HE100A: Health Issues I
Lecture 2:
The Human Body: An Overview
Copyright © 2026 by Dr. Nicolas Rouleau, Ph.D., Waterloo, Ontario
What is a body?
The human body is…
Multiscalar
Multisystemic
Early Eastern and Western philosophers discussed the
structures and functions of bodies and minds
Dual and non-dual models of body-mind were promoted
by different spiritual traditions
Empirical investigations of body structure and function
have always existed – usually because of wars
Dualism:
The body as a vessel
→ Dualism claims that body and mind are separate and distinct
→ Usually, the body is considered physical
→ Usually, the mind is considered non-physical
→ The body is therefore a vessel for mind
→ “Pneuma”, “Soul”, “Atman”, and other concepts place
the mind outside the body
Physicalism:
Mind is just body
→ Physicalists claim that mind is reducible to objective events
→ Neural activity is all that exists
→ Does not address the “Hard Problem” of consciousness
→ Considers mind to be a functional subset of body (brain)
→ This is the mainstream view in modern Western medicine
The body at different
spatial scales
→ The body’s structure is nested and self-similar
→ Scale is an important concept to learn (micro-to-macro)
→ Many molecules make up cells, many cells make up tissues
→ Many tissues make up organs, many organs make the organism
→ Cells are approximately 1 million times smaller than bodies
→ There are 9 orders of magnitude between proteins and people
The body at different
temporal scales
→ The human body is always changing
→ Embryonic development does not end at birth
→ Brain development continues for 25 years after birth
→ Sexual maturation occurs over decades
→ Trillions of molecular events occur continuously
→ There are 12 orders of magnitude between protein
synthesis and total lifespan
Embryogenesis
→ Our bodies come from single cells that express genes
to carry out morphogenesis
→ All cells have a complete body plan, but any given cell
only expresses a fraction of the blueprint
→ To grow 5 fingers on a hand, cells need to know where
to grow and not grow
→ After achieving adulthood, growth must stop
→ Cancer and regeneration are related to embryogenesis
Maturation and
senescence
→ The body is constantly “maturing”, growing
→ Bone, muscle, and fat densities change
→ Immune system function varies over time
→ Sexual maturity enables reproduction
→ Ultimately, cells senescence and are imperfectly replaced
→ Aging is a consequence of entropy
Cells are the building
blocks of the body
→ The cell is the fundamental unit of the body
→ It is 70% water, enclosed in a fatty membrane, and
contains many types of organelles
→ It synthesizes protein, communicates with other cells,
migrates, performs specialized functions, and can
auto-destruct
→ Cells form connections to build functionalized tissues
Multicellularity,
tissues, and organs
→ Multicellularity evolved from single-celled organisms
→ Because cells can cooperate rather than compete, they
can achieve highly complex functions
→ Adhering to each other triggers gene expression
→ Cytoarchitectures form with unique properties
→ Cells lose surface area when they become tissues,
requiring unique solutions (e.g., vasculature)
Molecular biology
→ The body can be viewed through the lens of molecular biology
→ Protein complexes and other structures are used to
signal or trigger cellular events
→ These events drive physiology
→ Because this paradigm is central to modern medicine,
our solutions are molecular in nature (e.g., pharmacotherapy)
Bioelectricity
→ Cells are also biophysical systems that are responsive
to more than chemicals
→ Physical compression, light, electric fields, magnetic fields,
and electric current drive cell functions
→ Neurons and cardiomyocytes are most conspicuous
→ However, all cells maintain a resting membrane potential
→ Stem cells, cancer cells, and regenerative cells share
bioelectric signatures
→ Novel therapies are exploiting bioelectricity
Identifying “us” and
“them”
→ Where does the body end and the environment begin?
