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Introduction to Physiology

• Physiology is defined as the study of function of living organism – so human physiology attempts to explain how and why humans function.

• Physiology is where we figure out how stuff works.

• How do muscles contract?• How do we run?• How does our heart beat?

Levels of Structure

• In order to understand how something is built and how something works, you must look at all of its components and analyze them both individually and together.

• In doing these collective and separate analyses, you must examine things at multiple structural levels, i.e., one must break them down from large to small – this is called reductionism

• An organism (such as a human being) may be broken down as illustrated on the left.

Organelle

Cell

Tissue

Organ

Organ System

Organism

Levels of Structure

• The basic unit of life is the cell.• All living organisms are composed of one or more

cells.• The human body contains about 100 trillion cells.• There are about 200 different types of cells in the

human body.• The different types of cells have different features but

for the most part, all cells are made up of organelles and various macromolecules (e.g., proteins, lipids, carbohydrates and nucleic acids).

• Organelles themselves are made of these macromolecules and macromolecules are polymers of smaller molecules which consist of atoms of various chemical elements.

A Prototypical Cell

Important Organelles

• Plasma Membrane → Separates the cell exterior from the cell interior (cytoplasm).

• Nucleus → Membrane bound structure that contains deoxyribonucleic acid (DNA) which is the set of instructions for the synthesis of all the body’s proteins.

• Mitochondria → Structure bound by a double membrane and the site at which the energy stored in sugars and other organic molecules is transferred to ATP, the chemical which acts as the “currency” for energy in the cell.

• Ribosomes → Not bound by a membrane. Sites of protein synthesis. May be free – floating in the cytoplasm – or bound to the endoplasmic reticulum.

Important Organelles

• Rough Endoplasmic Reticulum → Membranous set of tubes with ribosomes studded along its surface. Site of the synthesis of proteins that are destined to be exported from the cell.

• Smooth Endoplasmic Reticulum → ER w/o the attached ribosomes. Site of cellular lipid synthesis, among other things.

• Golgi Apparatus → Membrane bound organelle responsible for determining the direction of proteins synthesized in the rough ER.

• Lysosomes → Membrane bound organelle that houses digestive enzymes that can be used to break down ingested toxins or worn out cell parts.

More Levels of Structure

• Similar cells and cell products come together to form tissues.

• A structure made of 2 or more tissue types that perform a particular function is an organ.

• A group of organs with a unique collective function is an organ system.

Integumentary System

• Structures: - Skin, hair, sweat and oil glands

• Functions: - Forms the external body covering

- Protects deeper tissues from injury

- Involved in vitamin D synthesis

- Prevents desiccation, heat loss, and pathogen entry

- Site of pain and pressure receptors

Skeletal System

• Structures:– The 206 bones of the human body

• Functions:– Protects and supports body organs

– Provides a framework that muscles can use to create movement

– Hemopoiesis (synthesis of blood cells)

– Mineral storage• Bone contains 99% of the body’s store of

what mineral?

Muscular System

• Structures:– The 600+ muscles of

the body

• Functions:– Locomotion

– Maintaining posture

– Thermogenesis (generation of heat)

Nervous System

• Structures: – Brain, spinal cord, and

peripheral nerves

• Functions:– Fast-acting control system

of the body– Monitoring of the internal

and external environment and responding (when necessary) by initiating muscular or glandular activity

Endocrine System

• Structures:– Hormone-secreting glands

• Pituitary, Thyroid, Thymus, Pineal, Parathyroid, Adrenal, Pancreas, Small Intestine, Stomach, Testes, Ovaries, Kidneys, Heart

• Functions:– Long-term control system of the

body– Regulates growth, reproduction,

and nutrient use among other things.

Cardiovascular System

• Structures: – Heart, Blood vessels (arteries, veins,

and capillaries)

• Functions:– The heart pumps blood thru the

blood vessels.– Blood provides the transport

medium for nutrients (glucose, amino acids, lipids), gases (O2, CO2), wastes (urea, creatinine), signaling molecules (hormones), and heat.

Lymphatic/Immune System

• Structures:– Lymphatic vessels, Lymph nodes,

Spleen, Thymus, Red bone marrow

• Functions:– Disposal of debris

– Attacking and resisting foreign invaders (pathogens i.e., disease-causing organisms)

Respiratory System

• Structures:– Nasal cavity, pharynx,

trachea, bronchi, lungs

• Functions:– Constantly supply the

blood with O2, and remove CO2

– Regulate blood pH

Digestive System

• Structures:– Oral cavity, esophagus, stomach, small

intestine, large intestine, rectum, salivary glands, pancreas, liver, gallbladder

• Functions:– Ingestion and subsequent breakdown of food

into absorbable units that will enter the blood for distribution to the body’s cells

Urinary System

• Structures:– Kidneys, ureters,

urinary bladder,

urethra

• Functions:– Removal of nitrogenous

wastes

– Regulation of body’s levels of water, electrolytes, and acidity

Reproductive System

• Structures:– Male:

• Testes, scrotum, epididymis, vas deferens, urethra, prostate gland, seminal vesicles, penis

– Female:• Ovary, uterine tube, uterus,

cervix, vagina, mammary glands

• Functions:– Production of offspring

Stayin’ Alive

• Your body has about 100 trillion cells in it.• For your life to NOT end abruptly, these cells

need to have the correct amount of:• Oxygen

• Nutrients

• Waste removal

• Heat

• Ions (sodium, calcium, etc.)

