I was recently notified that one of my YouTube videos had been “age restricted.” In the video, I dissect a sheep’s brain and describe all of the relevant structures of the brain. I posted the video years ago for my online students to watch. I never made any money off of the video and I never intended to. It was for educational purposes.
Age restriction means that only adults who are signed in can watch the video. Adults who are not signed in (as well as children) cannot view the video. This is done in an effort to protect children from objectionable material… Which is great on paper, but in practice?
Is there any world in which it is reasonable to restrict basic knowledge? Is a sheep’s brain really so offensive? If your kids aren’t ready to see a sheep’s brain, wait until you tell them what’s in lamb chops.
Honestly, I cannot fathom how anyone came to the conclusion that it was best to prevent children from learning the structures of the brain. Recently, my daughter has started watching Lanky Box and SSSniper Wolf on YouTube. Their respective brands of idiocy are far more objectionable, in my opinion as a parent.
So, we’ve just described the cell’s Plasma Membrane (PM) and how it separates the water inside a cell from the water outside a cell. But no
man cell is an island. Except pancreatic islet cells, which is a joke we won’t cover until A&P 2.
Obviously we’re going to need to import nutrients and export waste products. How do we get things into and out of cells?
What separates passive from active transport is energy. Passive transport does not require energy, while active transport does require energy. Energy is provided by a molecule called ATP. We’ll get more into what ATP is in a later post, but for now, just know that it provides the energy for active transport.
To understand passive transport, you have to understand Diffusion. Basically, if things are highly concentrated in one area, they will tend to spread out to areas where they are less concentrated.
The example I always use is a shoe box with a bunch of marbles on one side. If you gently shake the box, the marbles will bounce into each other and spread out to fill the rest of the box. When the marbles are equally spread out, we would say they are at equilibrium.
Molecules are no different. When they’re highly concentrated in one area, they keep bouncing off of each other until they spread into areas with fewer of that molecule in it. We can see that happening here:
As you can see, the food dye is highly concentrated around the skittles. It dissolves in the water and spreads into new areas, where the concentration of dye is lower. Molecules will always move along their concentration gradient, which again, means from areas of high concentration to areas of low concentration.
Now that we understand diffusion, let’s jump in with Passive Transport methods! Passive transport doesn’t require energy because molecules are following their concentration gradient. Some molecules are able to freely cross the Plasma Membrane. We would say these molecules are membrane permeable. Membrane permeable molecules must be small and have no charge. They must be lipophilic, meaning they love the lipid interior of the Plasma Membrane. Examples include O2, CO2, ethanol (drinking alcohol), and steroid hormones. These molecules just follow their concentration gradient and diffuse into or out of the cell.
Most molecules are not membrane permeable. In fact, we would say they are membrane impermeable. Molecules that are large, charged, or both cannot cross the Plasma Membrane. We still need to get some of these molecules into the cell. To do this, we can insert a channel in the membrane, which lets them through. When we use a channel to allow substances to cross the Plasma Membrane, we call it Facilitated Diffusion because the diffusion was facilitated by a protein channel.
For example, remember that water has a partial charge. Therefore, water cannot cross the plasma membrane. We can let water into or out of a cell using protein channels called aquaporins. In some cells (ex: certain cells in the Renal Tubules) we don’t water to cross the plasma membrane, so we can remove the aquaporins in the membrane.
Other examples of molecules that require facilitated diffusion include ions (Sodium, Potassium, Chloride, Calcium) and even glucose. When our blood-sugar levels rise, we release the hormone insulin. Insulin causes glucose channels to be inserted in the Plasma Membrane, allowing glucose to diffuse into the cell. Type II Diabetes is an insensitivity to insulin — the cells don’t respond to insulin, so the channels don’t get inserted into the membrane and glucose can’t cross the PM. The blood-sugar stays high while the cells starve and cannibalize themselves.
