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Week 1 · Chapter

Chapter 1 — Body Organization, Homeostasis & Anatomical Terminology

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 1 (use the language of the body correctly — the anatomical position, the directional-term pairs, the three body planes, and the body cavities and abdominal regions — distinguish anatomy from physiology, climb the levels of structural organization, list the functions and requirements of human life, and trace a homeostatic feedback loop through its receptor, control center, and effector, telling negative feedback from positive feedback) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

This course studies the human body two ways at once: what each part is and where it sits, and how that part works to keep you alive. The first tool this week is homeostasis (the body keeping its inside steady while the outside keeps changing), run by small loops that sense a change and push back against it. The second tool is a shared map of the body, starting from one standard pose, the anatomical position (standing upright, palms facing forward). Direction words, cutting planes, and body spaces then let two people point to the same spot without confusion. By the end of the week you can read a temperature chart as a loop and describe any spot on the body so that someone else finds it.

Why you already care about this

Step out of a warm house into a cold morning. Within a minute your skin prickles into goosebumps, and if you stay out long enough you start to shiver. Nobody told your body to do that. Now picture the opposite: a long uphill hike on a hot afternoon, your shirt soaked through with sweat. Two weather extremes, two different responses, one goal. In both cases your body pushed its temperature back toward about 37 °C. On one illustrative clinic chart made for this course, three readings sit at 36.0 °C, 37.0 °C, and 38.5 °C, and by Section 5 you will read each one as a loop.

Now picture a clinic. A clinician writes on a chart that a scrape sits "on the left forearm, proximal to the wrist." Another clinician, who never saw the injury, reads the chart and finds the exact spot on the first try. That only works because both of them read the body from the same pose and use the same words for direction.

This chapter teaches both of those skills: reading the body's steady-state loops, and speaking the body's address language. Every system you meet later in this course, from bone to nerve to the senses, is described in this language and held steady by loops like these.

By the end of this chapter you will be able to:

  • tell anatomy from physiology and read a structure in the order structure, then function, then the clinical connection;
  • list the seven characteristics of life this course uses and the requirements for staying alive;
  • put the levels of structural organization in order and name the eleven organ systems;
  • trace a feedback loop through its receptor, control center, and effector, and compute how far a reading sits from its set point, with every step shown;
  • sort a loop as negative or positive feedback;
  • describe a location with the anatomical position, the five directional pairs, the three planes, and the body cavities and abdominal regions.

Carry this line with you: the receptor senses; the control center compares; the effector responds.

Where we are

  • This week: anatomy versus physiology, the characteristics and requirements of life, the levels of organization, homeostasis and feedback loops, and the language of position, direction, planes, and cavities.
  • How they connect: this chapter is the foundation; every later week uses the loop and the map you build here.
  • Where we're going next: Week 2, The Chemistry of Life, zooms in to the bottom rung of the ladder, the chemical level, and shows homeostasis at work again in the blood's pH.

This is the first week of the course, so nothing earlier is assumed. You need high-school biology and the arithmetic of subtracting one decimal from another. If a word is new, this chapter defines it the first time it appears. Next week you will meet atoms and molecules, the first rung of the ladder in Section 3.

1. Anatomy and physiology: two views of one body

Think about a pair of scissors. You can describe them two ways. You can say what they are: two blades, a pivot screw, two finger loops. Or you can say what they do: the blades slide past each other and cut. Both descriptions are about the same object. And the second depends on the first, because the blades cut only because of how they are shaped and joined.

The body works the same way, and the two descriptions have names.

  • Anatomy is the study of the body's structures: what the parts are, how each one is built, and where it sits.
  • Physiology is the study of how the body's parts work, alone and together, to keep a person alive.

These are not two separate subjects. They are two views of one thing. This course's spine fits in four words: structure determines function. A part's shape is tied to the job it does.

You will read every structure in this course in the same fixed order: structure, then function, then the clinical connection. First you say what the part is and where it sits. Then you say what it does and how its shape makes that possible. Then, in later weeks, you name one condition that shows the pairing in plain clinical terms, and you stop there. This course never gives medical advice.

