete mucus. We can only smell substances if they are dissolved in mucus and present in sufficient concentration.

When olfactory cells are stimulated, they send nerve impulses from the olfactory bulb, through the olfactory nerve, to the brain, which interprets the information and identifies the smell.

Classes of Smells

• Woody/Resinous
• Fragrant
• Fruity (non-citrus)
• Chemical
• Minty/Peppermint
• Sweet
• Lemon
• Popcorn
• Pungent
• Decayed

The Tongue: Taste and Gustation

Taste Perception

Responsible for our sense of taste. Its surface is covered in taste buds, which form papillae.

The taste buds contain chemoreceptors that detect substances in foods that are dissolved in saliva. They send nerve impulses through the gustatory nerve to the brain, which interprets and identifies tastes.

Basic Tastes

• Sweet
• Salty
• Sour
• Bitter

The Skeletal System: Bones and Joints

Made up of bones, to which muscles are attached, as well as cartilage.

Bones: Composition and Structure

Bone Composition

Hard structures formed of collagen, a protein, as well as phosphorus and calcium salts.

An adult human has 206 bones.

Composed mainly of:

• Compact bone tissue on the outside
• Spongy bone tissue at the core

Skeletal Divisions

Together, the bones form the skeleton, divided into two sections:

Axial Skeleton

Made up of the skull, thorax, and spinal column.

Appendicular Skeleton

Made up of the upper and lower limbs.

Types of Bones by Shape

Depending on their shape, there are three types of bones:

• Long Bones: E.g., femur.
• Short Bones: E.g., vertebrae, wrist, hand bones.
• Flat Bones: E.g., skull, scapula.

Structure of Long Bones

The long bones are made up of:

• Cartilage: A cartilaginous tissue that surrounds the epiphysis at the joint.
• Epiphysis: The thick end of a long bone, containing spongy bone tissue filled with red bone marrow (where blood cells are produced).
• Endosteum: A layer of connective tissue lining the internal cavity of the diaphysis.
• Diaphysis: The long middle section, composed of compact bone tissue filled with yellow bone marrow (made up of adipose tissue).
• Periosteum: A layer of connective tissue that surrounds the bone.

Functions of Bones

• Provide a framework for the body.
• Provide structures to which muscles are attached.
• Protect organs (such as the brain and spinal cord).
• Contain red bone marrow, which produces red blood cells.
• Store calcium, which is released into the blood if required.

Cartilage: Structure and Location

Cartilage is softer than bone and formed of cartilaginous tissue, found in:

• Nose
• Ears
• Intervertebral discs (between vertebrae)

Cartilage also forms the embryonic skeleton. Because cartilage is softer than bone, it is more flexible.

Joints: Connections Between Bones

Structures that connect two bones.

Types of Joints

Three types:

• Immovable Joints: E.g., in the skull.
• Slightly Movable Joints: E.g., in the spinal column.
• Movable Joints: E.g., knee, elbow, shoulder. These consist of:
  • Ligaments: Groups of fibers that connect bones.
  • Cartilage: Covers the surface to prevent friction.
  • Synovial Cavity: Space between bones, filled with viscous fluid (lubricant).

The Muscular System: Movement and Structure

Skeletal Muscles: Composition and Function

Made up of skeletal muscles. These muscles are formed of striated muscle tissue, which contains long cells with several nuclei, known as muscle fibers.

The human body contains approximately 650 skeletal muscles. They maintain posture and generate movement.

Muscle Structure Hierarchy

• Muscle (5): Surrounded by Epimysium (4). Formed of:
  • Muscle Fascicle (1): Surrounded by Perimysium (2). Formed of:
    • Muscle Fibers: Surrounded by Endomysium (3).
• Tendon (6): A strong structure of fibrous connective tissue that connects muscle to bone and transmits muscle contraction.

Types of Muscles by Shape

Fusiform Muscles

They are long and spindle-shaped. They are involved in movement.

Flat Muscles

They are square or fan-shaped. They cover organs.

Circular Muscles

They are ring-shaped. They open or close ducts or cavities.

Muscle Contraction and Relaxation

Striated muscle tissue, which forms muscles, contracts quickly and voluntarily. This process requires calcium, energy, and oxygen.

