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Fire is a chemical reaction that occurs very quickly, and in the process, gives off heat and light.

Three things are necessary for this reaction to occur- fuel, oxygen, and heat.

Fuel whether it is paper or wood-by itself will not catch fire. It is only when the fuel is heated, and becomes hot enough, that the oxygen in the air combines freely with it to burst into flames.

The reason, fire is hot is because it releases a lot of energy that has been stored in fuel. For example, energy from sunlight is stored in the wood that is used as fuel.

When energy is released very quickly, heat and light are produced.

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Growth Hormone levels

Growth hormone, also called somatotropin, is a large polypeptide hormone produced by somatotroph cells in the anterior pituitary that plays a signifi cant role in growth and metabolism. It primarily affects bone, muscle, and tissue growth.

Without sufficient growth hormone, an individual would suffer from short stature. Too much growth hormone would result in gigan tism. For normal growth to occur, the body requires energy, which growth hormone provides through protein synthesis and the breakdown of fats.



Feathers, are important parts of a bird’s flying equipment’. A bird’s tail feathers are used for lifting, steering, and breaking, and these are perfectly symmetrical, to allow a balanced and smooth flight.

Along the sides of a bird’s feathers are barbs, which if separated, look like a fringe, or even like the threads that stick out from the edge of a piece of unstitched cloth. Since these barbs end in hooks, they hook on to one another efficiently, making a strong, but light flying wing.

There here are two sources of feather colour- pigments, and the physical structure of the feather. Many feathers are coloured by a combination of these features. Pigments are chemical compounds that absorb certain wavelengths of light while reflecting others. The colours you see are those reflected back. Feathers coloured by pigments, range from crow black to canary yellow, and cardinal red.

Many colours, such as blue, are a result of feather structure. When light hits these feathers, it hits microscopic structures on the feather that act as prisms to reflect a colour.
No blue pigment is known in birds. Shimmering iridescent colours such as those found in peacocks, are caused by special structures, air bubbles, or films on feather surfaces.

These modifications interfere with the bending and scattering of light to strengthen some wavelengths, and cancel out others.


The Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland together serve as the command center of the endocrine system, and the core of the relationship between the endocrine and nervous systems. Together, they regulate virtually every physiological activity in the body.

As mentioned earlier, the nervous and endocrine systems also regulate each other: neurohormones from the hypothalamus direct the release of endocrine hormones, and hormones from the endocrine system regulate nervous system activity.



The pancreas is an irregular-shaped gland that is located just below the stomach and adjacent to the duodenum of the small intestine. It averages between 4.7 and 5.8 inches (12 and 15 centimeters) in length, and a little over 0.8 inches (2 centimeters) in thickness. For descriptive purposes, it is divided into three major sections, although there is little difference in the physiology of the sections. The head is located closest to the duodenum and is connected to the digestive tract by two ducts. The hepatopancreatic duct is a common duct formed by the linking of the bile duct and pancreatic ducts. A second duct, called the duct of Santorini, directly connects the pancreas to the duodenum. Moving away from the duodenum and the head of the pancreas are the regions called the body and tail.

The pancreas actually represents two separate organs, both of which contribute to digestion, which are integrated into a single structure. A por tion of the pancreas is an exocrine gland, meaning that it secretes com pounds into a cavity.

The second major area of the pancreas is the endocrine tissue, which secretes chemicals into the bloodstream. In general, the exocrine functions of the pancreas can be described as those directly involved with the processing of nutrients in the duodenum, while the endo crine is best described as those functions that involve hormones and the regulation of glucose homeostasis in the body. Both types of tissue exist throughout the pancreas.

Enzymatic Digestion

Enzymatic digestion is responsible for breaking organic material into smaller subunits that can be absorbed into the circulatory system.

The amount of enzymatic digestion within the oral cavity is small in comparison to the activity of the lower GI tract. However, there is some initial digestion of both carbohydrates and lipids in the oral cavity.

The salivary glands, primarily the submandibular and sublingual glands, secrete an enzyme called salivary amylase.

Recall that the nutrients are primarily absorbed from the digestive system in their simplest structure, or monomers. Salivary amylase belongs to a class of enzymes that digest complex carbohydrates, such as starch, into monosaccharides.

The monosaccharides are easily absorbed into the circulatory system, although little absorption occurs in the oral cavity. The salivary amylase is mixed into the food by the action of the tongue and cheeks and continues to break down the starches in the food for about an hour until deactivated by the acidic pH of the stomach. A second enzyme of the oral cavity is lingual lipase.