Worked example — reading a red blood cell. Read one tiny structure in the fixed order.

  1. Structure. A red blood cell is a flat, flexible disc.
  2. Function. Its job is to carry oxygen through the body.
  3. How the shape makes the job possible. Because the disc is flat and flexible, it can squeeze through a capillary, a very narrow blood vessel, and deliver its oxygen.

In words: the cell's flat, flexible shape is what lets it reach tight spaces and carry oxygen there. That is structure determining function.

Anatomists also sort structures by size. Gross anatomy is the study of structures large enough to see with the unaided eye, such as a bone or the heart. Microscopic anatomy is the study of structures you can see only with a microscope or other magnification, such as cells and tissues. A red blood cell belongs to microscopic anatomy; the thigh bone belongs to gross anatomy.

Misconception check. Students often think, "Anatomy and physiology are two separate subjects, and I can learn one without the other." It is tempting because the two words sound like two courses. But every function in this course depends on a structure, and every structure is explained by what it does. The cure is to ask the second question every time you name a part: what does it do, and how does its shape let it do that?

Memory hook: name the part; then ask what it does.

2. What life does and what it needs

Picture a puppy. In a single afternoon it eats, grows a little, chases a ball, startles at a door, and naps. Those everyday behaviors map onto a list biologists use to describe life. Conventions differ on exactly how the list is written. This course uses seven.

The characteristics of life are the seven properties this course uses to describe a living body:

  1. Organization. The body is built from parts arranged in an orderly way.
  2. Metabolism. Metabolism is all of the body's building-up (anabolic) and breaking-down (catabolic) chemical reactions taken together.
  3. Responsiveness. The body adjusts to changes inside and outside it.
  4. Movement. The body and its parts move.
  5. Development. The body changes over its lifetime.
  6. Growth. The body increases in size.
  7. Reproduction. The body can make new individuals.

Staying alive also has requirements. The body needs:

  • oxygen — and the air around you is only about 20% oxygen;
  • nutrients and water — water alone is up to about 60% of an adult's body mass;
  • a body temperature held in a narrow band near 37 °C;
  • suitable atmospheric pressure, the pressure of the air around the body.

Worked example — matching behavior to a characteristic. Match each moment from an ordinary day to the characteristic it shows.

  1. Your stomach breaks down lunch into small molecules. That is a breaking-down reaction, so it is metabolism.
  2. You pull your hand back from a hot pan. That is adjusting to a change outside the body, so it is responsiveness.
  3. A child's shoe size goes up over a year. That is an increase in body size, so it is growth.

In words: each everyday moment points to one item on the list, and a living body shows all seven.

Misconception check. Students often think the list is a fixed law, so a different source with eight or six items "must be wrong." Lists differ because authors group the same ideas in different ways. The honest caveat is that this course has chosen one convention, the seven above, and uses it every week. Learn these seven, and expect other sources to slice them differently.

3. Levels of organization: build up from chemical to organism

Think about a school band. One trumpet player can play a note. A full section can play a chord. Only the whole band can play the whole song. Each step up adds something the step below could not do alone.

The body is built the same way. The levels of structural organization, from smallest to largest, are:

chemical → cellular → tissue → organ → organ system → organism

Read the arrow as "builds up to." Atoms and molecules form the chemical level. They build cells. A tissue is a group of similar cells working together on one job. Tissues build an organ, such as the heart. An organ system is a group of organs that work together on a shared job. All the organ systems together make one organism, a whole person.

At each step, a new capability appears. A single heart cell can twitch. Only the whole organ, the heart, can pump.

This course names eleven organ systems: integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive. Conventions for grouping the systems differ, so other sources may count them a little differently.

Worked example — place it on the ladder. Put these in order from smallest to largest: the heart, a heart cell, a water molecule, a whole person, the cardiovascular system, and a sheet of heart-muscle tissue.

  1. A water molecule is at the chemical level.
  2. A heart cell is at the cellular level.
  3. A sheet of heart-muscle tissue is at the tissue level.
  4. The heart is at the organ level.
  5. The cardiovascular system is at the organ-system level.
  6. A whole person is at the organism level.