• Certain stimuli cause muscle fibers to shorten (contract).
• Muscles lengthen (relax) when the stimuli stops.

This generates movement. For example, to extend the forearm, the triceps contracts and the biceps relaxes. To flex the forearm, the biceps contracts and the triceps relaxes.

Diseases Affecting the Ears

Otitis (Ear Inflammation)

• Otitis Externa: Ear pain when touched or pulled.
• Otitis Media: Fluid behind the eardrum.
• Otitis Interna: Common symptom is vertigo.

Deafness (Hearing Impairment)

Partial or total inability to hear. Causes include:

• Hereditary factors
• Malformations
• Infections
• Tumors
• Traumatic Injuries
• Prolonged Exposure to Loud Noises

Diseases Affecting the Eyes

Conjunctivitis

Inflammation of the conjunctiva, caused by allergies, irritants, or infections.

Astigmatism

Optical defect caused by irregular curvature of the cornea or crystalline lens. Objects appear distorted.

Long Sight (Hypermetropia)

Inability to focus on near objects. Images are formed behind the retina, corrected with converging lenses.

Short Sight (Myopia)

Inability to focus on distant objects. Images are formed in front of the retina, corrected with diverging lenses.

Presbyopia

Loss of ability to accommodate in the crystalline lens. Causes poor near vision, associated with aging.

Blindness

Caused by problems in the nervous system or eyes.

Cataracts

Clouding of the crystalline lens or vitreous humor, can eventually cause blindness. Associated with aging and diabetes.

Glaucoma

Caused by increasing pressure of intraocular fluid. Affects vision, can cause light sensitivity and even blindness.

Color Blindness

Hereditary disease caused by a lack of one type of cone.

Diseases Affecting the Skin

Dermatitis

Inflammation of the skin, caused by allergies, irritants, or other diseases, treated with creams.

Psoriasis

Skin cell overproduction; cells are replaced every few days instead of every month. Other symptoms: flaking, inflammation, heat, itchiness, pain, and redness of the skin. Treated with medicines, creams, and phototherapy.

Diseases Affecting the Bones

Fissure

A crack in a bone.

Fracture

A break in a bone, causing intense pain, bruising, swelling, and limited mobility. Can be treated with a plaster cast or splint, sometimes requires surgery.

Osteoporosis

Caused by loss of bone mass and deterioration of bone structure. Bones fracture easily. Treatment involves medicines, exercise, and consuming enough calcium.

Spinal Defects

• Scoliosis: Sideways curvature of the spine.
• Hyperlordosis: Excessive curvature of the lumbar spine.
• Kyphosis: Excessive curvature of the dorsal spine (humpback).

Diseases Affecting the Joints

Arthritis

Inflammation of a joint. Causes pain, inflammation, redness, and joint deformities.

Osteoarthritis

Breakdown of cartilage increases friction in the joint. Treated with medicines, sometimes replacement of the joint with a prosthesis.

Gout

Type of arthritis caused by uric acid crystals forming in joints.

Sprain

Overstretched or torn ligament. Symptoms are swelling, pain, bruising, and limited mobility of the joint.

Dislocation

End of a bone leaves its normal position in the joint. Causes intense pain, inflammation, and a deformed joint, treated by relocating the bone.

Diseases Affecting Muscles and Tendons

Muscle Contracture

Painful, long-lasting, involuntary contraction. Treated with physiotherapy and applying heat.

Strained Muscle

Torn muscle fibers, caused by overstretching or impact.

Tendonitis

Inflammation of a tendon. Generally caused by overuse, bad posture, or too much exercise. Treated with medicines, rest, and physiotherapy.

Nerve Cells: Neurons and Neuroglia

Nervous tissue is made up of two types of cells: Neurons and Neuroglia.

Types of Nerve Cells

Neurons

• Highly specialized cells.
• Star-shaped cells unable to divide.
• Initiate and transmit nerve impulses.

Made up of:

• Soma (1)
• Dendrites (2)
• Axon (3)

Neuroglia

Located between neurons, they support, nourish, and protect them.

• Astrocytes (1): Nurture neurons and remove neurotransmitters.
• Oligodendrocytes (2): Wrap around several axons, forming an insulating layer called the myelin sheath.
• Schwann Cells (3): Wrap around a part of one axon, forming an insulating layer called the myelin sheath.

