BIO2305
Introduction to Physiology
Physiology – the science of body functions
Teleological vs. Mechanistic views:
Teleological – the why, explains purpose of a physiological process
Example: The Why-- shivering elevates a low body temperature
Mechanistic – the how, explained in terms of cause and effect of physiological process
Example: The How-- when body temperature drops below normal, a reflex pathway
causes involuntary oscillating skeletal muscle contractions which produce heat
The Hierarchy of Structural Organization
Levels of organization of the body
1. Chemical
2. Cellular
3. Tissue
4. Organ
5. System
6. Organism
Chemical - atomic and molecular level: C H N O P S (or SPONCH) carbohydrates, proteins, fats, and
nucleic acids, H2O
Cellular - smallest living unit of the body; review cell functions and Cell Theory
Tissue - Group of cells and surrounding materials that perform a common function
Four primary tissue types in the human body: epithelial, connective, muscle, nervous
Organ - consists of two or more types of primary tissues that function together to perform a particular
function or functions
1
Example: Stomach
Inside of stomach lined with epithelial tissue
Wall of stomach contains smooth muscle
Nervous tissue in stomach controls muscle contraction and gland secretion
Connective tissue binds all the above tissues together
System - collection of related organs with a common function; sometimes an organ is part of more than
one system
Organism - one living individual
Body Systems
The human body has 11 systems. Body Systems do not act in isolation from one another
Endocrine System
All hormone-secreting tissues:
hypothalamus, pituitary, pineal, thyroid, parathyroids, thymus, adrenals, pancreas, testes and ovaries, kidneys, intestine,
and heart
Nervous System
brain, cranial nerves, spinal cord, spinal nerves, and sensory receptors
Muscular System
skeletal, smooth, and cardiac muscle
Skeletal System
bones, bone marrow, joints, ligaments, tendons, cartilage
Cardiovascular System
heart, blood, arteries, arterioles, capillaries, veniole, veins
Integumentary System
epidermis, dermis, hypodermis, nails, hair, apocrine and eccrine sweat glands
Lymphatic System
lymph, lymph nodes, lymphatic vessels, mucosa-associated lymphoid tissue (m.a.l.t.)
Respiratory System
nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli
Digestive System
oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, as well as salivary glands, liver,
gallbladder, pancreas
Renal System
kidneys, ureters, urinary bladder, urethra
Reproductive System
ovaries, uterine tubes, uterus, vagina, testes, prostate gland, penis, mammary glands
2
Basic Cell Functions
Senses and responds to changes in surrounding environment
Controls the exchange of materials between the cell and its surrounding environment
Obtain nutrients and oxygen from surrounding environment
Eliminate carbon dioxide and other wastes to surrounding environment
Performs chemical reactions that provide energy for the cell
Synthesizes needed cellular components
Reproduces: divides mitotically (somatic cells) or meiotically (germ cells)
Exception: Nerve cells and muscle cells lose their ability to undergo mitosis during early
development
Homeostasis
Homeostasis – the maintenance of a relatively stable internal environment
Does not mean that composition, temperature, and other characteristics are absolutely
unchanging!
Homeostasis within the body is essential for survival and function of all cells
Most cells are not in direct contact with the external environment
Each cell contributes to homeostasis, although some more than others
Homeostasis
Body cells are contained in a watery internal environment through
which life-sustaining exchanges are made
Extracellular fluid (ECF) – the fluid environment in which the cells
live (fluid outside the cells)
Two components: of ECF:
Plasma – the liquid portion of blood
Interstitial fluid (ISF) – the fluid between cells
Intracellular fluid (ICF) – the fluid contained within all body cells
Balancing the Internal and External Environment
Cells, the fundamental units of life, exchange nutrients and wastes
with their surroundings:
The intracellular fluid (ICF) is conditioned by
the interstitial fluid (ISF), which is conditioned by
the plasma, which is conditioned by
the organ that it passes through
3
Homeostasis
Homeostasis involves dynamic mechanisms that detect and respond to deviations in physiological
variables from their “set point” values by initiating effector responses that restore those variables to
the optimal physiological range.
