Renal System Study Guide PDF

STUDY GUIDE FOR THE RENAL SYSTEM In order to understand the physiology of the renal system you have to understand the anatomical organization of the kidney and particularly the nephron. You should know that the kidney is composed of an outer layer called the cortex and an inner layer calle...

STUDY GUIDE FOR THE RENAL SYSTEM In order to understand the physiology of the renal system you have to understand the anatomical organization of the kidney and particularly the nephron. You should know that the kidney is composed of an outer layer called the cortex and an inner layer called the medulla. These layers surround what is called the renal pelvis that receives the urine and passes it on to the ureter, which passes it on to the bladder where it is stored until it is eliminated from the body through the urethra. The nephron is the functional unit of the kidney. Each nephron is composed of a vascular element and a tubular element. You should be familiar with the components of each of these elements. The vascular element includes the afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries (which includes the vesa recta). The tubular element consists of Bowman’s capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct. You should be able to relate each part of the tubular element to each part of the vascular element, and where each part lies within the cortex and medulla. There are four processes that occur in the kidneys in processing the filtrate. These processes are filtration, reabsorption, secretion, and excretion. Every other topic discussed in lecture falls under these four processes. Filtration of the blood occurs between the fenestrated capillaries of the glomerulus and Bowman’s capsule and consists of the bulk flow of fluid and solutes out of the blood and into the lumen of Bowman’s capsule. You should know the three forces (PH, Posm, and Pfluid) that contribute to the determination of the filtration pressure responsible for bulk flow. The volume of plasma filtered by the kidney per day is called the glomerular filtration rate (GFR). On average, the GFR is about 180 liter of plasma/day. The regulation of the GFR is carried out by three mechanisms: Antiregulatory mechanisms, Nervous System mechanisms, (mainly sympathetic), and Hormonal mechanisms. You should know the details of each of these. The minute-to-minute regulation of GFR is carried out by the two autoregulatory mechanisms: the myogenic mechanism and the tubuloglomerular mechanism. These two mechanisms are designed to maintain GFR at about 180 liters/day. These mechanisms work by adjusting blood flow in the afferent arteriole. Blood flow in the afferent arteriole affects PH, which alters filtration pressure. If filtration pressure is changed, GFR is changed. The nervous mechanisms involve the sympathetic division of the autonomic nervous system. This mechanism is designed to reduce volume loss when blood pressure drops as a result of hemorrhaging or dehydration. Many hormones also affect GFR. A hormone that is particularly relevant is angiotensin II. Angiotensin II is a potent vasoconstrictor and so reduces afferent arteriolar blood flow and GFR. Reabsorption is the movement of water and/or solutes from the filtrate to the interstitial fluid. You should know the two types of reabsorption and where each predominates. Because the filtrate is similar in composition to the plasma, solutes must be moved by transport processes. You should know and be able to describe the four types of transport processes that are found in the renal tubule and be able to give an example of a solute that is moved by each process. These processes involve mechanisms that are subject to saturation if the concentration of the solute that they are intended to reabsorb becomes too high. When saturation occurs, the concentration of the solute will increase in the urine.  Secretion is the transfer of solutes from the interstitial fluid to the filtrate. Like reabsorption, secretion relies on membrane transport systems to move solutes across the tubular epithelium. Examples of solutes that are secreted are Na+, K+, H+ and HCO3-. These ions are very important in water and ion balance and acid-base balance of the body fluids and so their concentrations in the body fluids are closely regulated. The mechanisms by which these ions are regulated will be covered on the next exam. Drugs, such as penicillin, are also removed from the body by secretion into the filtrate. Excretion is the elimination of solutes from the body in the urine. Analysis of the urine can tell us whether or not a particular solute is being excreted but it can’t tell us whether a particular solute is being secreted or reabsorbed by the kidney tubules. To understand how the tubule is handling a particular solute requires an understanding of renal clearance of the solute from the plasma. Clearance is defined as the volume of plasma that is cleared of a solute per unit time. You should know the formulas for calculating clearance, urine excretion rate, and filtration rate, and be able to use these formulas to determine whether a solute