As you can see, passive transport relies heavily on diffusion. Next time, we’ll pick it up with Active Transport, which will require energy (and more proteins!)
We usually start this topic by saying that the cell is “the smallest functional unit of life.” This means that nothing smaller than a cell is considered to be alive. There are a few criteria for determining if something is alive, including whether or not it can replicate and carry out its own metabolism. Viruses can’t do either of those things, so they technically aren’t alive.
But that does nothing to explain what a cell really is. We all know that the body is filled with water. And that cells are filled with water. So, how do you separate the water inside a cell from the water outside of a cell? It all starts with the Plasma Membrane or PM. You may see some textbooks call this the Cell Membrane.
You already know that oil and water don’t mix. If you haven’t tried mixing them before, just look at a bottle of vinaigrette salad dressing. The oil and the water separate out every time. This is because water is polar and has a slight charge, while the oil is neutral and has no charge.
The Plasma Membrane is made up of lipids, somewhat similar to those in oils. These lipids are called phospholipids. Phospholipids have a negatively charged head and a neutral tail. This means that like a compass, the head is always going to be drawn toward water and the tail will move away from water.
What does that mean? When these lipids are added to water, they will naturally form a layer where their heads are facing out (toward the water) and their tails are facing in (toward each other).
Plasma Membranes are more than just lipids, though. Proteins are essential to the stability and function of the PM. Some textbooks will tell you that the PM is 80% lipid and 20% protein. Here, they’re counting the number of molecules in the membrane. Others will tell you that the PM is 50% lipid and 50% protein. This is measured by weight. Proteins are fewer in number, but they’re bigger and heavier than lipids.
The main job of the Plasma Membrane is to control what gets into or out of the cell. Yes, it serves as a barrier between two different environments, but it also serves to move things from one area to another. For example, some bacteria have drug efflux pumps in their Plasma Membrane, which pump antibiotics out of the cell and make them resistant to that drug. We’ll talk more about how Human cells handle transport in the next post.
In this post, I’m only going to cover information that is relevant to people studying Anatomy & Physiology or Biology. Personally, I’m in a long-term love-hate relationship with Chemistry.
So, last time we discussed the atom. Why? Because atoms make up molecules and molecules are where it gets interesting. But what are molecules and how do we make them? Let’s address some terminology first:
A compound is any 2 (or more) elements, bonded together. A molecule is any 2 (or more) atoms bonded together. They sound very similar. It’s one of those “a square is a rectangle, but a rectangle is not a square” kind of things.
Basically, in a compound, the 2+ atoms have to be of different elements, but that’s not the case with molecules, which can be made of any element or elements. For example:
Textbooks always emphasize the difference between compounds and molecules. Maybe the distinction is significant to chemists, but guess what? It never comes up again in Physiology because all compounds, by definition, are molecules. We will only ever use the term molecule.
So how do we bring together Hydrogen and Oxygen, to make a molecule of water? Atoms have to be bonded to one another with… bonds. There are a number of different types of bonds, but the most important is called a covalent bond.
Remember that in an atom, we have a nucleus at the center, which contains protons and neutrons. Orbiting the nucleus, we have electrons. Those electrons are found in layers called shells. The first shell, which is the one closest to the nucleus, can hold up to 2 electrons. If an element has more than 2 electrons, we start filling up the next shell. This shell can hold up to 8 electrons. Some elements can be massive with over 100 electrons and several shells. But in Physiology and Biology, we mainly work with Hydrogen, Carbon, Nitrogen, and Oxygen — all of which are pretty small.
The outermost shell of an atom, whichever shell that may be, is called the valence shell. This shell is where covalent bonds will be formed. From the name, you might guess that these bonds involve sharing electrons in the valence shell.
The octet rule tells us that atoms will attempt to either fill or empty their valence shell. So, let’s take an example:
Hydrogen has 1 electron. It’s valence shell can hold 2 electrons. To fill the valence shell, how many electrons does Hydrogen need to pick up? Yeah, 1. You got it.