In words: the order is water molecule → heart cell → heart-muscle tissue → heart → cardiovascular system → person.

Misconception check. Students often swap the tissue and organ levels and say "the heart is a tissue." It is tempting because the heart is made of tissue. The cure is the band rule: an organ is several tissues working as one structure, the way a band is several sections playing one song. The heart contains tissues; it is an organ.

4. Trace the three parts of a feedback loop

Picture the thermostat in a home. You set it to a target. A sensor in the wall reads the room. A small computer compares the reading with your target. If the room is too cold, it switches on the heater; once the room warms up, it switches the heater off. That is a classroom model of how your body holds itself steady, and it is a good one to keep in mind.

Homeostasis is the maintenance of a stable internal environment, so that conditions inside the body stay steady while the outside changes. Two words describe the target:

  • A set point is the target value a regulated variable is held near. For body temperature, the set point is about 37 °C.
  • A normal range is the narrow band of values around a set point in which the body works best.

Every loop that holds a variable steady has three parts, always in this order:

  1. A receptor senses the current value and reports it.
  2. A control center compares the receptor's report with the set point and its normal range, and sends out a response.
  3. An effector acts on that response and reverses the change.

In one line: receptor → control center → effector.

Worked example — naming the parts of a home thermostat. Label each part of the home heating loop with its body word.

  1. The wall sensor that reads the room is the receptor.
  2. The small computer that compares the reading with the target is the control center.
  3. The heater that warms the room is the effector.
  4. The temperature you chose is the set point.

In words: the home heating loop has the same three parts in the same order as a body loop, which is why the thermostat is a useful model. The limitation is that a thermostat has one effector, while the body often uses several at once.

Misconception check. Students often think the control center is the part that senses the change. It is tempting because the control center seems to be "in charge." But sensing is the receptor's job. The control center only compares, and the effector only responds. The cure is the carry line: the receptor senses; the control center compares; the effector responds.

5. Read the body-temperature loop on a clinic chart

Now run the loop on real body language. The readings below are illustrative, constructed for this course; they are not real clinical data. Picture a clinic chart with three temperature readings on it. The set point is 37.0 °C. The chart uses a model band of 36.5 to 37.5 °C as its normal range. The band sits 37.0 − 36.5 = 0.5 °C below the set point and 37.5 − 37.0 = 0.5 °C above it.

To read each reading, you take three moves: find the gap from the set point, place the reading against the band, and name the loop's response.

Three illustrative temperature readings on one clinic chart, in degrees Celsius, read against a 37.0 set point and a 36.5 to 37.5 model band
Reading Gap from the set point Against the band Effectors that respond
36.0 °C 1.0 °C below below the band vasoconstriction, shivering, goosebumps
37.0 °C 0.0 °C inside the band none needed
38.5 °C 1.5 °C above above the band sweat that evaporates, vasodilation

Worked example — reading the low value. Read the 36.0 °C reading.

  1. Find the gap. The reading is below the set point, so subtract it from the set point. 37.0 − 36.0 = 1.0 °C below.
  2. Place it. 36.0 < 36.5, so the reading is below the band.
  3. Name the response. The receptors report cold. The control center compares the report with 37.0 °C. The effectors respond with vasoconstriction (the dermal arterioles, small blood vessels in the skin, narrow to conserve heat), shivering, and goosebumps.

In words: the reading is 1.0 °C too low, so the body conserves and makes heat, and the temperature rises back toward 37.0 °C.

Worked example — reading the middle value. Read the 37.0 °C reading: 37.0 − 37.0 = 0.0 °C. The reading sits at the set point and inside the band, so nothing needs correcting.

Worked example — reading the high value. Read the 38.5 °C reading.

  1. Find the gap. The reading is above the set point, so subtract the set point from it. 38.5 − 37.0 = 1.5 °C above.
  2. Place it. 38.5 > 37.5, so the reading is above the band.
  3. Name the response. The receptors report heat. The control center compares the report with 37.0 °C. The effectors respond with sweat that evaporates and vasodilation (the dermal arterioles widen to shed heat).

