Lesson 1.1 · Unit 1 · HS-LS1-2
Two questions, and why the answer to one predicts the answer to the other
Anatomy asks what a structure is and where it sits. Physiology asks what it does and how. They are separated for convenience and they are not actually separable, because in a body the shape of a thing is the explanation for its job. That claim is the single most useful tool in this course, and it will save you more memorization than any study technique.
- Anatomy is the study of structure. Gross anatomy is what you can see without a microscope; microscopic anatomy covers tissues (histology) and cells (cytology). Developmental anatomy follows structure as it changes over a lifetime.
- Physiology is the study of function. It is usually named by the system it addresses: neurophysiology, cardiac physiology, renal physiology. It is the harder of the two, because a function is a process and you cannot point at it.
- Structure predicts function, and function explains structure. This is the complementarity principle. Bone is hard because it must resist compression. The bladder is lined with a tissue that stretches because it has to. A red blood cell has no nucleus because the space is worth more as cargo room.
- Use the principle in both directions. Given a structure you have never seen, you can often predict what it does. Given a function you need explained, you can predict what the structure must look like and then check whether it does.
- The levels of organization run chemical, cellular, tissue, organ, organ system, organism. Each level is built from the one below and has properties the level below does not. A protein does not contract; a muscle cell does.
- Eleven organ systems make up a human. Integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, reproductive. Every one of them appears in this course.
- Organ systems are not separate machines. Nearly every physiological question in this course is answered by two or more systems acting together, which is why the last three writing tasks all cross system boundaries.
Where students lose marks: answering a "why" question with a location. "Why can the small intestine absorb so much?" is not answered by "it is between the stomach and the large intestine." It is answered by surface area. Watch for questions that look anatomical and are physiological.
The problem. You are shown an unlabeled tissue under a microscope. The cells are flat, thin, arranged in a single layer, and the whole sheet is wrapped around a tube so narrow that red blood cells pass through it in single file. Predict what the structure does, without being told.
Step one: list the structural facts, separately. Flat cells. Single layer. Very thin overall. Tube diameter about the width of one red blood cell, which is roughly 7 to 8 micrometers. Do not interpret yet; get the observations down first.
Step two: ask what each feature costs. A single layer of flat cells is structurally weak. It tears easily and resists almost nothing. Whatever this structure does, it is not protection or support, because the design gives up both.
Step three: ask what each feature buys. Thin means short. Anything crossing this wall crosses a very small distance. That is the only obvious advantage a single flat layer has over any other arrangement.
Step four: name the process that rewards short distances. Diffusion. The rate at which a substance diffuses falls off sharply as the distance increases, so a structure built for exchange should be as thin as it can survive being.
Step five: check the second observation against that hypothesis. The tube is so narrow that red cells pass single file. That has the same consequence: every red cell is forced against the wall, so no cell is far from the exchange surface, and the cells are slowed, which gives more time to exchange. Both features point the same way.
Step six: state the prediction. This is an exchange vessel, a capillary, and the tissue is simple squamous epithelium. It should be found wherever something must cross quickly between blood and tissue, and it should never be found where the wall must take abuse.
Step seven: test the prediction against where it actually appears. Simple squamous epithelium lines capillaries, the alveoli of the lungs and the filtration membrane of the kidney, which are the three great exchange surfaces in the body. It does not line the esophagus, the skin or the vagina, all of which take mechanical abuse and are lined with stratified squamous epithelium instead. The prediction holds in both directions, which is the test that matters.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Distinguish anatomy from physiology in one sentence each.
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Anatomy is the study of the structure of the body and its parts; physiology is the study of how those parts function
- Name the levels of organization in order from simplest to most complex.
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Chemical, cellular, tissue, organ, organ system, organism
- State the principle of complementarity of structure and function.
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What a structure can do is determined by its form, so structure predicts function and function explains structure
- How many organ systems are recognized in the human body?
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Eleven
- Name the branch of microscopic anatomy that studies tissues.
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Histology
- The walls of the heart's left ventricle are about three times thicker than those of
the right ventricle. Using the complementarity principle, predict the functional
difference this implies.
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Thicker muscle generates greater force and therefore greater pressure. If the two ventricles pump the same volume but one does so at much higher pressure, the thicker one must be pushing blood a much greater distance or against much greater resistance. The left ventricle supplies the entire body; the right supplies only the lungs, which sit next door. The left ventricle generates far higher pressure because it pumps to the whole body rather than just to the lungs
- A tissue must both stretch considerably and return to its original shape. Predict two
structural features it should have.
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Stretching requires that the cells or fibers be able to change shape or slide past one another rather than being locked in a rigid arrangement. Returning requires something that stores the energy of deformation and gives it back, which in the body means elastic fibers. Cells or fibers arranged so they can rearrange under load, plus elastic fibers in the matrix to restore the original shape
- Explain why the statement "the stomach digests food" is a physiological claim while
"the stomach lies in the upper left of the abdominal cavity" is an anatomical one.
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The first names a process the organ carries out over time, which is a function and cannot be seen by looking at a still organ. The second names position, which is structural and is established by observation alone. The first describes a function, the second describes structure
- A student claims the digestive system alone is responsible for getting nutrients to a
muscle cell. Identify what is missing.
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Digestion breaks food down and absorbs the products into the blood, but it does not move them anywhere. The cardiovascular system transports absorbed nutrients to the tissues, and the respiratory system supplies the oxygen the muscle needs to get energy out of them. This is the pattern for almost every question in this course: no single system completes a physiological task. The cardiovascular system must transport the absorbed nutrients, and the respiratory system must supply oxygen
- You are told a cell has an unusually large number of mitochondria. Predict what the
cell does, and name one cell type in the body that fits.
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Mitochondria produce ATP, so a cell packed with them has a high and sustained energy demand. That points to a cell doing continuous mechanical or transport work rather than sitting idle. Cardiac muscle fits: it contracts about once a second for a lifetime without rest. The cells of the kidney's proximal tubule also fit, because they run active transport pumps continuously. A cell with a high, continuous ATP demand, such as cardiac muscle or proximal tubule cells