is being secreted or reabsorbed by the tubule. The kidneys are responsible for maintaining the concentration of the body fluids by adjusting the concentration of the urine. The kidneys control the concentration of the urine by varying the amount of water and NaCl that is reabsorbed from the filtrate. The loop of Henle, distal tubule, and the collecting duct determine the concentration of urine. The loop of Henle receives filtrate from the proximal tubule with an osmolarity of 300mOsM. The descending limb of the loop of Henle concentrates the filtrate to 1200 mOsM, and then the ascending limb dilutes it to 100mOsM (which is hypotonic to the body fluids). You should understand how each part of the loop functions to achieve this and the role of the osmolarity of the medullary interstitial fluid in this process. You should also understand the role that the vesa recta plays in maintaining the high osmolarity of the medullar interstitial fluids. The ability of the nephron to regulate the final concentration of the urine depends on the variable water permeability of the walls of the distal tubule and collecting duct, and the high osmolarity of the medullar interstitial fluids. The permeability of the walls of the distal tubule and collecting duct is controlled by a hormone called anti-diuretic hormone (ADH). You should know the mechanism by which ADH works and the factors that control its release from the posterior pituitary. Water volume is not the only factor that is regulated in controlling the concentration of body fluids. The reabsorption and secretion of ions is also very important. The two most important ions in the body fluids are Na+ and K+. Sodium is the most abundant ion in the extracellular fluids and potassium is the most abundant intracellular ion. Both of these ions play important roles in many physiological functions, including muscle contraction and neuron depolarization, and their regulation is crucial in maintaining normal body functions. Sodium levels are not directly monitored and regulated. Instead regulation of Na is mediated primarily through a complex pathway called the renin-angiotensin- aldosterone system (RAAS). The stimuli that initiate this pathway are directly or indirectly related to low blood pressure. You should be able to summarize and discuss the RAAS pathway in detail for the exam. In this system, the hormone that acts on the kidney tubule to increase the reabsorption of sodium is aldosterone. The target cells of aldosterone are the P-cells of the collecting duct. You should be prepared to describe in detail how aldosterone acts on these cells to increase Na+ reabsorption You should also know the factors that control the release of aldosterone from the adrenal cortex. Both increased K+ levels in the extracellular fluids and the increased osmolarity of the extracellular fluids (which is usually due to increased Na+ levels) act directly on the adrenal cortex to influence the release of aldosterone. Increased K+ levels stimulate the release of aldosterone. The increased levels of aldosterone thus increase K+ secretion and Na+ reabsorption. Increased osmolarity of the extracellular fluids inhibits the release of aldosterone, which increases Na+ excretion. The increased Na+ excretion brings the osmolarity of the extracellular fluids down. Blood pressure and the flow of filtrate past the macula densa are referred to as indirect factors controlling aldosterone release. These factors mediate their effects through the RAAS system. The RAAS pathway begins when the juxtaglomerular cells of the juxtaglomerular complex are stimulated to release rennin. The three stimuli that control the release of rennin are: 1) low blood pressure 2) Increased sympathetic activity resulting from a drop in blood pressure, and 3) decreased fluid flow past the macula densa. When rennin enters the blood it converts a plasma protein called angiotensinogen into angiotensin I. Angiotensin I is subsequently converted into angiotensin II by angiotensin converting enzyme (ACE) which is found in the epithelium of blood vessels. Angiotensin II stimulates the adrenal cortex to synthesize and release aldosterone, which acts on the nephron to increase Na+ reabsorption. The increase in Na+ increases the osmolarity of the blood. The increased blood osmolarity stimulates thirst and fluid intake. The increased fluid intake raises blood volume and brings blood pressure up. Another hormone involved in Na_ and water balance is atrial natriuretic peptide (ANP). ANP is a hormone that is synthesized and released by atrial myocardial cells. For the exam you should know what controls the release of ANP and the ways that ANP acts to increase Na and water excretion by the kidney. The kidneys are also involved in acid-base balance of the body fluids. Normally the body fluids are maintained at a pH of around 7.4. You should know the three mechanisms that the body has to cope with disturbances in acid-base balance. Suggested End-of-chapter Questions: Chapter 19 7th ed: 2,4,5,6,7,8,9a,b,f, 10,11,12,13,15,16a,b,d, 17,18,21,24,25 Chapter 20 7th ed: 1,3,4,5,6,7,8,9,10,11,15,16,19,20,21,22,24b,d,