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Clinical Director, Howard University College of Medicine

They also supply materials to build certain cell structures, and they often are stored as reserve energy supplies. Carbohydrates are water-soluble molecules that include atoms of carbon, hydrogen, and oxygen. These molecules usually have twice as many hydrogen as oxygen atoms, the same ratio of hydrogen to oxygen as in C H. T h i s ratio is easy to s e e in the molecular f o r m u l a s of the carbohydrates glucose (C 5 H I 2 0 6) a n d sucrose (C ^ H ^ O,). S i m p l e carbohydrates, or sugars, i n c l u d e the m o n o s a c c h a r i d e s (single sugars) and d i s a c c h a r i d e s (d o u b l e sugars). A m o n o s a c charide m a y i n c l u d e f r o m tliree to seven carbon atoms, in a straight c h a i n or a r i n g (f i g. M o n o s a c c h a r i d e s i n c l u d e the five-carbon sugars ribose and d e o x y r i b o s e, as w e l l as the six-carbon sugars g l u c o s e, d e x r o s e (a f o r m o f g l u c o s e), f r u c o s e, a n d g a l a c o s e (f i g. C o m p l e x carbohydrates, also called polysaccharides, are built o f s i m p l e carbohydrates (fig, 2. C e l l u l o s e is a p o l y s a c c h a r i d e f o u n d in plants, m a d e o f m a n y g l u c o s e m o l e c u l e s, w h i c h h u m a n s cannot d i g e s t. Starch m o l e c u l e s consist o f h i g h l y b r a n c h e d chains o f g l u c o s e m o l e c u l e s connected d i f f e r e n l y than in c e l l u l o s. Animals, including humans, synthesize a polysacc h a r i d e s i m i l a r lo starch c a l l e d glycogen, w h i c h is stored in l i v e r and skeletal m u s c l e s. Its m o l e c u l e s also are b r a n c h e d chains o f sugar units: each branch consists o f up to a d o z e n g l u c o s e units. L i p i d s i n c l u d e a n u m b e r of c o m p o u n d s, such as fats, p h o s p h o l i p i d s, a n d s e r o i d s, that h a v e v i a l f u n c i o n s in c e l l s and are i m p o r a n c o n stituents o f c e l l m e m b r a n e s (s e e c h a p e r 3, p. T h e most c o m m o n l i p i d s are the fats, w h i c h are p r i m a r i l y used to s u p p l y e n e r g y f o r c e l l u l a r a c i v i i e s. Fat m o l e cules can s u p p l y m o r e energy gram for gram than can carbohydrate molecules. L i k e c a r b o h y d r a e s, fat m o l e c u l e s are c o m p o s e d o f carbon, h y d r o g e n, and o x y g e n atoms. H o w e v e r, fats h a v e a m u c h s m a l l e r p r o p o r i o n o f o x y g e n than d o c a r b o h y drates. T h e f o r m u l a f o r the fat tristearin, C 5 7 H, 0 O 6 illustrates these characteristic proportions. T h e b u i l d i n g b l o c k s of fat m o l e c u l e s are f a t y a c i d s a n d g l y c e r o l. A l h o u g h the g l y c e r o l p o r i o n o f e v e r y fat m o l e c u l e is the same, there are many kinds o f fatty a c i d s a n d, h e r e f o r e, m a n y k i n d s o f fats. A l l fatty a c i d m o l e c u l e s i n c l u d e a c a r b o x y l g r o u p (- C O O H) at the e n d o f a chain of carbon atoms. Fatty acids d i f f e r in the lengths o f their c a r b o n a o m c h a i n s, a l h o u g h s u c h c h a i n s u s u a l l y c o n a i n an e v e n n u m b e r o f c a r b o n a o m s. T h e fatty a c i d c h a i n s a l s o m a y vary in the w a y s h e c a r b o n a o m s }oin. I I I I c - c - C-C-C-C-C-C-C-C-C-H I I I I 1 I I I I I I H H H H H H H H (a) Saturated fatty acid 0. T h i s y p e o f f a t y a c i d is c a l l e d a s a u r a e d f a l y a c i d; that i s, e a c h c a r b o n a o m b i n d s a s m a n y h y d r o g e n a o m s a s p o s s i b l e a n d is h u s s a l u r a e d w i h h y d r o g e n atoms. Other fatty a c i d chains, rated falty acids, have one or more acids, double unsatubonds two or more acids [fig. A g l y c e r o l m o l e c u l e c o m b i n e s w i h three fatty acid m o l e c u l e s o f o r m a s i n g l e fat m o l e c u l e, o r triglyceride 2. T h e fatty acids o f a r i g l y c e r i d e m a y h a v e d i f f e r e n l e n g h s a n d d i f f e r e n d e g r e e s o f s a u r a i o n. A diet rich in saturated fat increases risk of atherosclerosis, which obstructs blood vessels.