The two primary systems that maintain homeostasis
1. Nervous system
2. Endocrine system
To regulate homeostasis, the body uses an electrical system (nervous) and a chemical system (endocrine - hormones). The
nervous system (milliseconds) is generally faster than the endocrine system (seconds to hours). Both systems primarily act
automatically, but there is some voluntary control over the nervous system.
Nervous system
Neurons control and coordinate bodily activities that require rapid responses
Detects and initiates reactions to changes in external environment
Endocrine system
Secreting endocrine glands regulate activities that require duration rather than speed
Controls concentration of nutrients and, by adjusting kidney function, controls internal
environment’s volume and electrolyte concentration
Homeostasis
Factors homeostatically regulated include:
Concentration of nutrient molecules
Concentration of water, salt, and other electrolytes
Concentration of waste products
Partial pressures of gases:
pO2 = 100 mmHg
pCO2 = 40 mmHg
pH = 7.35 – 7.45
Blood volume = 4 – 6 Liters
Blood pressure ≈ 120/80 mmHg
Temperature = 37° C
Control of Homeostasis
Homeostasis is continually being disrupted by:
External stimuli
Heat, cold, lack of oxygen, pathogens, toxins
Internal stimuli
Body temperature
Blood pressure
Concentration of water, glucose, salts, oxygen, etc.
Physical and psychological distresses
Disruptions can be mild to severe.
Pathophysiological states develop when body systems fail to
maintain an optimal internal environment
Serious homeostatic disruption leads to death
4
Homeostatic Control Systems
Networks of body components working together to maintain a given factor in the internal environment
directed at an optimal set level
In order to maintain homeostasis, control system must be able to:
Detect deviations from normal in the internal environment that need to be held within narrow
limits
Integrate this information with other relevant information
Make appropriate adjustments in order to restore factor to its desired value
Control systems are grouped into two classes:
Intrinsic controls
Local controls that are inherent within an organ
Involves detecting deviations and making corrections within the organ, often called
autoregulation
Tends to benefit the organ, not the organism
Extrinsic controls
Regulatory mechanisms initiated outside an organ
Accomplished by nervous and endocrine systems
Responses of an organ that are triggered by factors external to the organ or systems.
Can over-ride intrinsic regulation
Tends to benefit the organism, not necessarily individual organs
Homeostatic Control Systems
Feedforward – a term used for responses made in anticipation of a change
Example: the smell of food initiates gastric secretions and motility in anticipation of a meal
Feedback - refers to responses made after change has been detected
Two Types of Feedback:
Negative Feedback Loop (NFbL)
Changes that move a value away from the reference value are attenuated
(reduced)
Original stimulus is alleviated
Most feedback systems in the body are negative
Used for conditions that need frequent adjustment
Positive Feedback Loop (PFbL)
Changes that move a value away from the reference value are enhanced
Original stimulus is intensified
Far less common; Seen during normal childbirth and lactation
Feedback Loops: Basic Outline
Negative Feedback Loop (NFbL) original stimulus is alleviated
Positive Feedback Loop (PFbL) original stimulus is intensified
5
Components of a Negative Feedback System
Negative feedback loops consist of:
Stimulus – a change
Receptor – structures that monitor a controlled condition and detect the
change
Afferent Pathway – the means to communicate information about the
change from the receptor to the control center; can be neurons or blood
Control Center (Integration Center) – determines next action
Efferent Pathway – the means to send a signal from the integration
center to the effector organ
Effector – the organ or tissue that receives directions from the control