→ Our bodies are covered in microorganisms
→ Skin flora create an inhospitable environment for pathogens
→ Gut flora enable digestion and signal brain events
→ Eight (8) percent of the human genome is of viral origin
→ Mitochondria may have been symbiotic/parasitic
→ Every body in this room contains dormant viruses (e.g.,
shingles, HSV-1)
We are mostly “them”
→ As a matter of raw numbers, there are more non-human cells
in your body than human cells
→ However, human cells are 10-100 times larger
→ Recent decades have revealed the importance of the gut-brainaxis and other interactions
→ Fecal transplants and other therapies have developed as a
consequence of our appreciation of these microbes
And cancer is made of you
→ Cancers are generated from your own cells
→ While trillions of pathogens reside within your body
without harming you for decades, your own cells
will eventually go rogue
→ Nearly half of all people will develop cancer in their
lifetime
→ As lifespan increases, cancer rates will also increase
without interventions
The body as an
interconnected whole
→ The body is a highly interconnected system
→ Blood vessels and nerves travel everywhere, providing
nutrients, oxygen, and simulation to all other tissues
→ Immune cells perfuse all tissues, removing defective cells
and pathogens
→ Endocrine functions support tissues throughout the body
→ All tissues interact to some degree
→ We can think of the body as a highly complex single system
Before we go any further, let’s familiarize ourselves
with spatial planes
Transverse
Coronal
Sagittal
The body divided by
systems
→ For practical reasons, we have divided the body into discrete
systems
→ Each system is defined by cell types, tissue types, functions,
structures, and interactions
→ Circulatory, respiratory, digestive, musculoskeletal, nervous,
endocrine, integumentary, immune, renal, and reproductive
systems are some examples
Circulatory system
→ Three-dimensional tissues are made up of cells that have
sacrificed their surface area
→ Therefore, nutrients and oxygen must perfuse tissues
→ Vessels enable the full perfusion of 3D tissues
→ A heart serves to pump blood containing vital fluids and
gases throughout the body
→ The system forms a circuit that continuously regenerates
Respiratory system
→ The respiratory system helps regenerate oxygen-deprived
blood
→ Alveolar sacs create an interface to exchange oxygen with
carbon dioxide in blood
→ Newly oxygenated blood returns to the heart and then
the circulatory system to perfuse tissues
→ The processes repeats itself continuously
Vessels must go everywhere cells are in the body
Digestive system
→ The digestive system enables the breakdown of nutrient-rich
foods to be used as energy sources
→ In large part, the digestive system is a long tube that slows
down internal movement of nutrients
→ Physical and chemical digestion combine to extract nutrients
→ Nutrient is sorted from waste
→ Nutrients are distributed throughout blood or stored as fat
→ Waste is removed from the system
Energy is carefully managed throughout the body
Musculoskeletal
systems
→ Skeletal muscles manipulate the skeleton to ambulate
→ Cardiac muscles pump blood throughout the circulatory system
→ Smooth muscle aids with digestion and other visceral functions
→ Muscles consume high amounts of energy, generate heat
All movement and behaviour is
accomplished by activating muscles
Nervous systems
→ Nervous tissues are excitable, rapid, and long-range
→ The central nervous system gives rise to all thought and
behaviour
→ The peripheral nervous system manages incoming sensory
information and outgoing motor activations
→ The brain uses 20% of bodily energy but is only 2% of its mass
→ The brain is the most complex object in the known Universe
You are your brain – even your personality and
memories are encoded at synapses
Endocrine system
→ Long-range chemical signals travel from hormonesecreting organs to their destinations via the blood
→ Vasculature carries the hormone everywhere but
only receptor-expressing sites promote binding
→ Therefore, drugs that mimic hormones only have
regional effects where the receptors are expressed
→ Endocrine function controls everything from hunger,
to temperature, to sexual arousal, growth, etc.
The pituitary gland is the master endocrine
gland – it is highly vascularized
Integumentary and
exocrine systems
→ Barriers and complex sensory management tissues
exist throughout the body
→ Skin prevents infection, retains fluid, modulates sensory input
→ Exocrine glands move fluids from spaces within the body to
external/environmental spaces for different purposes
Immune systems
→ The immune system discriminates between
self and non-self
→ It targets and eliminates cancer cells, pathogens,
and other problematic structures
→ It is distributed throughout the body, paralleling vascular
networks to enable transport
→ Lymphatic fluids co-opt musculature to regulate flow
of cells and waste
Two distinct immune system divisions
help maintain bodily integrity
Renal and urinary
systems
→ The kidneys remove waste, filter salts, and manage
water concentrations in the blood
→ They extract and filter resources from waste in the blood
and form urine
→ Urine is stored and concentrated in the bladder
→ Then, urine is excreted
Water/salt retention, hormone release, and waste
management are accomplished
Reproductive systems
→ The human species is dependent upon proliferation
→ Reproductive systems provide the mechanisms to move
cell division outside the body (a new body)
→ Combining DNA from separate sources can provide new
opportunities for adaptation/selection
→ Sexual activity is reinforced by orgasm and other CNS
activities through experience, learning