Write the sequence of the complementary strand of D N A to the sequence A G C G A T G C A G C. What is the sequence of m R N A that would be transcribed from the given sequence Distinguish between the anaerobic reactions and the aerobic reactions of cellular respiration. Explain how the oxidation of molecules inside cells differs from the burning of substances outside cells. A statistical analysis called hierarchy clustering groups cells by similarities in gene expression. The results generally agree with what is known of histology (the study of tissues) from microscopy, but go farther. For example, in one experiment that probed 35 tissue types for the activity of 26,000 genes, the cells of various lymphoid tissues (tonsils, thymus, and spleen) had very similar gene expression profiles, as did the organs of the digestive system, A liver cell expressed genes that encode clotting factors, transporters of metals and lipids, and enzymes involved in detoxification and metabolism-all logical. But the microarray also revealed activity of four proteins whose roles in liver function are unknown. This new approach will not only provide baseline portraits of celts to which injured or diseased counterparts can be compared, but will reveal new points of therapeutic intervention. This unexpected finding suggests new points for drugs, perhaps even existing ones, to intervene. In the mid 1990s, technology was developed to display the genes that are expressed in particular cell types. Probes representing two cell sources can be linked to different fluorescent tags so that their gene expression patterns can be directly compared-such as a healthy and cancerous version of the same cell type. Now that the human genome has been sequenced, a microarray can scan for activity in all genes. Alternatively, microarrays can be customized to paint molecular portraits of specific functions. In all c o m p l e x organisms, cells are o r g a n i z e d into tissues, w h i c h are layers or groups o f similar cells w i h a c o m m o n f u n c i o n. Tissues can be d i s i n g u i s h e d f r o m each other b y v a r i a i o n s in c e l l size, shape, o r g a n i z a i o n, and f u n c i o n. T h e study o f tissues, histology, w i l l assist u n d e r s a n d i n g in later discussions of the p h y s i o l o g y of organs a n d organ systems. T h e tissues o f the h u m a n b o d y i n c l u d e f o u r m a j o r y p e s: epithelial, connective, muscle, and nervous. T h e s e tissues a s s o c i a e a n d interact to f o r m o r g a n s that h a v e s p e c i a l i z e d f u n c i o n s. T h i s chapter examines in detail epithelial and connect i v e tissues, a n d p r o v i d e s an i n r o d u c i o n to m u s c l e a n d nervous tissues. T h r o u g h o u this chapter, s i m p l i f i e d l i n e d r a w i n g s (f o r e x a m p l e, fig. Chapter 9 discusses muscle tissue in more detail, and chapters 10 and 11 detail nervous tissue. Because e p i h e l i u m c o v e r s the b o d y s u r f a c e a n d organs, f o r m s the i n n e r l i n i n g o f b o d y c a v i ties, a n d lines h o l l o w organs, it a l w a y s has a free (apical) surface-one that is e x p o s e d to the o u s i d e or to an o p e n s p a c e internally. T h e u n d e r s i d e o f this tissue is a n c h o r e d to c o n n e c i v e tissue by a thin, n o n l i v i n g l a y e r c a l l e d the basement m e m b r a n. H o w One of h e w a y s that c a n c e r cells s p r e a d is by s e c r e i n g a substance that dissolves basement membranes. Cancer cells also p r o d u c e f e w e r a d h e s i o n proteins, o r n o n e at all, w h i c h allows h e m o invade surrounding tissue. Because epithelial cells readily d i v i d e, injuries heal r a p i d l y as n e w c e l l s r e p l a c e l o s o r d a m a g e d o n e s. S k i n c e l l s a n d h e c e l l s that l i n e h e s o m a c h a n d i n e s i n e s a r e e p i h e l i a l c e l l s that a r e c o n i n u a l l y b e i n g d a m a g e d and replaced. In m a n y p l a c e s, d e s m o s o m e s attach o n e to another, e n a b l i n g these cells to f o r m e f f e c i v e protective barriers in such structures as the outer l a y e r o f h e skin and the inner l i n i n g o f the m o u h. T O C H A P E R 3, a n d c o v e r s the m e m b r a n e s that l i n e b o d y c a v i i e s, H o w e v e r. Simple Cuboidal Epithelium S i m p l e c u b o i d a l e p i h e l i u m consists of a single layer of c u b e - s h a p e d c e l l s.