Oxygen has 8 electrons. 2 go in the first electron shell. How many are left over? You got it — 6 electrons. If Oxygen has 6 electrons in its outermost shell, its valence shell, how many does it need to fill the valence shell? Remember, the second electron shell can hold 8 electrons, so Oxygen needs to pick up 2 more electrons.
And so, Oxygen and Hydrogen come together to form water. H2O is made when two hydrogens and one oxygen form covalent bonds. The two hydrogens each let the oxygen borrow their electrons (1 each) some of the time. This means that, some of the time, oxygen will have 8 electrons in its valence shell. And oxygen shares an electron with each of the hydrogens (some of the time) so they will have 2 electrons in their valence shells. This arrangement is energetically stable, so covalent bonds are usually pretty strong.
Atoms and molecules can be neutral or they can be electrically charged (positively or negatively charged). Typically, atoms are neutral because the number of positive charges (protons) is equal to the number of negative charges (electrons). They balance each other out. If an atom loses an electron, it becomes positively charged because it lost a negative charge.
When atoms or molecules, which have opposite charges, come together, this is called an ionic bond. They are bonded by their charge. You’ve heard that opposites attract, right? Positively and negatively charged atoms are attracted to each other, as well.
For example, take Sodium Chloride (NaCl). In this ionic compound, positively charged sodium (Na+) is attracted to negatively charged chloride (Cl-).
Sometimes a molecule will have a partial charge in one region and a different charge in another region. The molecule has different charges at different ends, so we call these ends poles and this phenomenon polarity. It may be helpful to consider another example:
In H2O, Oxygen is bigger than Hydrogen. It’s about 16 times heavier, too. It’s a bully, who made an arrangement to share electrons with the hydrogens, but it holds onto them longer than it should. In science words, we would say it is more electronegative than hydrogen. As a result, the end of the molecule where we find oxygen is going to have a partial negative charge. The end where we find the hydrogens will have a slight positive charge.
Water is polar and this polarity is one of the most important characteristics of water. This polarity causes water molecules to stick to one another — the slightly positive end of one water molecule is attracted to the slightly negative end of another water molecule. It also allows charged molecules, such as salt and sugar, to dissolve in water. Lastly, it means that electrically neutral molecules, such as the lipids that make up the cell’s membrane, cannot dissolve in water.
Let’s pick it up with the cell membrane in the next post!
The endocrine system involves glands that secrete hormones (chemical messages) directly into the bloodstream. You can compare them to exocrine glands, which secrete outside of the body (ex: sweat glands).
It’s more useful to compare the endocrine system to the nervous system. Both systems carry out similar functions.
However, the nervous system is much faster. Neurons are able to send and receive messages much more quickly than glands. This is because neurons are using fast, electrical signals and glands have to secrete their product into the bloodstream and wait for the blood to make its way through the body. In general, the effects of hormones are going to last much longer as well.
The endocrine system is really easy to learn and really difficult at the same time. It works in a straight-forward, logical way, but there is a ton to memorize! It’s a good idea to start by understanding negative and positive feedback loops, the hypothalamic-pituitary axis, and then start making a table or chart to memorize the hormones, their target cells/tissues, and the effects that we see.
Let’s start with the hypothalamic-pituitary axis. Basically, many hormones are going to involve a system where:
For example, the Hypothalamus releases Thyrotropin Releasing Hormone, which travels through the blood in the hypothalamic-hypophyseal portal system. This stimulates cells in the anterior pituitary gland to release Thyroid Stimulating Hormone (TSH), which travels through the bloodstream to the thyroid. The thyroid is stimulated to release Thyroxine (a.k.a. tetraiodothyronine or T4) and Triiodothyronine (a.k.a. T3).