Nerve Impulses and Synapses

Nerve Impulses

Excitation of a neuron generates a nerve impulse. This is an electric current passing along the neuron from dendrite to axon. It always travels in one direction: it enters through dendrites, travels to the soma, and leaves through the axon.

Synapses: Neuronal Communication

Nerve impulses can pass to other neurons through connections between axons and dendrites called Synapses.

Presynaptic Terminal

End of axon, contains synaptic vesicles that store the neurotransmitters.

Synaptic Cleft

Small space between two neurons; they never touch.

Postsynaptic Terminal

Dendrite or soma of the second neuron.

The Nervous System: Coordination and Control

Responsible for coordinating all functions of the body, intellectual function, emotions, and feelings.

Divisions of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• CNS: Brain + Spinal Cord
• PNS: All nerves + nerve ganglia throughout the body.

Central Nervous System (CNS)

Microscopic Structure of CNS

At a microscopic level, the CNS consists of:

• White Matter: Neuron axons with myelin sheath.
• Grey Matter: Soma and dendrites.

Protection of the CNS

The brain is protected by the skull, and the spinal cord is protected by the spinal column.

Meninges

For further protection, the CNS is enveloped by meninges:

• Pia Mater: Innermost layer.
• Arachnoid Mater: Middle layer. Both Pia Mater and Arachnoid Mater are surrounded by cerebrospinal fluid for protection.
• Dura Mater: Outermost layer.

The Brain: Cerebrum, Cerebellum, Brain Stem

Cerebrum

Two hemispheres, separated by the great longitudinal fissure and connected by the corpus callosum (white matter).

Cerebral Hemispheres and Cortex

The Cerebral Cortex is the outer layer, rough and full of folds and furrows, formed of grey matter. Its functions include:

• Receiving, interpreting, and processing information from sense organs.
• Initiating voluntary motor responses.
• Intellectual and mental functions (intelligence, memory, language, communication, learning, awareness, choices, etc.).

Limbic System

Contains the pituitary gland, which regulates the endocrine system. Responsible for emotions, feelings, and basic instincts.

Cerebellum

It has tightly folded grey matter with white matter on the inside. Responsible for motor coordination and balance.

Brain Stem

It is connected to the spinal cord. It is formed of white matter on the outside and grey matter on the inside. It regulates autonomic functions such as:

• Heartbeat
• Respiratory movements
• Blood pressure, etc.

The Spinal Cord

It is connected to the brain stem and extends down the back of the body, protected by the spinal canal formed by vertebrae.

It has white matter on the outside and grey matter on the inside.

Functions of the Spinal Cord

• Conduit Function: Transmits information from sense organs to the brain and responses from the brain to effector organs.
• Reflex Function: Initiates involuntary motor responses.

The Autonomic Nervous System

Subdivisions of the Autonomic Nervous System

Sympathetic Nervous System

Prepares the body for emergencies, reacting to stress, fear, etc., and increases energy expenditure. It dilates pupils, increases heartbeat, and inhibits the activity of the digestive system.

Parasympathetic Nervous System

Associated with situations of rest and calm, and decreases energy expenditure. It activates the digestive system and slows heartbeat.

Somatic Nervous System

This part is formed of:

Components and Control

• Sensory Nerves: Connect sense organs to the nerve centers.
• Motor Nerves: Connect nerve centers to skeletal muscles.

It controls voluntary actions and reflexes.

Voluntary Actions and Reflexes

Reflexes and the Reflex Arc

Reflexes are generated unconsciously in the spinal cord, involving a system of three neurons (reflex arc).

• 1. Receptor: Perceives the stimulus (e.g., a pin prick).
• 2. Sensory Neuron: Transmits the impulse to the spinal cord.
• 3. Interneuron: Located in the grey matter of the spinal cord, generates an immediate response order.
• 4. Motor Neuron: Transmits the order to the effector organ.
• 5. Effector Organ: E.g., a muscle, which executes the response (in this case, it contracts, and we pull our hand away).

Voluntary Actions

Voluntary actions are generated consciously in the cerebral cortex.