In words: the reading is 1.5 °C too high, so the body sheds heat, and the temperature falls back toward 37.0 °C.

Look at all three readings together. When the temperature fell, the response raised it. When it rose, the response lowered it. In every case the response opposed the change. That is the defining feature of the loop type in the next section.

One honesty note about this chart: it is a teaching example, and the 36.5 to 37.5 °C band is a model band chosen for it. It is not a norm to hold any one person to.

Misconception check. Students often subtract in the wrong direction and write the low reading as "−1.0 °C above." It is tempting because subtraction feels automatic. The cure is to say the direction in words first, "below" or "above," then subtract the smaller value from the larger one. The gap is a distance; the word tells you which side.

6. Compare negative and positive feedback

Picture two different kinds of story. In one, something goes wrong and gets fixed: a room gets cold and the heater brings it back. In the other, something starts and keeps building until it is finished: a snowball rolling downhill, growing as it goes, until it hits the bottom. Feedback loops come in these two kinds.

  • Negative feedback is a loop in which the response reverses a change and brings the variable back toward its set point. It runs most of physiology: body temperature, blood glucose, and blood pH are all held this way.
  • Positive feedback is a loop in which the response intensifies a change until an end event stops it.

The course's two positive-feedback examples:

  1. Childbirth. The hormone oxytocin drives contractions, and the contractions trigger the release of more oxytocin. The cycle builds until the end event, delivery.
  2. Blood clotting. Each clotting substance that is activated recruits more, until the clot forms.

The names mean direction, not good or bad. "Negative" means the response goes the opposite way from the change. "Positive" means it goes the same way.

Worked example — sort four scenarios. Decide whether each is negative or positive feedback.

  1. Blood sugar rises after a meal, and the body brings it back down. The response reverses the rise, so it is negative feedback.
  2. A cut on a finger clots. Each activated clotting substance recruits more until the clot forms, so it is positive feedback.
  3. A hiker overheats and starts sweating. The sweat cools the body and reverses the rise, so it is negative feedback.
  4. Labor contractions grow stronger until delivery. Oxytocin and contractions build on each other until an end event, so it is positive feedback.

In words: ask whether the response pushes back against the change or pushes it further. Pushing back is negative; pushing further, until an end event, is positive.

Misconception check. Students often think "negative feedback is bad and positive feedback is good." The words sound like grades. But negative feedback is the body's main way of staying healthy, and positive feedback can be helpful, as in clotting. The cure: read the names as directions, opposite or same, never as judgments.

Pause and think. Choose one:

  • (a) Describe one time you noticed your own body push back against a change, such as shivering or sweating. Which part of the loop did you notice: the receptor, the control center, or the effector?
  • (b) Describe one everyday machine that works like a negative-feedback loop. Name its receptor, control center, and effector.

7. Stand in the anatomical position and use direction words

Try this. Stand up, let your arms hang, and look at your hands. Most people's palms face their thighs. Now turn your palms forward. Look where your thumbs went: they now point away from your body. That small turn is the key to every direction word in this course.

The anatomical position is the standard reference pose for describing the body: standing upright, feet parallel and about shoulder-width apart, upper limbs at the sides, and palms facing forward. Every directional term is read from this pose, even if the person is lying down, sitting, or upside down.

The directional terms come in five pairs. Each pair is a set of opposites.

The five directional pairs read from the anatomical position, with one example of each
Pair Meaning Example
superior / inferior toward the head / toward the feet the head is superior to the neck
anterior / posterior toward the front / toward the back the nose is anterior to the ears
medial / lateral toward the midline / away from the midline the thumb is lateral to the other fingers
proximal / distal closer to / farther from where a limb attaches (limbs only) the elbow is proximal to the wrist
superficial / deep toward / away from the body's surface the skin is superficial to the bones

Two more rules make the pairs work. First, ventral is another word for anterior and dorsal is another word for posterior. Second, left and right are always the body's own left and right, never the viewer's. When you face someone, their left hand is on your right side.

