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Anatomy is the subdiscipline of biology that studies the structure of the body. It describes (and labels in Latin) the morphology of the body: shape, size, color and position of various body parts, with particular attention to the internal organs, as visible by the naked eye. Histology is a subset of anatomy that describes what can be seen only under the misroscope: how cells are organized into tissues and tissues into organs. (Classical) embryology describes the way tissues and organs change their shape, size, color and position during development.
Anatomy provides the map and the tools for the study of the function of organs in the body. It describes (but does not explain) the structure of the body. Physiology further describes how the body functions, while evolutionary biology provides the explanation of the structure and the function.
While details of human anatomy are essential in the education of physicians and nurses (and animal anatomy for veterinarians), we do not have time, nor do we need to pay too much attention to fine anatomical detail. We will pick up on relevant anatomy as we discuss the function of organs: physiology.
There are traditionally two ways to study (and teach) physiology. The first approach is medical/biochemical. The body is subdivided into organ systems (e.g., respiratory, digestive, circulatory, etc.) and each system is studied separately, starting with the physiology of the whole organism and gradually going down to the level of organs, tissues, cells and molecules, ending with the biochemistry of the physiological function. Only the human body is studied. Often, pathologies and disorders are used to illustrate how organs work – just like fixing a car engine by replacing a broken part helps us understand how the engine normally works, so studying diseases helps us understand how the healthy human body works.
The other approach is ecological/energetic. The physiological functions are divided not by organ system, but by the problem – imposed by the environment – that the body needs to solve in order to survive and reproduce, e.g., the problem of thermoregulation (body temperature), osmoregulation (salt/water balance), locomotion (movement), stress response, etc., each problem utilizing multiple organ systems. Important aspect of this approach is the study of the way the body utilizes energy: is the solution energetically optimal? Individuals that have solved a problem with a more energy-efficient physiological mechanism will be favoured by natural selection – thus this approach is also deeply rooted in an evolutionary context. Finally, this approach is very comparative – study of animals that live in particularly unusual or harsh environments helps us understand the origin and evolution of physiological mechanims both in uhmans and in other animals.
The textbook is unusually good (for an Introductory Biology textbook) in trying to bridge and combine both approaches. Unfortunately, we do not have enough time to cover all of the systems and all of the problems in detail, so we will stick to the first, medical approach and cover just a few of the systems of the human body, but I urge you to read the relevant textbook chapters in order to understand the ecological and evolutionary aspects of physiology as well (not to mention some really cool examples of problem-solving by animal bodies). Hint: use the “Self Test” questions at the end of each chapter and if you answer them correctly, you are ready for the exam.
Let’s start out by looking at a couple of important basic principles that pertain to all of physiology. One such principle is that of scaling, for which you should read the handout that we will discuss in class next time. The second important principle in physiology is the phenomenon of feedback loops: both negative and positive feedback loops.
Negative feedback loop works in a way very similar to the graph we drew when we discussed behavior. The body has a Sensor that monitors the state of the body – the internal environment (as opposed to external environment we talked about when discussing behavior), e.g,. the blood levels of oxygen and carbon dioxide, blood pressure, tension in the muscles, etc. If something in the internal environment changes from the normal, optimal values, the sensor informs the Integrator (usually the nervous system) which initiates action (via an Effector) to bring back the body back to its normal state.
Thus, an event A leads to response B which leads to the countering and elimination of the event A. Almost every function in the body operates like a negative feedback loop. For instance, if a hormone is secreted, along with the functional effect of that hormone, there will also be a trigger of a negative feedback loop that will stop the further secretion of that hormone.
There are very few functions in the body that follow a different pattern – the positive feedback loop. There, an event A leads to response B which leads to re-initiation and intensification of the event A which leads to a stronger response B…and so on, until a treshold is reached or the final goal is accomplished, when everything goes abruptly back to normal.
We will take a look at an example of the positive feedback loop that happens in the nervous system next week. For now, let’s list some other notable positive feedback loops in humans.
First, the blood clotting mechanism is a cascade of biochemical reactions that operates acording to this principle. An injury stimulates production of a molecule that triggers production of another molecule which triggers production of another molecule as well as production of more of the first molecule, and so on, until the injury has completely closed.
Childbirth is another example of the positive feedback loop. When the baby is ready to go out (and there’s no stopping it at this point!), it releases a hormone that triggers the first contraction of the uterus. The contraction of the uterus pushes the baby out a little. That movement of the baby stretches the wall of the uterus. The wall of the uterus contains stretch receptors which send signals to the brain. In response to the signal, the brain (actually the posterior portion of the pituitary gland, which is an outgrowth of the brain) releases hormone oxytocin. Oxytocin gets into the bloodstream and reaches the uterus triggering the next contraction which, in turn, moves the baby which further stretches the wall of the uterus, which results in more release of oxytocin…and so on, until the baby is expelled, when everything returns to normal.
