Introduction The digestive tract absorbs nutrients, as well as sub- stances that we cannot metabolize. Also, metabolism

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Introduction The digestive tract absorbs nutrients, as well as sub- stances that we cannot metabolize. Also, metabolism produces waste molecules when we use nutrients for energy or other cellular functions. Urea, a nitrogen-containing waste, is an example of a waste product of amino acid metabolism. These wastes must be removed from the blood so that they do not build up to toxic levels. The kidneys and the urinary system not only provide this crucial service of waste removal, but they also contribute significantly to body fluid homeostasis. In this exercise, we will first examine the organs of the urinary system, and then we will focus on the process of urine formation. Organs of the Urinary System The Kidneys The kidneys are bean-shaped organs about the size of your fist, and they are located near the posterior abdominal wall (Figure 18.1a). Like some other organs we have encountered in the study of the human body, the kidneys have an outer cortex and an inner medulla. The cortex is where most of the blood is filtered and enters the microscopic kidney tubule system, called the nephron. The medulla mainly contains tubules and ducts involved in urine formation and concentration (Figure 18.1b). The tiny collecting ducts of the medulla (Figure 18.1c) eventually dump the urine they produce into the renal pelvis, which collects and funnels urine into the ureter (Figure 18.1b). The ureters from each of the two kidneys lead down to the urinary bladder, a saclike organ that stores urine. Both the ureters and urinary bladder have smooth muscle in their walls. In the ureters, the muscle produces wavelike contractions that push urine toward the bladder. In the urinary blad- der, the smooth muscle contracts as part of a reflex, which helps to push urine through the urethra to the outside of the body (Figure 18.1a). As you can see in Figure 18.2, the urethra is longer in males because it passes through the penis. In this figure, you can also see the two sphine- ter muscles that control urination. The internal urethral sphincter, which is near the base of the bladder, is involuntary. The external urethral sphincter, which is 1 to 2 inches farther down the urethra, is voluntary. ACTIVITY 1 Observing the Organs of the Urinary System Materials for This Activity Fetal pig Dissection equipment Disposable gloves Human torso models Although your fetal pigs should be open from pre- vious exercises, you may need to cut farther down in the pelvic cavity to view the urethra. See Figure 18.3 for views of the male and female urinary sys- tems of the fetal pig. Review the safety information provided by your instructor and the precautionary information for performing dissections presented in Exercise 13. 1. Move the digestive organs over to one side of the pig's abdominopelvic cavity, and look for the bean-shaped kidney located near the posterior abdominal wall. You may need to break and remove the plasticlike connective tissue that is covering the kidney to see it clearly. Repeat this process to find the kidney on the other side. Speculate as to why animals have two kidneys, as well as duplicates of other important organs. 2. Find the ureter, which will look like a piece of thin, wavy string emerging from the middle of the kidney. Follow each ureter down to its point of attachment to the urinary bladder. Does this tube use simple gravity feed, or does it use muscle to push urine into the bladder? 3. Find the urinary bladder, which looks more like a long, muscular tube than a saclike organ. It is located between the two umbilical arteries, which helps to make the urinary bladder rela- tively easy to find. 4. Use a scalpel to cut through the ventral body wall, slightly off center. Find the start of the ure- thra at the base of the urinary bladder, and fol- low it to where it reaches the surface of the skin. The urethra is the portion of the urinary system that is most different in males and females.
-Kidney Renal artery anal vein -A . Kidney -erior The components of the urinary system Renal cortex -Badder -Unha Henal artery Penal vein- Ranal pelvis b) Internal structure of t kidney 5. Using Figures 18.1 and 18.2 as references, find the following structures on the human torso models. FIGURE 18.1 The human urinary system. a) Structures of the urinary system within the body b) Internal structure of the kidney e) Nephrons within the cortex and medulla in relation to the collecting duct, through which urine produced by the nephron passes. Corte Moduls • Renal medulla • Renal pelvis . Ureter . Urethra Nephrons c) The cortex and medulis of the kidney are composed of numerous nephrons Urinary bladder -Madula -Collecting dun
228 Laboratory Manual for Human B Prostate gland Penis a) The male. The Nephron and Urine Formation Kidney- b) The female FIGURE 18.2 The bladder, the urethra, and associated organs in the human male and female. a) The urethra is longer in men than in women. b) Women generally have a smaller bladder capacity because their bladders are compressed by the uterus. A kidney typically contains about 1 million nephrons, which are the microscopic tubule systems that do the work of the kidney. A knot of leaky capillaries called the glomerulus is surrounded by the nephron's glomerular capsule, which is also known as Bow- man's capsule (Figure 18.4). The combination of the glomerulus and glomerular capsule is found in the Ureter Urinary- bladder Umbilical- cord Penis Interior vena cava -Aorta Prostate Ductus (vas) deferens Epididymis Urethra Bulbourethral- gland a) Urinary system of a male fetal pig. FIGURE 18.3 Male and female fetal pig urinary systems. Urinary bladder Plectum Testis Anus Internal uretral sphincter External- urethral sphincter Urethra- Interior vena- cave Kidney cortex of the kidney, which is where blood is filtered for processing by the nephron. The fluid that leaks out of the glomerulus, called glomerular filtrate, is picked up by the glomerular capsule and funneled into the proximal tubule. This tubule is sometimes referred to as the proximal convoluted tubule due to its wavy appearance (Figure 18.4). This is a hardworking tubule, with cells loaded with mitochondria to provide energy for active trans- port. Most valuable molecules are reabsorbed by the Uterine horn Urinary- bladder -Uterus Vagina -Rectum Vagina Urogenital sinus- b) Urinary system of a female fetal pig. phot Aorta Ovary Uterus body Urethra KAMUS Anus
1. Move the digestive organs over to one side of the pig's abdominopelvic cavity, and look for the bean-shaped kidney located near the posterior abdominal wall. You may need to break and remove the plasticlike connective tissue that is covering the kidney to see it clearly. Repeat this process to find the kidney on the other side. Speculate as to why animals have two kidneys, as well as duplicates of other important organs. 2. Find the ureter, which will look like a piece of thin, wavy string emerging from the middle of the kidney. Follow each ureter down to its point of attachment to the urinary bladder. Does this tube use simple gravity feed, or does it use muscle to push urine into the bladder?