Organs and Organ Systems of the Human Body
Musculoskeletal
system
Human skeleton
- 270 bones at birth, 206 in adults.
- Longest bone – Femur (thighbone).
- Smallest bone – Stapes (in the ear).
- Other bones – Humerus (upper arm), Fibula
(lower leg), Tibia (shinbone).
Joints
- Connection between bones in the body.
Ligaments
- Connective tissue between bones.
- Made of collagen.
Muscular system
- Skeletal, smooth and cardiac muscles.
- Largest muscle – Gluteus Maximus (buttock).
- Smallest muscle – Stapedius (in the ear).
Tendons
- Connective tissue between muscle and bone.
- Made of collagen.
Digestive system
Mouth
- Lower Jawbone is called mandible, upper is
called maxilla.
- First portion of the alimentary canal.
Teeth
- 20 baby (or milk) teeth, 32 adult (permanent)
teeth.
- Incisors (8), Canines (4), Premolars or
Bicuspids (8), Molars (12).
- Made of enamel.
Salivary glands
- Exocrine glands (glands with ducts).
- Produce saliva from amylase etc.
- Parotid glands, Submandibular glands,
Sublingual glands, Von Ibner glands etc.
Pharynx
- Nasopharynx, Oropharynx, Laryngopharynx.
- Important part of Digestive system,
Respiratory System, Vocalization
Esophagus
- Aka Food pipe or gullet.
- During swallowing, epiglottis tilts backwards
to prevent food from going into lungs.
Stomach
- Secretes digestive enzymes.
- Pyloric Sphincter controls passage of
partially digested food into duodenum.
Small intestine
- Three parts – Duodenum, Jejunum, Ileum
- Duodenum receives bile and pancreatic juice.
- Average length in adult human male is 6.9 m.
Large intestine
- Also known as bowels.
- Average length is about 1.5 m.
- Parts – Cecum, Ascending Colon, Transverse
Colon, Descending Colon, Sigmoid Colon, Rectum, Anal Canal.
Liver
- Largest internal organ in human body.
- Largest gland in human body.
- Secretes the enzyme – bile.
- Located in upper right quadrant of abdominal
cavity.
- Has four lobes.
- Can be fractured. Can be regenerated from
part.
Gallbladder
- Stores bile before being released into small
intestine.
- Can survive without gall bladder.
- Three parts – fundus, body, neck.
Pancreas
- An endocrine gland that produces several
important hormones.
- Chief among these hormones are insulin,
glucagon, somatostatin, pancreatic polypeptide.
- It also secretes digestive enzymes called
pancreatic juices.
- Insulin is produced from near the area called
Islets of Langerhans).
Respiratory
system
Nasal cavity
- Aka nasal fossa.
- Divided into two by vertical fin called nasal
septum.
Larynx
- Commonly called Voice box.
- Involved in protecting trachea, manipulating
pitch and volume of sound.
- Thyroid Cartilage is called Adam’s apple.
Trachea
- Aka Windpipe.
- Cartilaginous tube that connects larynx and
pharynx to the lungs.
Bronchi
- Airway in respiratory tract that allows air
into lungs.
- Primary bronchus – right and left.
- These branch into smaller secondary and
tertiary bronchi.
- No gas exchange takes place in bronchi.
Lungs
- Primary respiratory organs.
- Two lungs (left and right) – each divided into
two lobes.
- They extract oxygen from atmosphere and
transfer to the bloodstream. They release carbon dioxide from bloodstream
into atmosphere.
- Right lung is bigger than left lung.
- Protected by the ribcage.
Diaphragm
- Primary muscle that drives breathing.
- Separates thoracic cavity (containing heart
and lungs) from abdominal cavity.
Urinary system
Kidneys
- Regulates balance of electrolytes in blood.
- Also maintains pH homeostasis.
- Also removes excessive organic molecules from
blood. Thus it removes waste products of metabolism.
- Also produce hormones – renin, calcitriol and
erythropoietin.
- Two bean shaped organs made up of cells called
nephrons.
Ureters
- Tubes that take urine from kidneys to bladder.
Bladder
- Hollow muscle that collects urine from kidneys
before urination.
- Typical capacity of bladder is between 300 and
500 ml.
Urethra
- Tube that connects urinary bladder to urinary
meatus.
- It helps in removal of fluids from the body.
Reproductive
organs – Female reproductive system
Ovaries
- Produces and periodically releases eggs in the
female body.
- They are both gonads and endocrine glands.
