Water Balance PDF - Biology Past Paper

Water Balance 7.6 The body adjusts for increased water intake by increasing urine output. Conversely, it adjusts for increased exercise or decreased water intake by reducing urine output. These adjustments involve the interaction of the body’s two communication systems: the...

Water Balance 7.6 The body adjusts for increased water intake by increasing urine output. Conversely, it adjusts for increased exercise or decreased water intake by reducing urine output. These adjustments involve the interaction of the body’s two communication systems: the nervous system and the endocrine system. Regulating ADH A hormone—antidiuretic hormone (ADH)—helps regulate the osmotic pressure of antidiuretic hormone (ADH) body fluids by causing the kidneys to increase water reabsorption. When ADH is released, causes the kidneys to increase a more concentrated urine is produced, thereby conserving body water. ADH, produced water reabsorption by specialized nerve cells in the hypothalamus, moves along specialized fibres from the hypothalamus to the pituitary gland, which stores and releases ADH into the blood. Specialized nerve receptors, called osmoreceptors, located in the hypothalamus detect osmoreceptors specialized nerve changes in osmotic pressure. When you decrease water intake or increase water loss— cells in the hypothalamus that detect by sweating, for example—blood solutes become more concentrated. This increases the changes in the osmotic pressure of the blood and surrounding extracel- blood’s osmotic pressure. Consequently, water moves into the bloodstream, causing the lular fluids (ECF) cells of the hypothalamus to shrink (Figure 1). When this happens, a nerve message is sent to the pituitary, signalling the release of ADH, which is carried by the bloodstream to the kidneys. By reabsorbing more water, the kidneys produce a more concentrated urine, preventing the osmotic pressure of the body fluids from increasing any further. osmoreceptors located behavioual response: in the hypothalamus sensation of thirst detect changes in cells of hypothalamus shrink; osmotic pressure nerve message sent to drinking pituitary lowers the gland allows increased osmotic water absorption pressure of the blood water reabsorption prevents the osmotic pituitary release of ADH pressure of body fluids gland stores from increasing any and releases ADH further—prevention into blood collecting duct carries urine from nephrons to of dehydration the pelvis of a kidney high Figure 1 Homeostasis: By increasing water reabsorption increase in in the kidneys, ADH helps conserve blood osmotic pressure Blood osmotic pressure body water. The osmoreceptors in (e.g., sweating, dehydration) the hypothalamus initiate the thirst response. The shrinking of the cells of the hypothalamus also initiates a behavioural response: the sensation of thirst. If more water is taken in, it is absorbed by the blood and the con- centration of solutes in the blood decreases. The greater the volume of water consumed, the lower the osmotic pressure of the blood. As the blood becomes more dilute, fluids move from the blood into the hypothalamus. The cells of the hypothalamus swell, and nerve messages to the pituitary stop. Less ADH is released, and less water is reabsorbed from the nephrons. NEL Maintaining an Internal Balance 353 DID YOU KNOW ? ADH and the Nephron Approximately 85% of the water filtered into the nephron is reabsorbed in the prox- Alcohol Affects Fluid Levels Alcohol consumption decreases the imal tubule. Although the proximal tubule is very permeable to water, this permeability release of ADH. Some of the symp- does not extend to other segments of the nephron. (Refer to Figure 1 in section 7.5.) toms experienced the day following The descending loop of Henle is permeable to water and ions, but the ascending tubule excessive alcohol consumption can is only permeable to NaCl. Active transport of Na ions from the ascending section of be attributed to increased water loss by urine and decreased body fluid the loop concentrates solutes within the medulla of the kidney. Without ADH, the rest levels. of the tubule remains impermeable to water, but continues to actively transport Na ions from the tubules. The remaining 15% of the water filtered into the nephron will be lost if no ADH is present. INVESTIGATION 7.6.1 ADH makes the upper part of the distal tubule and collecting duct permeable to water. Do Sports Drinks When ADH makes the cell membranes permeable, the high concentration of NaCl in the Really Work? (p. 363) intercellular spaces creates an osmotic pressure that draws water from the upper sec- Are sports drinks any better than tion of the distal tubule and collecting duct. As water passes from the nephron to the inter- water and sugar? How can you cellular spaces and the blood, the urine remaining in the nephron becomes more determine whether a sports drink is able to restore the electrolytes concentrated. It is important to note that the kidneys only control the last 15% of the water essential for the operation of nerves found in the nephron. By varying water reabsorption, the kidneys regulate the osmotic and muscles? concentrations of body fluids. Kidneys and Blood Pressure The kidneys play a role in the regulation of blood pressure by adjusting for blood vol- aldosterone hormone that umes. A hormone called aldosterone acts on the nephrons to increase Na reabsorp- increases Na reabsorption from tion (Figure 2). The hormone is produced in the cortex of the adrenal glands which lies the distal tubule and collecting duct above the kidneys. Not surprisingly, as NaCl reabsorption increases, the osmotic gra- dient increases and more water moves out of the nephron by osmosis. angiotensin has two important functions low blood pressure detector: stimulates juxtaglomerular apparatus the release of aldosterone from the adrenal renin converts gland causes angiotensinogen constriction —a plasma protein— of blood into angiotensin vessels specialized aldosterone cells within is carried in the apparatus blood to the release renin kidneys Homeostasis: decrease in acts on Blood pressure blood pressure nephrons and blood or blood volume Figure 2 to increase volume (e.g., dehydration, low blood loss) The hormone aldosterone maintains Na and H2O homeostasis by increasing Na and reabsorption water reabsorption. 