Every directional statement names the two structures being compared. "The wrist is distal" is incomplete. "The wrist is distal to the elbow" is a full statement.

Worked example — the thumb. Is the thumb medial or lateral to the index finger?

  1. Put the body in the anatomical position: palms facing forward.
  2. Find the midline, the imaginary line down the center of the body.
  3. In this pose the thumb sits on the outer edge of the hand, farther from the midline than the index finger.

In words: the thumb is lateral to the index finger.

Worked example — the knee and the ankle. Use the pair that fits a limb. The knee is closer to where the leg attaches to the trunk, so the knee is proximal to the ankle, and the ankle is distal to the knee.

Misconception check. Students often say "the thumb is medial" because, with the arms relaxed, the thumb faces forward and feels like it is on the inside. The cure is to always turn the palms forward first. From the anatomical position, the thumb is lateral. A second common slip is using proximal and distal for the head or trunk; those two words are for limbs only. Use superior and inferior there instead.

Memory hook: palms forward first — the thumb is lateral, not medial.

8. Body planes: name each cut by its two parts

Picture a loaf of bread on a kitchen counter. You can slice it lengthwise down the middle into a left half and a right half. You can slice it so you have a front piece and a back piece. Or you can cut it across, into a top and a bottom. Anatomists slice the body, on paper and in scans, in the same three ways.

A plane is a flat cut through the body, and each one is named by the two parts it makes:

  • A sagittal plane is a vertical cut that divides the body into left and right parts. A midsagittal plane passes through the midline and makes equal halves.
  • A frontal plane, also called the coronal plane, is a vertical cut that divides the body into front (anterior) and back (posterior) parts.
  • A transverse plane is a horizontal cut that divides the body into top (superior) and bottom (inferior) parts. A single transverse cut gives a cross-section.

Worked example — choose the plane. Name the plane each cut uses.

  1. A cut that separates the right eye from the left eye. It makes left and right parts, so it is a sagittal plane.
  2. A cut that separates the face from the back of the head. It makes front and back parts, so it is a frontal plane.
  3. A cut through the waist that separates the chest from the hips. It makes top and bottom parts, so it is a transverse plane.

In words: name the two parts the cut leaves, and the plane follows.

Misconception check. Students often say "the sagittal plane separates front and back." It is tempting because "sagittal" sounds like it could mean anything. The cure: sagittal makes left and right; frontal makes front and back; transverse makes top and bottom. Say the two parts first, then the name.

9. Body cavities: find where each organ sits

Think of a house with two main wings. One wing at the back holds the most protected rooms. A larger wing at the front is split by a floor into an upstairs and a downstairs. The body's cavities, its large closed spaces, are laid out much like that.

The dorsal cavity is the body cavity at the back. It holds two spaces:

  • the cranial cavity, which holds the brain;
  • the vertebral cavity, which holds the spinal cord.

The ventral cavity is the body cavity at the front. The diaphragm is the divider that splits it into two parts:

  • the thoracic cavity, above the diaphragm, which holds two pleural cavities (which hold the lungs), the pericardial cavity (around the heart), and the mediastinum, the middle region between the two pleural cavities;
  • the abdominopelvic cavity, below the diaphragm.

Many organs in these cavities sit inside a serous membrane, a double membrane in a closed cavity. Its parietal layer lines the cavity wall, and its visceral layer covers the organ. You need these names for orientation only this week.

For clinical description, the abdomen is divided into four quadrants or nine regions. Together, the abdominal quadrants and regions are the clinic's address system. They let a clinician say where a sensation or finding is, the way a street address says where a house is. As everywhere in this course, any right or left in that address is the body's own, never the viewer's.

Worked example — find the cavity. Name the cavity for each structure and place it against the diaphragm.

  1. The heart. It sits in the pericardial cavity, inside the thoracic cavity, so it is superior to the diaphragm.
  2. The lungs. Each sits in a pleural cavity, inside the thoracic cavity, so they are superior to the diaphragm.
  3. The brain. It sits in the cranial cavity, part of the dorsal cavity.
  4. The spinal cord. It sits in the vertebral cavity, part of the dorsal cavity.