Lingual lipase is secreted from glands on the surface of the tongue. This enzyme acts on triglycerides in the food, breaking them down into monoglycerides and fatty acids. How ever, the action of this enzyme is relatively minor and it does not make a major contribution to overall lipid digestion.



Vitamins are similar to the energy nutrients in that they are organic mole cules, but differ in the fact that the body does not get energy directly from these molecules. Instead, vitamins serve as enzyme assistants, or coen zymes. Some vitamins, specifically the B-complex vitamins, are directly involved in the processing of energy nutrients, specifically lipids and carbohydrates. Certain vitamins serve as protectors of the delicate cellular machinery. These are called the antioxidants and are best represented by vitamins C and E. Others aid in the vision pathways (vitamin A), or in the building of healthy bones (vitamins D and A). Nutritionists divide the vitamins into two groups based upon how they interact with the body.

The first are the water-soluble vitamins, a group that consists of vitamin C and the B vitamins. These vitamins are readily absorbed by the digestive system and, with a few exceptions, do not require special processing. The other class, known as the fat-soluble vitamins (vitamins A, D, E, and K), are frequently treated in the same manner as the triglycerides, meaning that they are packaged into specialized lipoproteins and transported by the lymphatic system. In general, both classes are required in relatively small quantities (micrograms or less) daily by the body.


Digestion at the Cellular and Molecular Level

When the term digestion is mentioned, it is natural to think about the actions of the mouth, stomach, and small intestine in the processing of food for energy. While these actions are no doubt important in the breakdown of food, they actually are the result of complex processing mechanisms at the cellular level. This chapter will examine the physiology of the digestive system at the organ level. However, to effectively understand the structure and function of the digestive system, we must first understand the cellular and molecular basis of nutrient processing.

The purpose of digestion is to process food by breaking the chemical bonds that hold the nutrients together. This is necessary so that the body has an adequate source of energy for daily activity, as well as materials for the construction of new cells and tissues. Since these nutrients arrive in the digestive system as the tissues of previously living organisms, they are rarely in the precise molecular structure needed by a human body. For example, the blood of cows and chickens has evolved over time to meet the precise metabolic needs of the organism. When the tissues of these animals are consumed, our bodies must chemically alter the proteins and other nutrients found in the animal’s blood to form human blood proteins such as hemoglobin.

As is the case with almost all the nutrients (with the exception of water, minerals, and some vitamins), the body breaks down the nutrient into its fundamental building blocks, transports the digested nutrient into circulatory and lymphatic systems, and eventually uses these nutrients in the cells of the body for either energy or metabolic processes.

Cardiac Muscle

The heart is made out of cardiac muscle (also known as myocardium), a tissue that is unlike the smooth or striated muscle seen elsewhere in the human body. Striated muscle is the tissue that a person uses to move his or her legs or fingers. Because the individual can control it, it is also known as voluntary muscle.

This tissue has light and dark bands, called striations, which give skel etal muscle yet another name: striated muscle. Smooth muscle, like that in blood vessels, is known as involuntary muscle, because a person cannot direct its movements like he or she can control skeletal muscle. Instead, the autonomic nervous system controls its action.

Falling somewhere in the middle of these two types of tissue is cardiac muscle. Cardiac muscle has the striations seen in skeletal muscle, but it takes its direction from the autonomic nervous system like the smooth muscle does. Unlike either stri ated (also known as skeletal) or smooth muscle, cardiac muscle cells are very closely linked to one another and have fibers that interconnect one cell to the next. As will be shown in the section on electrical activity later in this chapter, this is vital in making the heart beat as a unit. In addition, cardiac muscle does not tire out like skeletal muscle does, and it requires a shorter resting time between contractions. It is easy to assume that skeletal muscle can contract for a very long time, especially when considering how a body maintains muscle tone. A closer look reveals that different groups of skeletal muscle alternately shorten to give the appearance of constant contraction, even when the muscle cells are individually contracting and relaxing. In the heart, conversely, all of the cardiac cells contract at the same time.

Capillary System

Although the capillaries are the smallest vessels in the circulatory system, they represent the main exchange site between the blood and the tissues. They can be viewed as both the ultimate destination of the arterial system and the starting point of the venous system. From the heart, blood travels through the arteries to the arterioles, and then to the capilla ries, where exchange occurs.

Nutrients, oxygen, and other materials carried by the blood are traded for waste products from tissue cells. Blood contin ues down the capillaries, soon entering the venules and then the veins on its return trip to the heart.