The discs may even collapse on and i m p a i r s h e a b i l i y o f h e s o f c e n e r s o f h e d i s c s o absorb themselves s l i g h l y, c o n r i b u i n g o h e loss o f h e i g h i n h e e l d e r l y. T h e s i f f e n i n g s p i n e g r a d u a l l y restricts the range o f m o i o n. L o s s o f f u n c i o n in s y n o v i a l joints b e g i n s in the third d e c a d e o f l i f e, but progresses s l o w l y. F e w e r capillaries servi n g the s y n o v i a l m e m b r a n e s l o w s the circulation o f s y n o v i a l fluid, and the m e m b r a n e may b e c o m e infiltrated w i h fibrous material and cartilage. M o r e c o l l a g e n cross-links shorten a n d s i f f e n l i g a m e n s, a f f e c i n g the range o f m o i o n. T h i s may, in turn, upset balance and retard the ability to respond in a p r o e c i v e w a y to f a l l i n g, w h i c h m a y e x p l a i n w h y o l d e r p e o p l e are m o r e l i k e l y to b e injured in a fall than y o i m g e r i n d i v i d u a l s. U s i n g j o i n s, h r o u g h a c i v i y and e x e r c i s e, can k e e p h e m f u n c i o n a l l o n g e r. D i s u s e h a m p e r s the b l o o d s u p p l y to j o i n s, w h i c h hastens s i f f e n i n g. P a r a d o x i c a l l y, this can k e e p p e o p l e f r o m e x e r c i s i n g, w h e n this is e x a c l y w h a they should be doing. Classification of Joints (page 262) Joints are classified according to the type of tissue that binds the bones togedier. Bones at fibrous joints are tightly fastened to each other by a layer o f dense connective tissue with many collagenous fibers. These joints include articular cartilage, a joint capsule, and a synovial membrane. A s y n o v i a] m e m b r a n e that secretes s y n o v i a l fluid lines the i n n e r layer o f a joint capsule. S y n o v i a l fluid moistens, p r o v i d e s nutrients, and lubricates h e articular surfaces. M e n i s c i d i v i d e s o m e s y n o v i a l joints into c o m p a r m e n s. Bursae are usually located b e w e e n the skin a n d underlying bony prominences. T h e s h o u l d e r joint is a ball-and-socket joint that consists o f the head o f the humerus and the g l e n o i d c a v i y o f h e scapula. A c y l i n d r i c a l joint capsule e n v e l o p s the joint, (1) h e capsule is loose and b y itself cannot k e e p the articular surfaces together. Because its parts are l o o s e l y attached, h e s h o u l d e r joint p e r m i s a w i d e range o f m o v e m e n s. T h e e l b o w has a h i n g e joint b e w e e n the h u m e r u s a n d the ulna and a g l i d i n g joint b e w e e n the h u m e r u s and the radius. A s y n o v i a l m e m b r a n e partially d i v i d e s the joint c a v i y into w o portions. T h e joint b e w e e n h e humerus and the ulna p e r m i s flexion a n d e x e n s i o n o n l y. T h e h i p joint is a ball-and-socket joint b e w e e n the f e m u r and the c o x a. T h e articular surfaces are held together by a h e a v y joint c a p s u l e that is r e i n f o r c e d by ligaments. T h e k n e e joint i n c l u d e s w o c o n d y l o i d joints b e w e e n the f e m u r and h e tibia and a g l i d i n g joint b e w e e n h e f e m u r and the patella. S e v e r a l l i g a m e n s, s o m e of w h i c h are w i h i n h e joint capsule, bind articular surfaces. T w o m e n i s c i separate the articulating surfaces o f the f e m u r and the tibia. T h e knee joint p e r m i s flexion and e x e n s i o n; w h e n h e l o w e r l i m b is f l e x e d at the knee, s o m e rotation is possible. In a ball-and-socket joint, the g l o b u l a r head o f a b o n e fits into the cup-shaped cavity o f another bone. T h e hip and s h o u l d e r are ball-and-socket joints, C o n d y l o i d joints a. A c o n d y l o i d joint consists o f an o v o i d c o n d y l e o f o n e b o n e fitting into an e l l i p i c a l c a v i y of another bone. In a hinge joint, the c o n v e x surface o f o n e b o n e fits into the c o n c a v e surface o f another bone.