center and produces
Response – the output that removes the original stimulus
Components of a Positive Feedback System
Positive feedback loops consist of:
Stimulus – a change
Receptor – structures that monitor a controlled condition and detect the
change
Afferent Pathway – the means to communicate information about the
change from the receptor to the control center; can be neurons or blood
Control Center (Integration Center) - determines next action
Efferent Pathway – the means to send a signal from the integration
center to the effector organ
Effector – the organ or tissue that receives directions from the control
center and produces
Response – the output that increases the original stimulus
6
Homeostasis: Blood Glucose NFbL
Complex Neuroendocrine Reflex
Stimulus: ↑ pH & stretch in stomach indicates
imminent ↑ BGL
Receptor: Mechanoreceptors and
chemoreceptors in stomach
Afferent Pathway: Vagus nerve (sensory)
Neural Integrating Center: Medulla oblongata
Efferent Pathway: Vagus nerve (motor)
Endocrine Integrating Center: Pancreas
(releases insulin)
Efferent Pathway: Bloodstream
Effector: Target body cells
Response: Insulin (hormone) released from the
pancreatic β cells stimulates the transport of
glucose into target body cells
Negative Feedback: ↓ BGL
Simple Endocrine Reflex
Stimulus: ↑ BGL
Receptor: β cells in the pancreatic Islets of
Langerhans
Afferent Pathway: none
Endocrine Integrating Center: Pancreas (releases insulin)
Efferent pathway: Bloodstream
Effector: Target body cells
Response: Insulin (hormone) released from the pancreatic β cells
stimulates the transport of glucose into target body cells
Negative Feedback: ↓ BGL
Homeostasis: Thyroid Hormone NFbL
Stimulus: Low serum T3, T4
Receptors: T3, T4 binding sites on Hypothalamus
Neural Integrating Center: Hypothalamus releases TRH
Efferent Pathway: Hypothalamic-Pituitary Portal Veins
Endocrine Integrating Center: Anterior Pituitary releases TSH
Efferent Pathway: Bloodstream
Endocrine Integrating Center: Thyroid Gland (releasing T3, T4)
Efferent Pathway: Bloodstream
Effector: Various body tissues
Response: Increased serum sT3, T4, relieves original stimulus
7
Homeostasis: High Blood Pressure NFbL
Stimulus: ↑ BP (hypertension)
Receptors: Baroreceptors in walls of blood vessels detect ↑BP
Afferent Pathway: Vagus nerve (sensory)
Integrating Center: CNS (medulla oblongata)
Efferent Pathway: Vagus nerve (motor)
Effector: Cardiac muscle of the heart
Response: ↓HR
Feedback: ↓ BP, relieving initial stimulus
Homeostasis: Childbirth PFbL
Stimulus: Amniotic sac breaks, fetus drops lower in uterus
Receptors: Stretch receptors in walls of uterus
Afferent Pathway: Vagus nerve (sensory) to Hypothalamus
Integrating Center: Hypothalamus
Efferent Pathway: Hypothalamo-neurohypophyseal nerve tract
Integrating Center: Neurohypophysis releases oxytocin
Efferent Pathway: Bloodstream to uterine smooth muscle
Effector: Uterine contractions cause cervix to dilate & fetus to descend in
birth canal
Response: More stretch… more oxytocin release… more contractions….
repeat
PFbL ends with birth of the baby & ↓ uterine stretch
8
Homeostasis: Milk Production & Let Down PFbL
Stimulus: Breastfeeding; suckling
Receptors: Mechanoreceptors in and around areola of breast
Afferent Pathway: Spinal nerves
Integrating Center: Hypothalamus
Efferent Pathway: Hypophyseal
portal system
↓ release of PIH (Prolactin
Inhibiting Hormone)
↑ release of TSH (Thyrotropinreleasing Hormone)
Integrating Center:
Adenohypophysis releases
Prolactin
Efferent Pathway: Bloodstream
Effector: Glandular tissue in
breast
Response: Lactation (milk
production)  Breastfeeding 
More prolactin release
Efferent Pathway: Hypothalamohypophyseal nerve tract
Integrating Center:
Neurohypophysis releases
oxytocin hormone into
bloodstream
Efferent Pathway: Bloodstream
Effector: Smooth muscle in
breast. Oxytocin stimulates milk
'let down'
Response: Let down allows for
breastfeeding, which stimulates
more oxytocin release, which
stimulates further let down.
9