Science Weblecture for Unit 24
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Anatomy looks at the organs and systems of the body in terms of structure. An anatomist tries to identify the different organs from their appearance and location. The ability to do this more accurately, thanks to the illustrations in texts like those of Vesalius, made it possible for scientists in different areas to compare and communicate their observations. As their observations became more detailed, they also became more interested in how the different organs and tissues of the body function. This study of function is physiology.
In this unit, you have a kind of biology "grammar" lesson in four different systems of the human body and their functions. We still need to do anatomy, and learn what organs belong to each system and where they reside in human bodies, but this time, we will focus on the physiology, that is, on the processes that each system makes possible. A process is an ordered series of steps, so for respiration, we need to ask: what happens first when you breath? What happens next? Is the process cyclical and if so, what is the last step that triggers the first one again?
We have looked the bone, muscle, and nervous systems in terms of their structure, their parts and location. Now we look at four other systems of the body in terms of their function: the circulatory system, the respiratory system, and the digestive system.
One of the best ways to remember a process like digestion is to follow an object through the process. Food enters the mouth, where it is chewed and enzymes in saliva (secreted from the parotid gland behind the tongue) begin to break down starches. The food moves through the pharynx and down the esophagus to the stomach, where proteins are digested by acids and pepsin. Then it moves on into the small intestine, where the fats are broken down as well, and all of the now simple molecules are absorbed into the blood stream. Wastes that are not used by the body move on into the large intestine, and are eventually eliminated. Since each organ in the system produces enzymes or chemicals to break down a specific type of food, the breakdown processes don't interfere with each other.
In order to increase the rate at which nutrients can be absorbed, the lining of the small intestines has millions of bumps called villi, so that it resembles a terry cloth towel. This increases the surface area through which the nutrients can move.
The many enzymes used in digestion come from different glands in the endocrine system. Enzymes from the liver and gall bladder break down fats. The liver also metabolizes carbohydrates into sugars that form the source of energy for neurons in the brain, and for blood cells to store iron. Enzymes from the pancreas, particularly those in insulin, break down sugars.
Digestive systems vary because different organisms get their energy from different sources. Autotrophes produce their own food. While we normally think of plants in this category, there are some bacteria that use photosynthesis to produce energy from light. Herbivores like cows and horses get their nutrition from plants, carnivores get their nutrition from animals (this includes animals like cats but also plants like the Venus flytrap!) and omnivores like humans can eat both plant and animal food sources. Animals also take their food into the body in different ways. Bulk ingesters like humans take bites of food, but filter feeders like whales and most mollusks sift small food particles out of liquid sources.
Air enters the mouth, flows through the larynx and down the trachea, then through the bronchus into the lungs, where oxygen is transferred into the blood and carbon dioxide absorbed from the blood. The flow is controlled by a process called diffusion. Whenever we have a concentration of something, whether it is oxygen or salt in water or perfume in the air, the concentrated substance will try to spread to areas of lesser concentration. In the lungs, there is more oxygen in the air than there is in the blood flowing through the blood vessels that surround the many small alveoli or bubble-like pockets in the lungs. The pressure of the concentrated oxygen pushes some of it into the blood. At the same time, the high concentration of carbon dioxide (a product of cells doing work) in the blood pushes the carbon dioxide out of the blood into the air.
In order to get air into the lungs, a large muscle called the diaphragm, which lies between the lungs and the intestines, pushes up and down. When it moves down, the lungs expand and draw oxygen-bearing air in. When it contracts, the lungs are compressed and carbon-dioxide laden air is pushed out.
The rate at which we breath is determined by the amount of oxygen we need for the current tasks. When we run or exercise, we need more oxygen, so we breath faster to push oxygen into the bloodstream. When we are resting or asleep, our breathing slows down.
The circulatory system is responsible for carrying oxygen and nutrients to the cells of the body, and waste carbon dioxide back to the lungs. The heart is the main organ of this system. It has four chambers. Blood flows into the right atrium until that chamber fills. It is then pushed into the right ventricle, which is a bit larger and has a stronger muscle. These pushes put pressure on the blood vessels which you feel as your pulse at points in your wrists, the inside of your elbows, and your throat.
If your blood vessels are constricted in any way, the heart has to push harder, raising the pressure. We measure blood pressure as a ratio between the pump pressure (systolic pressure) and the relaxed state of the heart, or diastolic pressure, measured as the pressure's ability to push a column of mercury upwards. Normal blood pressure is "120 over 80".
When the right ventricle contracts, the blood leaves the heart and circulates through the pulmonary circuit. Here the blood flowing through small capillaries around alveoli can absorb oxygen from the air in the sac.
The blood flows back to the heart and into the left atrium. When enough blood has gathered, it is pushed into the left ventricle, which contracts and strongly shoves through the aorta to the arteries.
Arteries carry the blood all over the body through ever smaller arterioles until it reaches narrow capillaries. The thin walls of the capillary tubes let oxygen flow out along with nutrients, and carbon dioxide flow in. The deoxygenated blood flows back toward the heart through veins. Some of the veins pass through the lining of the small intestines, where they can absorb nutrients processed by the digestive system. The blood then flows back into the heart through the veins, and begins its journey again toward the pulmonary circuit to dump its carbon dioxide "waste" and pick up more oxygen.
|Red Cells||Also called erythrocytes, red cells carry oxygen to cells of the body and may carry carbon dioxide away from cells. Produced in bone marrow, they age quickly and have to be constantly replenished.|
|White Cells||Also called leukocytes, white cells are the primary defense against infectious disease. There are different kinds to handle different types of invasive cells.|
|Plasma||Plasma is the liquid component of blood. Besides the red and white blood cells that float in plasma, the liquid also contains clotting factors and transports most of the nutrients.|
By now, you realize that these systems are integrated with each other: digestion depends on blood flow from the circulatory system, not just to carry off the nutrients it extracts from the food you eat, but also for delivery of the hormones and enzymes that break down that food, and it eventually passes wastes off to the internal control systems' excretory organs like the kidneys.
We have not studied the other major body systems in detail, because of lack of time. But you should also note that human bodies possess a reproductive system for producing children, an endocrine system, for producing the hormones that trigger growth and development and regulate body functions, an immune system for eliminating alien substances and diseased cells that would otherwise harm the whole body, and an excretion system to rid the body of waste products from digestion and other metabolic functions. If you have questions about these systems, you may raise them in class.
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