- They secrete estrogen, testosterone, progesterone.
Fallopian tubes
- Aka Uterine tubes.
- They allow passage of eggs from the ovaries to
the uterus.
Uterus
- Aka womb.
- It is a hormone-responsive reproductive sex
organ in females.
- One end connects to the fallopian tubes. The
other end, the cervix, opens into the vagina.
- It is within the uterus that the egg is
fertilized, and the fetus is developed.
Vagina
- It is the opening of the female genital tract.
- It allows for sexual intercourse, childbirth
and channels menstrual flow.
Vulva
- This is the external genital organ of the
woman.
Clitoris
- It is a female sex organ.
Placenta
- Aka afterbirth.
- Organ that connects developing foetus to the
uterine wall.
- It allows for nutrient uptake,
thermoregulation to the foetus, waste elimination and gas exchange via mother’s
blood supply.
Reproductive
organs – Male reproductive system
Testes
- They are the male gonads producing sperms.
- They are also endocrine glands producing
androgens, primarily testosterone.
- Typically there are two.
Prostate
- It is an exocrine gland in males.
- It produces a white fluid that constitutes 30%
of the seminal fluid.
Penis
- It is the external male sexual organ.
- It also serves as the urinal duct.
Scrotum
- It is the male reproductive organ.
- It consists of a pair of suspended sacs.
Endocrine glands
Pituitary gland
- Aka hypophysis.
- It is an endocrine gland, the size of a pea.
- It is found at the base of the brain.
- It secretes hormones that regulate growth,
blood pressure, certain functions of sex organs, thyroid glands,
temperature regulation, pain relief, metabolism, and some aspects of
pregnancy, childbirth nursing etc.
Pineal gland
- Aka conarium or epiphysis cerebri.
- Small endocrine gland found near the center of
the brain, between the two hemispheres.
- It resembles a tiny pine cone.
Thyroid gland
- One of the largest endocrine glands in the
body.
- It consists of two connected lobes.
- It is found in the neck.
- Controls rate of use of energy sources,
protein synthesis, and controls body’s sensitivity to other hormones.
- They help regulate growth and rate of function
of many other systems in the body.
- T3 and T4 are synthesized from iodine and
tyrosine.
- It also produces calcitonin which plays a role
in calcium homeostasis.
- Hormonal output is regulated by thyroid
stimulating hormone (TSH).
Parathyroid
glands
- Small endocrine glands in the neck.
- It produces parathyroid hormone.
Adrenal glands
- Aka suprarenal glands.
- Endocrine glands that produce many hormones.
- These include adrenaline and the steroids
cortisol and aldosterone.
- They are found above the kidneys.
Circulatory
system – Cardiovascular system
Heart
- Muscular organ that pumps blood through blood
vessels to entire body.
- Divided into 4 chambers – Left atrium (top),
right atrium (top), left ventricle (bottom), right ventricle (bottom).
- In healthy heart, blood flows one way through
heart due to heart valves, which prevent backflow.
- Enclosed by a protective sac called
pericardium.
- Wall of heart made up of – epicardium,
myocardium, endocardium.
- Blood low in oxygen enters right atrium from
superior and inferior venae cavae.
- From here it passes to right ventricle.
- From here it is sent to the lungs, where it
receives oxygen and gives off CO2.
- Oxygenated blood returns to left atrium and
passes to left ventricle.
- From here it leaves heart through aorta.
- Heart is a fist-sized organ that is placed in
the middle of the chest cavity.
- It is slightly offset to the left. The left
heart is stronger so heartbeats felt more strongly on the left.
Arteries
- Blood vessels that carry blood away from
heart.
- Most carry oxygenated blood.
- Exceptions are pulmonary and umbilical
arteries.
Veins
- Blood vessels that carry blood towards the
heart.
- Most carry deoxygenated blood from tissues.
- Exceptions are pulmonary and umbilical veins.
- Less muscular than arteries and closer to
skin.
- Valves in veins prevent backflow.
Capillaries
- Smallest blood vessels in the human body.
- Make up microcirculation.
- They connect arterioles and venules.
- Help enable exchange of O2, CO2, other
nutrients and waste substances between blood and tissues.
- Lymph capillaries drain lymph from lymph
vessels.
Circulatory
system – Lymphatic system
Lymphatic vessel
- Thin walled, valved structures that carry
lymph.
- Lymph is a fluid that lies between body
tissues.
Lymph node
- Oval or kidney shaped organ.
- Present at multiple locations throughout body
including armpits, neck and groin.