354 Chapter 7 NEL  Section 7.6 Conditions that lead to increased fluid loss can decrease blood pressure, reducing the delivery of oxygen and nutrients to tissues. Blood pressure receptors in the juxtaglomerular apparatus (found near the glomerulus) detect low blood pressure. Specialized cells within the structure release renin, an enzyme that converts angiotensinogen, a plasma protein produced by the liver, into angiotensin. Angiotensin has two important functions. First, the activated enzyme causes con- striction of blood vessels. Blood pressure increases when the diameter of blood vessels is reduced. Second, angiotensin stimulates the release of aldosterone from the adrenal gland. Aldosterone is then carried in the blood to the kidneys, where it acts on the cells of the distal tubule and collecting duct to increase Na transport. This causes the fluid level of the body to increase. pH Balance In addition to regulating body fluid volumes and maintaining the composition of salts in the blood, the kidneys maintain pH balance. Despite the variety of foods and fluids consumed with varying pH levels, the pH of the body remains relatively constant, between 7.3 and 7.5. In addition, during cellular respiration, cells produce carbon dioxide which forms carbonic acid. Carbonic acid and other excess acids ionize to produce H ions. The buildup of H ions lowers the pH. An acid–base balance is maintained by buffer systems that absorb excess H ions or ions that act as bases. Excess H ions from metabolic processes are buffered by bicarbonate ions in the blood. Carbonic acid, a weak acid, is produced. In turn, the carbonic acid breaks down to form carbon dioxide and water. The carbon dioxide is then transported to the lungs where much of it is exhaled. The following reaction shows one type of buffer system, called the bicarbonate–carbon dioxide buffer system (Figure 3): H2O  CO2 H2CO3 HCO3  H carbonic acid bicarbonate ion carbonic acid/bicarbonate buffer Bicarbonate ions, HCO3 , eliminate the excess H ions, preventing a change in pH. peritubular capillary CO2 Na+ + HCO3 interstitial fluid HCO3  Na+ tubule cells CO2  H2O H2CO3 HCO3  H+ CO2  H2O H2CO3 HCO3  H+ NaHCO3 HCO3  Na+ tubule lumen H  ammonia (NH3) Figure 3 H  phosphate (HPO42) The bicarbonate-carbon dioxide filtrate buffer system maintains the pH balance. NEL Maintaining an Internal Balance 355  The buffer system of the blood removes excess H ions; however, the buffer must be restored if the body is to be protected. The kidneys help restore the buffer by reversing the reaction. As shown in Figure 3 (page 355), carbon dioxide is actively transported from the peritubular capillaries, which surround the nephron, into the cells that line the nephron. The carbon dioxide combines with water to initiate the reverse reaction, gen- erating HCO3 and H ions. The bicarbonate ions diffuse back into the blood, thereby restoring the buffer. The H ions recombine with either phosphate ions or ammonia and are excreted with the filtrate from the nephron. SUMMARY Water Balance Antidiuretic hormone (ADH) helps regulate osmotic pressure of body fluids and fluid volume. Aldosterone regulates body fluid volume. ADH and aldosterone are regulated by negative feedback. Kidneys restore buffers by excreting excess H ions or restoring more HCO3 ions. Section 7.6 Questions Understanding Concepts Table 1 1. What is ADH and where is it produced? Where is ADH Substance Blood Glomerular Urine stored? found in plasma from filtrate from 2. Describe the mechanism that regulates the release of ADH. body fluid afferent Bowman’s 3. Where is the thirst centre located? arteriole capsule 4. Describe the physiological adjustment to increased osmotic protein 7.00 0.00 0.00 pressure in body fluids. urea 0.04 0.04 2.00 5. Describe the behavioural adjustment in humans to increased glucose 0.10 0.10 0.00 osmotic pressure in body fluids. sodium ions 0.32 0.32 0.35 6. Discuss the mechanism by which aldosterone helps to maintain blood pressure. chloride ions 0.38 0.38 0.60 7. What role do the kidneys serve in maintaining the pH of Quantities are in g/100 mL. body fluids? 8. Using the HCO3 buffering system, explain what would (b) Which substance provides evidence of secretion? happen if the kidneys failed to excrete H ions. Provide reasons for your response. (c) Which substance provides evidence of reabsorption? Applying Inquiry Skills Justify your answer. 9. Draw a flow chart that shows why the release of ADH is a negative feedback mechanism. Making Connections 10. A micropipette was used to extract fluids from various 11. Using the Internet and other resources, conduct research to structures within the kidney. The data in Table 1 shows explain how urine might be recycled on a space flight. an analysis of substances collected. GO www.science.nelson.com (a) According to the data provided, which substance is not filtered from the blood into the Bowman’s capsule? Give reasons for your answer. 356 Chapter 7 NEL