Medical brain notes 36

Pancreas, Insulin, and Diabetes The endocrine part of the  pancreas  (pan′ kr ē -as) consists of  pan-creatic islets  (islets of Langerhans), which are dispersed through-out the exocrine portion of the pancreas (figure 10.22). The islets secrete three hormones—insulin, glucagon, and somatostatin— which help regulate the blood levels of nutrients, especially glu-cose (table 10.3).  Alpha cells  secrete glucagon,  beta cells  secrete insulin, and delta cells secrete somatostatin.   It is very important to maintain blood glucose levels within a normal range (figure 10.23). A below-normal blood glucose level causes the nervous system to malfunction because glucose is the nervous system’s main source of energy. When blood glucose decreases, other tissues rapidly break down lipids and proteins to provide an alternative energy source. As lipids are broken down, the liver converts some of the fatty acids to acidic  ketones  (k ē ′ t ō nz...

Testes and Ovaries The testes of the male and the ovaries of the female secrete sex hormones, in addition to producing sperm cells or oocytes, respectively. The hormones produced by these organs play impor-tant roles in the development of sexual characteristics. Structural and functional differences between males and females, as well as the ability to reproduce, depend on the sex hormones (see table 10.2).  The main sex hormone in the male is  testosterone  (tes′ tos′ t ĕ -r ō n), which is secreted by the testes. It is responsible for the growth and development of the male reproductive structures, muscle enlargement, the growth of body hair, voice changes, and the male sexual drive.  In the female, two main classes of sex hormones, secreted by the ovaries, affect sexual characteristics:  estrogen  (es′ trō-jen) and  progesterone  (prō-jes′ ter-ōn). Together, these hormones contribute to the development and function of female reproduc-tive str...

Thymus The  thymus  lies in the upper part of the thoracic cavity (see figure 10.1 and table 10.2). It is important in the function of the immune system. The thymus secretes a hormone called  thymosin  (th ı̄ ′  m ō -sin), which aids the development of white blood cells called T cells. T cells help protect the body against infection by foreign organisms. The thymus is most important early in life; if an infant is born without a thymus, the immune system does not develop normally, and the body is less capable of fighting infec-tions . Pineal Gland The  pineal  (pin ′  ē-ăl; pinecone)  gland  is a small, pinecone-shaped structure located superior and posterior to the thalamus of the brain . The pineal gland produces a hormone called  melatonin  (mel-ă-tōn ′  in), which is thought to decrease the secre-tion of LH and FSH by decreasing the release of hypothalamic-releasing hormones (see table 10.2). Thus, melatonin inhibi...

  Other Hormones Cells in the lining of the stomach and small intestine secrete hor-mones that stimulate the production of digestive juices from the stom-ach, pancreas, and liver. This secretion occurs when food is present in the digestive system, but not at other times. Hormones secreted from the small intestine also help regulate the rate at which food passes from the stomach into the small intestine .  Prostaglandins are widely distributed in tissues of the body, where they function as intercellular signals. Unlike most hor-mones, they are usually not transported long distances in the blood but function mainly as autocrine or paracrine chemical signals (see table 10.1). Thus, their effects occur in the tissues where they are produced. Some prostaglandins cause relaxation of smooth muscle, such as dilation of blood vessels. Others cause contraction of smooth muscle, such as contraction of the uterus during the deliv-ery of a baby. Because of their action on the uterus, prost...

  Effects of Aging on the Endocrine System Age-related changes to the endocrine system include a gradual decrease in the secretion of some, but not all, endocrine glands. Some of the decreases in secretion may be due to the fact that older people commonly engage in less physical activity.  GH secretion decreases as people age, but the decrease is great-est in those who do not exercise, and it may not occur at all in older people who exercise regularly. Decreasing GH levels may explain the gradual decrease in bone and muscle mass and the increase in adipose tissue seen in many elderly people. So far, administering GH to slow or prevent the consequences of aging has not been found to be effective, and unwanted side effects are possible.  A decrease in melatonin secretion may influence age-related changes in sleep patterns, as well as the decreased secretion of some hormones, such as GH and testosterone.  The secretion of thyroid hormones decreases slightly with age. Ag...

  FUNCTIONS OF BLOOD Blood has always fascinated humans, and throughout history they have speculated about its function. Some societies consider blood the “essence of life” because the uncontrolled loss of it can result in death. Many cultures around the world, both ancient and modern, believe blood has magical qualities. Blood has also been thought to define our character and emotions. For example, people of a noble bloodline are sometimes described as “blue bloods,” whereas criminals are said to have “bad” blood. People commonly say that anger causes their blood to “boil,” whereas fear makes it “curdle.” The scientific study of blood reveals characteristics as fascinating as any of these fantasies. Blood performs many functions essential to life and can reveal much about our health. The heart pumps blood through blood vessels that extend throughout the body. Blood helps maintain homeostasis in several ways: 1.            ...

  COMPOSITION OF BLOOD Blood is a type of connective tissue that consists of a liquid matrix containing cells and cell fragments. The liquid matrix is the plasma  (plaz′ m ă ), and the cells and cell fragments are the  formed elements  (figure 11.1). The plasma accounts for slightlymore than half of the total blood volume, and the formed elements account for slightly less than half. The total blood volume in the average adult is about 4–5 liters (L) in females and 5–6 L in males. Blood makes up about 8% of total body weight.  

  PLASMA Plasma is a pale yellow fluid that consists of about 91% water, 7% proteins, and 2% other components, such as ions, nutrients, gases, waste products, and regulatory substances (figure 11.1 and table 11.1). Unlike the fibrous proteins found in other connective tissues, such as loose connective tissue, plasma contains dis-solved proteins. Plasma proteins include albumin, globulins, and fibrinogen. Albumin  (al-b ū ′ min) makes up 58% of the plasma proteins. Although the osmotic pressure  of blood Figure 11.1   Composition of Blood results primarily from sodium chloride, albumin also makes an important contribution. The water balance between the blood and the tissues is determined by the movement of water into and out of the blood by osmosis.  Globulins  (glob′ ū -linz; globule) account for 38% of the plasma proteins. Some globulins, such as antibodies and complement, are part of the immune system . Other globulins and albumin function as trans...

Formed Elements   About 95% of the volume of the formed elements consists of  redblood cells (RBCs),  or erythrocytes ( ĕ -rith′r ō -s ı̄ tz; erythro-, red+  kytos,  cell). The remaining 5% of the volume of the formed elements consists of white blood cells (WBCs),  or  leukocytes (loo′k ō -s ı̄ tz;  leuko -, white), and cell fragments called  platelets  (pl ā t′letz), or  thrombocytes  (throm′b ō -s ı̄ tz;  thrombo -, clot). Redblood cells are 700 times more numerous than white blood cells and 17 times more numerous than platelets. Table 11.2 illustrates the formed elements of the blood. Production of Formed Elements The process of blood cell production is called  hematopoiesis  (hē′mă-tō-poy-ē′sis;  hemato-,  blood +  poiesis,  making). In the fetus, hematopoiesis occurs in several tissues, including the liver, thymus, spleen, lymph nodes, and red bone marrow. After birth, hematopoie...