When studying, it may be helpful to make a list that looks something like this:
To remember Anterior Pituitary Hormones, I use the mnemonic “Pro ATHletes GOt To GROW”
For the Posterior Pituitary, there are only 2 hormones. These hormones are produced in the Hypothalamus, but stored in the Posterior Pituitary. They are released when cells in the Posterior Pituitary are stimulated by neurons coming from the Hypothalamus:
Other Glandular Structures include (but are not limited to):
A long time ago, in ancient Greece, a philosopher named Democritus was trying to figure out what matter was made of. He thought that if divided a block of salt in half, then divided it again, and again, and again, that eventually he would reach a point where the salt could no longer be divided. He called this unit, atomos, meaning “indivisible.”
Democritus was astonishingly close, although he supposed that all of these atoms were identical. In reality, there are at least 118 known types of atoms, called elements. In Physiology, we really only deal with a handful of them: Hydrogen, Carbon, Oxygen, and Nitrogen. You’ll also see a few others, including Calcium, Phosphorous, Sodium, Potassium, and Chloride.
An atom is made up of 3 types of subatomic particles:
Normally, the number of electrons (negative charges) and protons (positive charges) in an atom are the same, so the charge of the atom is balanced.
If something happens and an atom gains or loses electrons, then the atom will have a positive or negative charge. For example, if Lithium has 3 protons (+) and 3 electrons (-), the charge of the atom is neutral. But if it loses an electron (-), then the Lithium atom will have a positive charge. This point confuses a lot of people so take a moment to take it in — if you lose a negative charge, you become positive.
Atoms (or molecules) that have a charge are called ions. Positively charged ions are called cations and negatively charged ions are called anions.
In the Lithium example, we asked,”what would happen if an atom gained or lost electrons?” But what if it gained or lost protons? Well, it wouldn’t. Sort of. If an atom of an element gains or loses a proton, then it is no longer an atom of that element. It becomes a different element.
The number of protons in an atom determines which element it is. In fact, if you look at the Periodic Table of Elements, they’re arranged by their atomic number, which is the number of protons each element has. Let’s take Carbon and Nitrogen as an example. Carbon has 6 protons and Nitrogen has 7. If Carbon gained a proton, it would become Nitrogen. If Nitrogen lost a proton, it would become Carbon. It’s not very common for atoms to gain or lose protons, but it does happen, for example, when solar radiation excites Nitrogen to lose a proton and become radioactive Carbon-14.
So by now, you have to be asking “what happens when an atom gains or loses neutrons?” Well, it becomes an isotope. The textbook tells us that isotopes are atoms of an element that differ in the number of neutrons. You could think about isotopes as different versions of an element. If we say that Windows and MacOS are different elements, then Windows 7 and Windows 10 would be different isotopes of Windows.
The weight of an atom is called its atomic weight or atomic mass and it’s measured in units called Atomic Mass Units (AMU). 1 AMU is equal to the weight of normal Hydrogen atom. Protons and neutrons each weigh 1 AMU. We don’t factor in electrons because they have very little mass.
Carbon, which has 6 protons and (normally) 6 neutrons, weighs 12 AMU. This isotope of Carbon is the most common and it’s known as Carbon-12. Different isotopes of Carbon can have different numbers of neutrons. For instance, Carbon-13 has 7 neutrons and Carbon-14 has 8 neutrons. Remember, when we see different isotopes of an element, only the number of neutrons will change. The number of protons cannot change, because then it would be a different element.
That’s it for now! Join us next time when we cover how these atoms are involved in reactions and build molecules.
First, what are Anatomy & Physiology? “Anatomy” comes from Greek, meaning “to cut up”. Anatomy is the study of the structures in the Body. Physiology is the study of how those structures function. They’re technically two different, distinct fields of study but we always talk about them together because form follows function. In other words, the way something works, depends on its form or shape.
For example, studying the shape of bone features (all of the bumps, grooves, rough spots, etc.) will tell you something about how that bone interacts with other bones. All of the macroscopic structures (things that are visible to the naked eye) are thoroughly well understood at this point. Modern Anatomists and Physiologists study the body at the cellular or molecular level.