Next example of the positive feedback loop is also related to babies – nursing. When the infant is hungry, mother brings its mouth to the nipple of the breast. When the baby latches onto the nipple and tries to suck, this stimulates the receptors in the nipple which notify the brain. The brain releases hormone oxytocin from the posterior pituitary gland. Oxytocin gets into the bloodstream and stumulates the mammary gland to release milk (not to synthetize milk – it is already stored in the breats). Release of milk at the nipple stimulates the baby to start suckling vigorously, which stimulates the receptors in the nipple even more, so there is even more oxytocin released from the pituitary and even more milk is released by the mammary gland, and so on, until the baby is satiated and unlatches from the breast, when everything goes back to normal.
Next example of the positive feedback loop is also related to babies, but nine months earlier. Copulation – yes, having sex – is an example of a positive feedback loop, both in females and in males. Initial stimulation of the genitals stimulates the touch receptors which notify the brain which, in turn, stimulates continuation (and gradual speeding up) of movement, which provides further tactile stimulation, and so on, until the orgasm, after which everything goes back to normal (afterglow nothwithstanding).
The last example also applies to the nether regions of the body. Micturition (urination) is also a positive feedback loop. The wall of the urinary bladder is built in such a way that there are several layers of cells. As the bladder fills up, the wall stretches and these cells move around until the wall is only a single cell thick. At this point, urination is inevitable (cannot be stopped by voluntary control). Beginning of the urination starts the movement of the cells back from single-layer state to multi-layer state. This contracts the bladder further which forces urine out even more which contracts the wall of the bladder even more, and so on until the bladder is completely empty again and everything goes back to normal.
The concept of feedback loops is essential for the understanding of the principle of homeostasis. Homeostatic mechanisms ensure that the internal environment remains constant and all the parameters are kept at their optimal levels (e.g,. temperature, pH, salt/water balance, etc) over time. If a change in the environment (e.g., exposure to heat or cold) results in the change of internal body temperature, this is sensed by thermoreceptors in the body. This triggers corrective mechanisms: if the body is overheated, the capillaries in the skin expand and radiate heat and the sweat gland release sweat; if the body is too cold, the capillaries in the skin contract, the muscles start shivering, the hairs stand up (goosebumps), and the thyroid hormones are released, resulting in opening of poers in the membranes of mitochondria in the muscles, thus reducing the effciency of the break-down of glucose to water and carbon-dioxide, thus producing excess heat. Either way, the body temperature will be returned to its optimal level (around 37 degrees Celsius), which is called the set-point for body temperature. Each aspect of the internal environment has its own set-point which is defended by homeostatic mechanisms.
While essentially correct, there is a problem with the concept of homeostasis. One of the problems with the term “homeostasis” is linguistic: the very term homeostasis is misleading. “Homeo” means ‘similar, same’ and “stasis” means ‘stability’. Thus, the word homeostasis (coined by Walter Cannon in the early 20th century) suggests strong and absolute constancy. Imagine that you were told to draw a graphical representation of the concept of homeostasis in 10 seconds. Without sufficient time to think, you would probably draw something like this:
The main characteristic of this graph is that the set-point is constant over time. But that is not how it works in the real world. The graph above is correct only if the time-scale (on the X-axis) spans only seconds to minutes. If it is expanded to hours, days or years, the graph would be erroneous – the line would not be straight and horizontal any more. The set point changes in a predictable and well-controlled manner. For instance, the set-point for testosterone levels in the blood in human males over the course of a lifetime may look like this:
That would be an example of developmental control of a set-point. At each point in time, that set-point is defended by homeostatic mechanisms, but the set-point value is itself controlled by other physiological processes. Another example of controlled change of a set-point may look like this:
This would be an example of an oscillatory control of a set-point. In the early 1980s, Nicholas Mrosovsky coined a new term to replace ‘homeostasis’ and specifically to denote controlled changes in set-points of all biochemical, physiological and behavioral values – rheostasis.
Almost every aspect of physiology (and behavior) exhibits rheostasis, both developmental and oscillatory (daily and/or yearly rhtyhms). Some notable exceptions are blood pH (which has to be kept within very narrow range 7.35-7.45) and blood levels of Calcium. If pH or Calcium levels move too far away from the optimal value, cells in the body (most notably nerve cells, muscles and heart cells) cannot function properly and the body is in danger of immediate death.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 31.
“Medicine Needs Evolution” by Nesse, Stearns and Omenn (http://www.sciencemag.org/cgi/content/summary/311/5764/1071)
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