- Important for proper functioning of immune
system.
- Act as filters for foreign particles and
cancer cells.
Bone marrow
- Flexible tissue in the interior of bones.
- Red blood cells produced by bone marrow in
heads of long bones.
- Bone marrow is a key component of the
lymphatic system.
- It produces lymphocytes that supports the
body’s immune system.
Thymus
- Specialized primary lymphoid organ of immune
system.
- Within thymus, T cells or T lymphocytes mature.
- These are key to adaptive immune system.
- Consists of two identical lobes located in
front of the heart.
Spleen
- Similar in structure to large lymph node.
- Acts as a blood filter.
- Removes old red blood cells and holds a
reserve of blood.
- It recycles iron.
- It also synthesizes antibodies in its white
pulp.
Nervous system
Brain
- Main organ of human central nervous system and
the human body.
- Located in head, protected by skull.
- Composed neurons, glial cells and blood
vessels.
- Divided into three parts – forebrain, midbrain
and hindbrain.
- Dominant feature of human brain is the
wrinkling of the cerebral cortex. The cerebral cortex is so large that it
overshadows all other parts of the human brain.
- Three parts of cerebral cortex – cerebrum
(forebrain), cerebellum (hindbrain), and brainstem.
- There are two hemispheres in the brain – left
and right.
- The mind is an emergent property of the brain.
The brainstem
- Posterior part of the brain.
- Consists of – Midbrain, Pons, and Medulla
Oblongata.
- Medulla Oblongata is responsible for
involuntary functions like sneezing, breathing, heart rate, blood pressure
etc.
- Pons has a role in sleep and dreams. Also
deals with swallowing, bladder control, equilibrium, hearing, taste, eye
movement, facial expressions and posture.
- Midbrain is associated with vision, hearing,
motor control, sleep/wake, alertness, temperature regulation.
Cerebellum
- Important in motor control, cognitive
functions like language and attention.
- Also regulates fear and pleasure responses.
Spinal cord
- Long, thin, tubular bundle of nervous tissue
and support cells.
- Extends from brainstem to pelvis.
- Brain and spinal cord together make up central
nervous system.
Nerves
- Enclosed, cable-like bundles of axons (nerve
fibres).
- Part of the peripheral nervous system.
- Provides pathway for electrochemical nerve
impulses to and from peripheral organs.
- Categorized in three groups based on direction
of signals – afferent nerves (from sensory neurons to central nervous
system), efferent nerves (from CNS to muscle and glands) and mixed nerves
(contains both signals).
- Categorized in two groups based on where they
connect to CNS – spinal nerves (connect to spinal cord), and cranial
nerves (connect directly to brain).
Sensory organs
Eye
- Eye is a sense organ.
- Contains rod cells (for light perception) and
cone cells (for colour perception).
- Made of three coats enclosing three
transparent structures.
- Outermost layer composed of cornea and sclera.
- Middle layer consists of choroid, ciliary body
and iris.
- Innermost layer is the retina, a light
sensitive layer of tissue.
- Within these coats are the aqueous humour
(clear fluid), the vitreous body (clear jelly), and the flexible lens.
- The lens is suspended to the ciliary body by
the suspensory ligament (Zonule of Zinn).
- The cornea is the transparent front part of
the eye that covers the iris and pupil. It refracts light.
- The iris controls the size and diameter of the
pupil, which is a hole that lets light in.
Ear
- Ear is an organ for hearing and balance.
- Consists of three parts – outer ear, middle
ear and inner ear.
- Outer ear consists of auricle (visible part of
the ear) and ear canal. It gathers and focusses sound energy on to the
eardrum.
- Ear drum is a membrane that separates external
ear from middle ear.
- Middle ear contains three ossicles which
transfer vibrations from ear drum to inner ear.
- Inner ear is a bony labyrinth. It has two main
functional parts – cochlea and vestibular system.
Olfactory
epithelium
- Specialized epithelial tissue in the nasal
cavity.
- Involved in smell and detecting odors.
Tongue
- Taste receptors or taste buds on different
parts of the tongue.
- Taste buds are also found in the soft palate,
upper esophagus, cheek and epiglottis.
- They are involved in detecting five elements
of taste perception – salty, sour, bitter, sweet and umami.
Integumentary
system
Mammary glands
- It is an exocrine gland that produces milk.
Skin
- Largest organ of the integumentary system.
- Contains pigment melanin that gives skin its
colour.
- Composed of three primary layers – epidermis,
dermis and hypodermis.