We always start A&P courses with an introduction to anatomical terminology. It involves memorizing a lot of new terms, most of which are in Latin, Greek, or both. There are 3 basic areas that we have to understand before moving on:
These terms are used to refer to regions or locations in the body. They can be broad or specific. For example, Cephalic refers to the head, Facial refers to the face, and Cranial refers to the bony vault that surrounds the brain. Cephalic would be pretty broad, while Facial and Cranial are more specific.
Directional Terms are used to describe the location of a structure, compared to other structures. For instance, I could say “the elbow is closer to the trunk of the body than the wrist is.” But it wouldn’t be science if it wasn’t in Latin, so we say “the elbow is proximal to the wrist.” I guess that’s easier to say, too.
The above example explains the general idea somewhat well, but it’s really not useful. A better example would be: There are 3 bones in most fingers. The one closest to the trunk in the Proximal Phalanx, the one furthest from the trunk is the Distal Phalanx, and the one in the middle is the Middle Phalanx.
Body planes are imaginary lines that divide the body. What good could that possibly do? Understanding body planes is important when trying to make sense of medical imaging, such as MRI or CT scans (or even X-rays). These imaging devices take a “slice” of the body and if we want to understand what we’re seeing, it’s important to know how the image is oriented.
Sections are actual cuts that we make in the body. These are useful for understanding tubular structures on a microscope slide.
Wow. That’s kind of a lot, when you look at it all at once. Take some time, review the terms, and quiz yourself often. Next up, we’ll cover Chemistry. Not… much of it — just enough to understand basic physiology. I promise.
My name is Jack and I have taught Biology, Anatomy, and Physiology at the college level for 7 years, at both public and private universities.
I earned a Master’s of Science in Biology from the University of Texas at San Antonio and a Master’s of Science in Physiology from the University of Louisville School of Medicine. In early 2020, I took a break from my Ph.D. when we welcomed the birth of our newest child. When the pandemic got into full swing a few weeks later, I decided to not go back.
My research has been primarily focused on a skin condition called Hidradenitis Suppurativa. I have also worked in bioinformatics and microbial genomics.
I’ve always been overweight, but I used to enjoy a lot of sports. Before moving to Texas, I would go snowboarding and play hockey. I loved martial arts; I studied Taekwondo, Hapkido, Aikido, Kumdo, and Judo. I never thought I could be a runner, but in January 2013, I started and by the end of the year I ran three Half-Marathons.
I decided to start this blog with a few goals in mind:
Fitness and Weight Loss: In the summer of 2020, I hit my highest weight, ever. I also had lost considerable range of movement and stamina. With lockdown in full effect and a newborn baby, most of our time was spent sitting on the couch. The amount of time we spent on the couch actually lead me to develop severe lower back pain and leg numbness. I couldn’t walk more than a quarter mile without taking a break. While I was waiting in the doctor’s office, writhing in pain, I could only think of two things: I want this pain to stop immediately — and I really want to go for a run.
Sports and Goals: I have a lot of athletic interests and I love meeting goals. I’ve registered for the Air Force Half-Marathon in September and I intend to be ready. Before I moved to Tokyo, I earned my red belt in Taekwondo — and later this year, I will start training again to reach black. I want to start backpacking again and I want to start bikepacking.
Educate: There are a lot of good resources out there to help people learn about anatomy and physiology. But when it comes to health and nutrition, there is a lot of bad advice. My goal is to bridge the gap and make the underlying science more accessible to people who have a genuine interest in understanding how their body works.
I believe in a traditional approach to diet and exercise. While I do hope to travel eventually, I’m a stay-at-home dad, so my focus is on exercises I can complete at or around home. Whether you’re here for Anatomy & Physiology or Health & Fitness, I hope you’ll stick around, hear me out, and share your experience and expertise.