• 1. Receptor: Perceives the stimulus.
• 2. Sensory Neuron: Transmits the impulse to the spinal cord and then to the cerebral cortex.
• 3. Cerebral Cortex: Interprets the information into a conscious sensation (image, shape, feeling of cold, etc.) and uses this, along with stored prior experiences, to generate a response order. This order is then sent through the spinal cord to a motor neuron.
• 4. Motor Neuron: Transmits the order to the effector organ.
• 5. Effector Organ: E.g., a muscle, which contracts voluntarily.

The Endocrine System: Hormones and Regulation

System for coordination and control of body functions. Made up of endocrine glands that manufacture and secrete hormones: Chemical messengers.

Endocrine Glands and Hormones

• Released directly into the bloodstream.
• Transported to their target organs.

Nerve vs. Endocrine Information

Nerve Information

They provide a quick response that lasts a short time.

Endocrine Information

Their response is slower and lasts longer.

Regulation by the Hypothalamic-Pituitary Axis

Its function is regulated by the nervous system through the hypothalamic-pituitary axis:

• The hypothalamus sends out releasing hormones, which stimulate the secretion of pituitary hormones.
• The pituitary hormones act on their target organs.
• When enough of these final hormones are released, they act on the hypothalamus and pituitary gland, preventing the secretion of their respective hormones. This stops the cascade of hormone release.

The Male Reproductive System

Penis: Structure and Function

It is formed of erectile tissue, a connective tissue with large spaces that fill with blood when erect.

The erectile tissue is found in the three regions that form the penis:

• Two Corpora Cavernosa
• Corpus Spongiosum

Corpus Spongiosum

Surrounds the urethra, keeping it open at all times. It widens at the end to form:

• Glans: A very sensitive area, protected by the foreskin.

Testicles (Testes): Location and Function

Two oval-shaped organs located inside a sac of skin (scrotum) and outside the abdominal cavity.

Each testicle is formed of millions of seminiferous tubules. The walls of the seminiferous tubules contain sperm precursor cells.

The temperature of the testicles is about 3ºC lower than body temperature, which is essential for spermatogenesis.

There are other endocrine cells between the seminiferous tubules that produce androgens (testosterone).

Male Genital Tract: Ducts for Sperm Transport

It consists of ducts that transport sperm outside the body:

Epididymis

• Long, coiled tube attached to the top of each testicle.
• Sperm is stored here while it matures.

Vas Deferens

• Part of the epididymis, passes through the prostate and empties into the urethra.
• Secretions from accessory glands empty into the vas deferens.

Urethra

• Duct shared by the reproductive and excretory systems.
• Expels semen and urine from the body.

Male Accessory Glands: Semen Formation

These glands secrete substances that mix with sperm to form semen:

Seminal Vesicles

• Located behind the urinary bladder.
• Empty into the final part of the vas deferens.
• Produce seminal fluid rich in fructose, providing energy for sperm.

Prostate Gland

• Located underneath the bladder, surrounding the urethra.
• Produces prostatic fluid.
• Activates sperm and neutralizes the acidity of female mucous secretions.

Cowper’s Glands (Bulbourethral Glands)

• Located at the base of the penis.
• Secrete lubricating fluid that neutralizes the acidity of the urethra.

The Female Reproductive System

External Genitalia (Vulva)

Called the vulva, it is formed of:

• Labia Majora: Two folds of skin that cover the
• Labia Minora: Two thinner folds.
• Clitoris: A very sensitive organ near the top junction of the labia minora; it is the female equivalent of the male glans.

Female Genital Tract: Egg Transport

Formed of the ducts that transport the eggs outside:

Fallopian Tubes (Oviducts)

• Two ducts that empty into the uterus.
• Connected to the ovaries by the infundibulum.

Uterus

• A hollow, muscular organ, shaped like an upside-down pear.
• The narrow section at the bottom is the cervix, connected to the vagina.
• The wall of the uterus is formed of the myometrium, a thick layer of smooth muscle.
• The inside is covered with the endometrium, epithelial cells with many blood vessels.

Vagina

• An elastic duct formed of muscular walls.
• Connects the cervix to the outside.
• Glands on the vaginal wall secrete lubricants.
• The external opening is partially covered by a membrane (hymen).

