Organ systems

Foundational Concept 3: Complex systems of tissues and organs sense the internal and external environments of multicellular organisms, and through integrated functioning, maintain a stable internal environment within an ever-changing external environment.

Biological basis of behavior: The nervous system

3A: The very fact that you are able to understand this sentence means that neurons in your brain (85 billion in total) are talking to each other. Neurons are the living substance of the nervous system, which extends beyond the brain to the spinal cord and peripherally, allows you to think and process, make decisions, stand up straight, maintain your heart rate, rest and digest. You will come to appreciate the structure and function of the nervous system as we delve into its anatomy and physiology, from the gray and white matter to the cerebellum to the neurons.

Neuron membrane potentials

3A: As you visually process this webpage, countless action potentials are taking place across the neurons in the occipital lobe of your brain. An action potential is an extremely quick change in the membrane potential of the neuron that can propagate toward other neurons. It’s like one neuron is telling another neuron “Hey, wake up! We have work to do!” You will soon gain an appreciation for the mechanism of the action potential, which allows for communication between neurons of all flavors in your body- from those responsible for the maintenance of memory to those functioning in sensation.

Neuronal synapses

3A: Synapses are the means by which neurons, just like other cells of the body, communicate with each other as well as other cells, like muscles and glands. They are the equivalent of neural telephone lines allowing for the transmission of a neurotransmitter, that is, the message. Thus, you are able to tell a dog from a cat. and a ball from a rock. Without all this cross-talk, you might look at an object without truly seeing or hear a sound without actually listening.


3A: The human body is composed of about 100 trillion cells (this is not counting your bacterial buddies, who actually outnumber your cells 10 to 1!) Your cells must speak to each other to coordinate this massive symphony of life. In this tutorial, you will learn about the molecular basis of cellular signaling that makes this vast network speedy and efficient.

Endocrine system

3A: Glands are special organs that secrete chemical messages called hormones, which seep into the blood - it’s like putting a tea bag in hot water. As the heart pumps, this blood carries these chemical messages throughout the body, allowing the hormones to interact with specific target cells and organs. Endocrine glands help us to maintain our appetites, grow up, metabolize molecules, concentrate urine,- and oh, so much more! We will examine how these variegated hormones play a role in homeostasis as the body responds to a changing environment.

Circulatory system

3B: With every beat, the heart pumps blood throughout the body through an intricate system of blood vessels to provide oxygenation to tissues - from about 4 weeks after conception until the day you die. Over your lifetime, it will pump about 175 million liters of blood (that’s only the amount of water that falls over the Niagara in a few minutes). This little pump in the middle of your chest is only the size of a clenched fist (if you’re an adult), and yet it does so much.

Respiratory system

3B: If you place your hand on your ribs and take a deep breath, you’ll notice that your chest expands and your back straightens. As this occurs, air is rushing through your windpipe and branches off to either your left or right lung. After about 20-30 more branch points, oxygen in the air ends up in the alveoli where it diffuses into the surrounding liquid, slipping into the blood. This rapid microscopic gas exchange allows oxygen to be taken up during inhalation as carbon dioxide leaves during exhalation.

Lymphatic system

3B: Your heart pumps roughly 20 L of blood throughout the day to your tissues. The plasma component of blood (not containing blood cells) leaks out through capillaries (the tiniest of blood vessels) and is mostly reabsorbed. However, about 3L of the plasma is left behind in fluid surrounding tissues, and it is the job of the hard-working lymphatic system to bring back this fluid to the circulatory system. The lymphatic system moves fluid in one direction, but without the force of a pump like the heart.

Immune system

3B: Chances are , you’ve had a fever or a cough at least once in your life (unless you live in a bubble, in which case you should probably go out more!) Have you ever wondered why your body reacts this way? Your body has a deadly arsenal of weapons against microbial invaders, ranging from bacteria and viruses to protozoans and fungi. We have specialized cells that destroy foreign bodies through mechanisms such as consumption, expulsion, and degradation. You will become acquainted with the interplay of the numerous soldiers in your body’s army and how they keep you healthy!

Renal system

3B: If you want to learn more about the renal system, then urine the right place! (Pun aside, the kidneys are about more than just making urine). Every thirty minutes, your kidneys filter the entire blood supply in your body. Imagine a dirty pool filled with algae. Placing a filter in this pool will cause the algae to be flushed out, and after a time you’ll have a clean, crisp blue pool to enjoy. Just like the filter for a pool, our kidneys filter the blood and remove toxic wastes. These paired organs are key to maintaining electrolyte and water homeostasis in your body.

Renal regulation of blood pressure

3B: In addition to fluid and electrolyte balance, the kidneys play an essential role in regulating blood pressure. When the nephron, the functional unit of the kidney, senses that it is not receiving much blood flow, it knows something is up. So begins a cascade which allows the nephron to reabsorb more fluid from the urine, in attempt to fill up the perceived dip in blood volume. Here you will learn the mechanism of ADH and the renin-angiotensin-aldosterone system in sustaining this homeostasis.

Gastrointestinal system

3B: You begin with mechanical digestion in your mouth, where starches are also broken down. The bolus of food travels down your esophagus into the stomach, where it sits and churns, further allowing for breakdown of biomolecules like fat and protein.The contents of the stomach then enter the small intestine, which is where the bulk of digestion and absorption of nutrients occurs through the work of shear mechanical (peristalsis) and chemical (digestive enzymes) force. Then our food commutes through the large intestine (AKA the colon), where most of the fluid absorption in the GI tract occurs. Phew! Eating sure is a lot of work! By Raja Narayan.

Muscular system

3B: Muscles never sleep (literally). If you have ever taken a breath, you have benefited from the work of the diaphragm, which contracts to create an area of low pressure within your thoracic cavity, allowing air in. How exactly are some weightlifters able to support 717 lbs without breaking anything more than a little sweat? Fun fact: the largest muscle in your body is the gluteus maximus (that’s your butt) while the smallest skeletal muscle is the stapedius (it stabilizes the smallest bone, the stapes, which is in your middle ear). By Raja Narayan.

Skeletal system

3B: Were it not for your skeleton, you and I would be a mere sack of flesh. You will come to appreciate that the bones, together with muscles, are a scaffolding for your body. We will also explore their endocrine function, especially with regards to calcium and phosphate homeostasis. Fun fact: the bone most broken is the clavicle (AKA collar bone).