In words: the thoracic organs are above the diaphragm, and the brain and spinal cord are in the dorsal cavity.

Misconception check. Students often place the heart and lungs in the abdominal cavity, because the whole trunk feels like one big space. The cure is the diaphragm: the heart and lungs sit in the thoracic cavity, superior to the diaphragm. The abdominopelvic cavity starts below it.

Key terms

  • Anatomy — The study of the body's structures: what the parts are, how each one is built, and where it sits. (first used: 1. Anatomy and physiology: two views of one body)
  • Physiology — The study of how the body's parts work, alone and together, to keep a person alive. (first used: 1. Anatomy and physiology: two views of one body)
  • Gross anatomy — The study of body structures large enough to see with the unaided eye, such as a bone or the heart. (first used: 1. Anatomy and physiology: two views of one body)
  • Microscopic anatomy — The study of body structures that can be seen only with a microscope or other magnification, such as cells and tissues. (first used: 1. Anatomy and physiology: two views of one body)
  • Characteristics of life — The seven properties this course uses to describe a living body: it is organized, carries out metabolism, responds to change, moves, develops, grows, and reproduces. (first used: 2. What life does and what it needs)
  • Metabolism — All of the body's building-up (anabolic) and breaking-down (catabolic) chemical reactions taken together. (first used: 2. What life does and what it needs)
  • Levels of structural organization — The ordered ladder used to place any body structure, from smallest to largest: chemical, cellular, tissue, organ, organ system, and organism. (first used: 3. Levels of organization: build up from chemical to organism)
  • Tissue — A group of similar cells working together on one job. (first used: 3. Levels of organization: build up from chemical to organism)
  • Organ system — A group of organs that work together on a shared job, such as the skeletal or nervous system; this course names eleven of them. (first used: 3. Levels of organization: build up from chemical to organism)
  • Homeostasis — The maintenance of a stable internal environment, so that conditions inside the body stay steady while the outside changes. (first used: 4. Trace the three parts of a feedback loop)
  • Set point — The target value a regulated variable is held near; for body temperature it is about 37 degrees Celsius. (first used: 4. Trace the three parts of a feedback loop)
  • Normal range — The narrow band of values around a set point in which the body works best. (first used: 4. Trace the three parts of a feedback loop)
  • Receptor — The part of a feedback loop that senses the current value of a regulated variable and reports it. (first used: 4. Trace the three parts of a feedback loop)
  • Control center — The part of a feedback loop that compares the receptor's report with the set point and its normal range and sends out a response. (first used: 4. Trace the three parts of a feedback loop)
  • Effector — The part of a feedback loop that acts on the control center's signal to change the regulated variable. (first used: 4. Trace the three parts of a feedback loop)
  • Negative feedback — A feedback loop in which the response reverses a change and brings the variable back toward its set point; it runs most of physiology. (first used: 6. Compare negative and positive feedback)
  • Positive feedback — A feedback loop in which the response intensifies a change until an end event stops it, as in childbirth or blood clotting. (first used: 6. Compare negative and positive feedback)
  • Anatomical position — The standard reference pose for describing the body: standing upright, feet parallel and about shoulder-width apart, upper limbs at the sides, and palms facing forward. (first used: 7. Stand in the anatomical position and use direction words)
  • Superior/inferior — A directional pair meaning toward the head (superior) and toward the feet (inferior). (first used: 7. Stand in the anatomical position and use direction words)
  • Anterior/posterior — A directional pair meaning toward the front (anterior, or ventral) and toward the back (posterior, or dorsal). (first used: 7. Stand in the anatomical position and use direction words)
  • Medial/lateral — A directional pair meaning toward the body's midline (medial) and away from the midline (lateral). (first used: 7. Stand in the anatomical position and use direction words)
  • Proximal/distal — A directional pair, used for limbs only, meaning closer to (proximal) or farther from (distal) the point where the limb attaches to the trunk. (first used: 7. Stand in the anatomical position and use direction words)
  • Superficial/deep — A directional pair meaning toward the body's surface (superficial) and away from the surface (deep). (first used: 7. Stand in the anatomical position and use direction words)
  • Sagittal plane — A vertical cut that divides the body into left and right parts; a midsagittal plane passes through the midline. (first used: 8. Body planes: name each cut by its two parts)
  • Frontal plane — A vertical cut, also called the coronal plane, that divides the body into front (anterior) and back (posterior) parts. (first used: 8. Body planes: name each cut by its two parts)
  • Transverse plane — A horizontal cut that divides the body into top (superior) and bottom (inferior) parts, producing a cross-section. (first used: 8. Body planes: name each cut by its two parts)
  • Dorsal cavity — The body cavity at the back that holds the cranial cavity, around the brain, and the vertebral cavity, around the spinal cord. (first used: 9. Body cavities: find where each organ sits)
  • Ventral cavity — The body cavity at the front, split by the diaphragm into the thoracic cavity above and the abdominopelvic cavity below. (first used: 9. Body cavities: find where each organ sits)
  • Diaphragm — The divider that splits the ventral cavity into the thoracic cavity above it and the abdominopelvic cavity below it. (first used: 9. Body cavities: find where each organ sits)
  • Mediastinum — The middle region of the thoracic cavity, lying between the two pleural cavities. (first used: 9. Body cavities: find where each organ sits)
  • Serous membrane — A double membrane in a closed body cavity, with a parietal layer lining the cavity wall and a visceral layer covering the organ. (first used: 9. Body cavities: find where each organ sits)
  • Abdominal quadrants and regions — The address system that divides the abdomen into four quadrants or nine regions so a location can be described precisely. (first used: 9. Body cavities: find where each organ sits)