Ovaries: Egg and Hormone Production

Size and shape of an almond. Suspended in the abdominal cavity by small ligaments.

Produce eggs and female hormones (estrogens and progesterone).

Made up of the medulla (inner part) and cortex (outer part). The cortex contains numerous spherical cavities called follicles. Follicles contain immature eggs (oocytes).

Estrogens and progesterones are synthesized in follicles at different times during the menstrual cycle.

When a mature follicle releases an egg, it converts into a corpus luteum.

Formation of Gametes

Spermatogenesis: Sperm Formation

Sperm formation takes place in the testicles. The process begins in puberty. The beginning of puberty is marked by the first ejaculation.

From that moment, there is continuous sperm production, which decreases with testosterone levels at an older age.

Phases of Spermatogenesis

• 1. Multiplication Phase: Spermatogonia divide to form new spermatogonia.
• 2. Growth Phase: Spermatogonia increase in size and become spermatocytes.
• 3. Maturation Phase: Spermatocytes divide (meiosis) to form four spermatids.
• 4. Differentiation Phase: Spermatids transform to become sperm.

Sperm Structure

Sperm consist of three parts:

Head

• Contains nucleus with 23 chromosomes.
• A human has 46 chromosomes, so half of them come from the egg.
• Contains the acrosome, an organelle containing enzymes to dissolve the outer layer of the ovum.

Midpiece

• Contains numerous mitochondria.

Tail

• Moves thanks to energy provided by mitochondria.

Oogenesis: Egg Formation

Egg formation (oogenesis) occurs inside the ovarian follicles. The process begins before birth and stops at the oocyte stage. It restarts in puberty.

Oocytes mature to become eggs in a cyclical process: the ovarian cycle. The ovarian cycle occurs approximately every 28 days, alternating between the two ovaries.

The cycle starts with menarche (first period) during adolescence and continues until menopause.

Phases of Oogenesis

• 1. Multiplication Phase: Oogonia, cells in the ovarian cortex, divide and form new oogonia.
• 2. Growth Phase: Oogonia grow, increasing in size to become oocytes.
• 3. Maturation Phase: Oocytes divide (meiosis) to form four cells; only one becomes an ootid, while the other three are polar bodies that disappear.
• 4. Differentiation Phase: The ootid undergoes small changes to become an egg.

The Ovarian and Menstrual Cycles

Regulation and Overview

They are regulated by the pituitary gland and ovarian hormones. They occur approximately every 28 days if no fertilization has occurred.

The Ovarian Cycle

Changes that occur in the ovary so that an egg can be produced, mature, and be released. It also produces female sex hormones. It occurs during oogenesis, alternating between the two ovaries.

The Menstrual Cycle

Changes that occur in the endometrium of the uterus to prepare it for implantation of the embryo. It occurs periodically.

Phases of the Ovarian Cycle

1st Follicular Phase

• Starts on the first day of a period and lasts about 14 days.
• The pituitary gland produces FSH, which stimulates the ovaries, leading to the maturation of a follicle and the oocyte it contains.
• As the follicle grows, it starts to produce estrogens.

2nd Ovulation

• Occurs approximately on day 14, when the levels of FSH and estrogens are at their highest.
• The pituitary gland starts to release LH, causing the mature follicle to rupture and release an egg into the Fallopian tubes.
• During and just after this period, the woman is fertile.

3rd Luteal Phase

• Lasts approximately between day 15 and day 28.
• If fertilization has not taken place, on day 28 the woman starts her period.
• The broken follicle transforms to become the corpus luteum.
• LH stimulates the corpus luteum to secrete progesterone.

Phases of the Menstrual Cycle

1st Proliferative Phase

• Begins when menstruation ends and lasts until ovulation.
• The estrogens produced by the ovarian follicle stimulate the endometrium in the uterus, causing it to thicken.

2nd Secretory Phase

• Starts after ovulation and continues until the start of the next period.
• Progesterone secreted by the corpus luteum causes maximum thickening of the endometrium, preparing it for possible implantation of a fertilized ovum.

3rd Menstruation

• Starts on the first day of the cycle and lasts between five and seven days.
• If fertilization has not taken place during the previous cycle, the corpus luteum breaks down in the ovary and stops producing progesterone. This causes the endometrium to break away.