However, supplemental oxygen should be provided if needed to maintain values in the targeted range (see Chap. Suctioning of the pharynx should be done carefully, and oral feedings should be avoided. Positions of extreme flexion or extension of the neck should be avoided to reduce the likelihood of airway obstruction. Treatment with caffeine, a methylxanthine, markedly reduces the number of apneic spells and the need for mechanical ventilation. Mechanisms by which methylxanthines may decrease apnea include (i) respiratory center stimulation; (ii) antagonism of adenosine, a neurotransmitter that can cause respiratory depression; and (iii) improvement of diaphragmatic contractility. We therefore begin caffeine citrate treatment in all infants less than 1,250 g birth weight soon after birth and continue until it is deemed no longer necessary to treat apnea. In preterm infants more than 1,250 g birth weight who require mechanical ventilation, we begin caffeine treatment prior to extubation. In other infants with apnea of prematurity, we begin caffeine to treat frequent and/or severe apnea. We use a loading dose of 20 mg/kg of caffeine citrate (10 mg/kg caffeine base) orally or intravenously over 30 minutes, followed by maintenance doses of 5 to 8 mg/kg (2. If apnea continues, we give an additional dose of 10 mg/kg caffeine citrate and increase the maintenance dose by 20%. The effect of caffeine likely remains for approximately 1 week after it has been discontinued. We continue monitoring until no apnea has been detected for at least 5 days after that period. Mean percentiles for growth parameters were similar at 18 to 21 months corrected age. Most reports of side effects of methylxanthines in newborns are based on experience with theophylline. We do not use doxapram, a respiratory stimulant that may reduce apnea if methylxanthine therapy has failed. Whether blood transfusion reduces the frequency of apneic spells in some infants is controversial. Mechanical ventilation may be required if the other interventions are unsuccessful. There is no consensus on the appropriate management of these infants, but efforts are directed at reducing the risk of apneic spells so that the child can be cared for at home. Continued use of caffeine may be helpful in infants whose spells recur when the drug is discontinued. Attempts to withdraw the drug can be made at intervals of approximately 2 months while the child is closely monitored. Some infants are cared for with cardiorespiratory monitoring at home, although few data are available on its effectiveness. The exceptions include preterm infants with respiratory disease, infants with symptomatic gastroesophageal reflux, and infants with craniofacial abnormalities or evidence of upper airway obstruction. The disorder is characterized by tachypnea with signs of mild respiratory distress, including retractions and cyanosis; decreased oxygen saturation is usually alleviated by supplemental oxygen with FiO2 0. To accommodate the transition to breathing air at birth, the lungs must switch from a secretory mode, which provides the fetal lung fluid required for normal lung growth and development in utero, to an absorptive mode. Amiloride-sensitive sodium channels expressed in the apical membrane of the alveolar epithelium play an important role in lung fluid clearance. Interstitial lung fluid pools in perivascular cuffs of tissue and in the interlobar fissures and is then cleared into pulmonary capillaries and lung lymphatics. Compression of the compliant airways by fluid accumulated in the interstitium can lead to airway obstruction, air trapping, and ventilation-perfusion mismatch. These have been attributed to delayed or abnormal fetal lung fluid clearance due to the absence of the hormonal changes that accompany spontaneous labor.