Summary

  • Anatomy is structure and physiology is function: two views of one body.
  • Read every structure in order: structure, then function, then the clinical connection.
  • This course uses seven characteristics of life; other sources list them differently.
  • Life needs oxygen, nutrients and water, a narrow temperature band near 37 degrees Celsius, and suitable air pressure.
  • The levels run chemical, cellular, tissue, organ, organ system, organism.
  • Every loop runs receptor, then control center, then effector.
  • Negative feedback reverses a change; positive feedback builds it until an end event.
  • The names mean direction, not good or bad.
  • Read every direction from the anatomical position, palms forward; the thumb is lateral.
  • Sagittal makes left and right; frontal makes front and back; transverse makes top and bottom.
  • The diaphragm divides the thoracic cavity above from the abdominopelvic cavity below.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

  1. An illustrative aquarium heater is set to hold the water at 25 °C. A sensor reads the water at 23.0 °C, a small circuit compares the reading with the setting, and the heating coil switches on. Name the receptor, the control center, and the effector, and find how far the water sits from its setting.
  2. A runner's heart beats harder and the runner breathes faster during a sprint, and both return to their resting pace afterward. Is the return to resting pace an example of negative or positive feedback? Explain in one sentence.
  3. Complete each statement with the correct directional term: the hip is ___ to the knee; the navel is ___ to the spine; the ears are ___ to the nose.
  4. A scan shows the body cut so that the left half and the right half are equal. Which plane is this, and which cavity would the cut pass through at the level of the heart?

Answers (worked reasoning — read after attempting)

  1. The sensor that reads the water is the receptor. The circuit that compares the reading with the setting is the control center. The heating coil is the effector. The water is below its setting, so subtract the reading from the setting: 25 − 23.0 = 2.0 °C below. The coil warms the water back toward 25 °C, so the loop is negative feedback.
  2. It is negative feedback, because the response brings the heart rate and breathing back toward their resting values instead of pushing them further.
  3. The hip is proximal to the knee, because both are on the lower limb and the hip is closer to where the limb attaches to the trunk. The navel is anterior to the spine, because it sits toward the front. The ears are posterior to the nose, because they sit toward the back of the head.
  4. Equal left and right halves come from a midsagittal plane, the sagittal plane through the midline. At the level of the heart, that cut passes through the thoracic cavity, superior to the diaphragm.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week the chemical level, the first rung of the ladder, shows homeostasis at work again in the blood's pH rule.

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