Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

a variety of organisms. A water-based skeleton provides the structure necessary for movement in worms. The hard external skeleton (exoskeleton) not only provides a protective mechanism for many organisms but also assists in movement of insects. The internal skeleton (endoskeleton) present in many animals provides the structural network for support, protection, and movement.

 

Hydrostatic skeleton

 

Many animals have a water-based skeleton, or hydrostatic skeleton. Hydrostatic skeletons do not contain hard structures, such as bone, for muscles to pull against. Rather, the muscles surround a fluid-filled body cavity. In a worm, for example, movement occurs when muscle cells contract, and the contractions squeeze internal fluid (the hydrostatic skeleton) against the skin, causing the worm to stiffen and the body to shorten and widen. The squirming motion of a worm also depends on a hydrostatic skeleton.

 

Exoskeleton

 

A second type of skeleton, the exoskeleton, exists in arthropods and mollusks. In mollusks, the exoskeleton is a hard, protective outer covering. An example is a clamshell; when a clam’s muscles contract, they close the shell rapidly, creating a spurt of water that propels the clam. In arthropods, the exoskeleton also provides protection and movement. Usually, wings are attached by muscles in the hard body surface, which provides a foundation for the muscle contractions. Muscle contractions raise and lower the wings, allowing flight.

 

Endoskeleton

 

Vertebrates have an internal skeleton called the endoskeleton, a framework of bones and cartilage (see Figure 26-1) that serves as a point of attachment for muscle. The endoskeleton thus transmits the force of muscle contractions. The endoskeleton also provides support for the body (for example, the legs) and protection (the skull).

 

Bone contains concentric rings of tissue in which bone cells called osteoblasts produce the inorganic materials (fibers and matrix) of bone. Much of this material is calcium phosphate, formed from calcium and phosphorus delivered by the blood. Living, mature bone cells called osteocytes are also located in the bone. Bone-destroying cells called osteoclasts break down bone, thus providing a turnover of bone material needed in other areas. The combination of bone cells and bone tissue comprises a unit called a Haversian system. Blood vessels and nerves also exist within the Haversian system.

 

Bones come together to form a joint, which may be immovable, such as in the sutures of the skull, or movable, such as in the joints of the elbow and shoulder. In a movable joint, a capsule of synovial fluid provides lubrication. Tough, fibrous tissues, known as ligaments, link bones to one another. Connective tissues, called tendons, attach muscles to bones.

 

 

Figure 26-1   The human skeleton showing the major bones of the body.

Chapter 27: Chemical Coordination

  Hormones and Glands

 

The animal body has two levels of coordination: nervous coordinatio and chemical coordination. Chemical coordination is centered in a system of glands known as endocrine glands. These glands are situated throughout the animal body and include such organs as the pancreas, thyroid gland, and adrenal gland. The glands secrete hormones, a series of chemical substances composed of protein or sterol lipids.

 

Hormones bring about changes that help coordinate body systems in a general way. For example, the pancreas secretes insulin, which facilitates the passage of glucose into all body cells for use in energy metabolism. Another example is thyroxine, a thyroid gland secretion that regulates overall body metabolism. In contrast to chemical coordination, the nervous system coordinates functions in the animal body on a more localized level as it delivers nerve impulses to contract body muscles or regulate gland activities.

 

The endocrine glands secrete their hormones into the bloodstream, where the blood carries the hormones to the target organs. Because the endocrine glands have no ducts, they are often called ductless glands. Other glands of the body (such as the enzyme-secreting salivary glands) deliver their enzymes via ducts and are referred to as exocrine glands.

 

The structure and physiology of hormones and endocrine glands are relatively similar in all animals; the emphasis in this chapter is on the human endocrine system.

 

Human Endocrine System

 

The human endocrine system modulates several processes of the body by the function of hormones. The endocrine system secretes hormones that control how bodily functions work. Thus, the human endocrine system watches over and coordinates all the systems of the body with the use of hormones.

 

Pituitary gland

 

The pituitary gland is located at the base of the human brain. The gland consists of two parts: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis).

 

The anterior lobe secretes at least seven hormones. One hormone, the human growth hormone (HGH), promotes body growth by accelerating protein synthesis. This hormone is also known as somatotropin. A deficiency of the hormone results in dwarfism; an oversecretion results in gigantism.

 

Another hormone of the anterior pituitary is prolactin, also called lactogenic hormone (LH). This hormone promotes breast development and milk secretion in females. A third hormone is thyroid-stimulating hormone (TSH). The function of TSH is to control secretions of hormones from the thyroid gland. A fourth hormone is adrenocorticotropic hormone (ACTH). This hormone controls the secretion of hormones from the adrenal glands.

 

There are three more hormones produced in the anterior lobe of the pituitary gland. The first is follicle-stimulating hormone (FSH). In females, FSH stimulates the development of a follicle, which contains the egg cell; in males, the hormone stimulates sperm production. The next hormone is luteinizing hormone (LH). In females, LH completes the maturation of the follicle and stimulates the formation of the corpus luteum, which temporarily secretes female hormones. In males, LH is interstitial cell-stimulating hormone (ICSH), which stimulates the production of male hormones in the testes. The final hormone is melanocyte-stimulating hormone (MSH), which stimulates production of the pigment melanin.