• Lots of other stuff

The Cell’s Environment

• In order to keep the right amount of stuff in the cell, we’ve got to make sure that all the fluid surrounding our cells (i.e., the extracellular fluid) has the right assortment of nutrients, ions, etc.

• We keep both our cells and the fluid surrounding our cells in a dynamically stable environment via a process called HOMEOSTASIS.

Homeostasis

• Defined as the body’s ability to maintain stable internal conditions in spite of the changing external conditions.

• Our body needs to have the right amount of stuff (i.e., temperature, blood glucose, pH etc.) at all times in order to function properly.

• First, let’s refer to all this stuff as “different variables”

Let’s use a thermostat as an example

• In order to keep the temperature in my house at the right level, the thermostat must first measure the current temperature in the house.

• After the thermostat measures the temperature, it compares the current value to a preset standard value. – If there is no difference then there’s nothing to do.– However, if it’s too hot or too cold, the thermostat has

to send a signal to the furnace or air conditioner to change the temperature of the house so that it equals the standard value.

Let’s clarify some stuff.

• In the previous example we had a:1. Variable temperature2. Measuring implement thermostat3. Control center also the thermostat4. A preset or standard value for the variable5. Effectors the air conditioner and furnace

• Similar situations arise in the human body where there are lots of variables that we want to maintain at certain precise levels

Blood Pressure

• BP is a variable that we’ve got to maintain at a certain level

• We have sensory receptors that measure the BP in the body. They’re located in the aorta (the big blood vessel coming out of the heart) and in the carotid arteries (the large vessels that bring blood to the brain).

• These pressure receptors measure BP and then send the info (we can call this input) to a control center in the brain – the particular BP control center is in the medulla oblongata of the brain

Blood Pressure

• We call the connection btwn the receptor and the control center the afferent pathway.

• In the control center, the input BP is compared with a set value.

• If there is a difference between the current BP value and the reference BP value then we’ve got an error.

• And we’ve got to fix that error!

Blood Pressure

• The control center will signal effector organs – such as the heart in this case – to alter their activity. This process is called output.

• The connection between the control center and the effector organ is called the efferent pathway.

Blood Pressure

• Suppose the current BP is too high.• The effector must act in a way to decrease it – so

the medulla oblongata (the control center) would signal the heart to decrease the force and rate of its contractions; this would decrease BP.

• Notice that the original stimulus was an Increase in BP and the body’s response was to act so as to Decrease BP.

• The stimulus is opposite the response!

Negative Feedback

• The movement of a variable in one direction causes the body to enact processes that cause the variable to move in the opposite direction (so as to return the value to the correct level) – we call it negative feedback

• Let’s look at BP again:

Increased BP

Sensed by pressure receptors in aortic arch and carotid sinus

Input sent via afferent pathway to medulla oblongata

Current BP compared with set point and error signal generated

Output sent along efferent pathway to heart and blood vessels

Heart rate & force of contraction decrease

Blood vessel diameter increases

BP DECREASES

Why is Negative Feedback so common in the body?

• Every time a variable starts changing too much, we’ve got to bring it back to normal. We’ve got to counteract its change.

• THAT’S NEGATIVE FEEDBACK

• Other examples you will encounter:– Maintenance of blood [Ca2+], blood [Glucose],

blood pH, and many others

When does a negative feedback process end?

• A negative feedback process begins when a particular variable leaves its homeostatic range.

• The process ends when that variable is back within its normal range.

• Negative feedback processes (or loops) are self-terminating.

Homeostasis is Important!

• Most of the physiological processes that occur in your body are designed to maintain homeostasis.

What this means is that the homeostatic variables are NOT kept rigidly fixed upon a single value. They are kept within a certain range, and when they exit that range – that’s when negative feedback loops turn on to bring them back.

What about Positive Feedback?

• Positive feedback occurs when the response magnifies the stimulus that produced it.

• In other words, a variable is altered and then the body’s response alters that variable even more in the same direction.

Positive Feedbackin Blood Clotting

Dangerous Positive Feedback

Rise in body temperature

Increase in body metabolism

Increase in body heat production

What stops a positive feedback loop?

Water, water everywhere!• About 60% of the human body is water• 2/3 of this water is found within your

cells so we refer to it as intracellular fluid (ICF)

• The other 1/3 is outside your cells so we call it extracellular fluid (ECF)

• The 2 main types of ECF are:1. The fluid that surrounds the cells – the tissue

fluid or interstitial fluid2. Blood!

• Minor types of ECF include cerebrospinal fluid and intraocular fluid