 

The posterior pituitary gland stores and then releases two hormones that are produced in the hypothalamus of the brain. The first hormone is antidiuretic hormone (ADH). This hormone stimulates water reabsorption in the kidneys. It is also called vasopressin. The second hormone is oxytocin, which stimulates contractions in the muscles of the uterus during birth.

 

Thyroid gland

 

The thyroid gland lies against the pharynx at the base of the neck. It consists of two lateral lobes connected by an isthmus. The gland produces thyroxine, a hormone that regulates the rate of metabolism in the body. It also produces a second hormone, calcitonin, which regulates the level of calcium in the blood.

Thyroxine production depends on the availability of iodine. A deficiency of iodine causes thyroid gland enlargement, a condition called goiter. An undersecretion of thyroxine during childhood development results in a condition known as cretinism (dwarfism with abnormal body proportions and possible mental retardation). In adults, an undersecretion can also result in myxedema (physical and mental sluggishness). Thyroxine oversecretion results in a high metabolic rate and Graves’ disease.

 

Parathyroid glands

 

The parathyroid glands are located on the posterior surfaces of the thyroid gland. They are tiny masses of glandular tissue that produce parathyroid hormone, also called parathormone. Parathyroid hormone regulates calcium metabolism in the body by increasing calcium reabsorption in the kidneys and by increasing the uptake of calcium from the digestive system.

 

Adrenal glands

 

The adrenal glands are two pyramid-shaped glands lying atop the kidneys. The adrenal glands consist of an outer portion, the adrenal cortex, and an inner portion, the medulla.

The adrenal cortex secretes a family of steroids called corticosteroids. The two main types of steroid hormones are mineralocorticoids and glucocorticoids. Mineralocorticoids such as aldosterone control mineral metabolism in the body. They accelerate mineral reabsorption in the kidneys. Mineralocorticoid secretion is regulated by ACTH from the pituitary gland. Glucocorticoids such as cortisol and cortisone control glucose metabolism and protein synthesis in the body. Glucocorticoids are also anti-inflammatory agents.

 

The adrenal medulla produces two hormones: epinephrine (adrenaline) and norepinephrine (noradrenaline). Epinephrine increases heart rate, blood pressure, and the blood supply to skeletal muscles. Epinephrine functions in stressful situations to promote the fight-or-flight response. Norepinephrine intensifies the effects of epinephrine. Both hormones prolong and intensify the effects of the sympathetic nervous system.

 

Pancreas

 

The pancreas is located just behind the stomach. Its endocrine portion consists of cell clusters called the islets of Langerhans.

 

The pancreas produces two hormones: insulin and glucagon. Insulin is a protein that promotes the passage of glucose molecules into the body cells and regulates glucose metabolism. In the absence of insulin, glucose is removed from the blood and excreted in the kidney, a condition called diabetes mellitus. Diabetes mellitus is characterized by glucose in the urine, heavy urination, excessive thirst, and a generally sluggish body metabolism.

 

The second pancreatic hormone, glucagon, stimulates the breakdown of glycogen to glucose in the liver. It also releases fat from the adipose tissue so the fat can be used for the production of carbohydrates.

 

Other endocrine glands

 

Among the other endocrine glands are the ovaries and testes. The ovaries secrete estrogens, which encourage the development of secondary female characteristics. The testes secrete androgens, which promote secondary male characteristics. Testosterone is an important androgen.

 

The pineal gland is a tiny gland in the midbrain. Its functions are largely unknown, but it seems to regulate mating behaviors and day-night cycles. The thymus gland is located in the neck tissues. It secretes thymosins, which influence the development of the T-lymphocytes of the immune system.

 

Prostaglandins are hormones secreted by various tissue cells. These hormones produce their effects on smooth muscles, on various glands, and in reproductive physiology. Erythropoietin is a hormone produced by the kidney cells. Erythropoietin functions in the production of red blood cells. Gastrin and secretin are hormones produced by digestive glands to influence digestive processes.

Chapter 28: Nervous Coordination

  Animal Nerve Cells

 

Nervous coordination enables an organism’s rapid response to an external or internal stimulus. Characteristic of animals only, nervous coordination is the function of the nervous system. The receptors for nervous coordination are generally located in the sense organs at the body surface, while the response in nervous coordination generally involves a gland or muscle. The function of coordination is accomplished by means of a set of signals conducted along a series of nerve cells.

 

Animal nerve cells are specialized cells called neurons. Depending upon function, these cells can be divided into sensory neurons, interneurons, and motor neurons. These three types of nerve cells coordinate with each other to receive external stimuli and to transmit the impulse to muscles or glands of the body for an appropriate response to the stimulus.

 

Neuron

 

The neuron is the nerve cell. (Approximately 12 billion neurons exist in the human body, the great majority of them in the brain and spinal cord.) The main portion of the neuron is the cell body. Protruding from the cell body are one or more short extensions called dendrites and one long extension called the axon. Axons are covered by