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Unformatted text preview: Chapter 37. The Renal System and Its Contribution to Homeostas is All higher animals have developed specialized excretory systems that function to preserve the internal environment. In mammals, the renal system contributes to water balance, ion balance or "mineral metabolism," removal of nonvolatile metabolic wastes and chemically-modified toxins, and pH homeostasis. Kidneys participate in support of other physiologic systems as well. The kidney produces erythropoietin in response to renal hypoxemia, thereby stimulating production of red blood cells. Renal arterial hypotension causes renin secretion and ultimately the production of angiotensin II, the potent vasoconstrictor agent, described earlier in Chapter 34. Additionally, angiotensin causes release of aldosterone, the major mineralocorticoid from the adrenal cortex. This steroid increases sodium retention and increases potassium excretion by its action back on the kidney. The main functions of the kidneys to be considered here concern formation of urine. Urine is stored in the bladder and is periodically eliminated from the body by urination, or micturition . A. Anatomy and Structure The urinary system is made up of a set of abdominal and pelvic viscera: the kidneys , the ureters , the bladder , and the urethra (Figure 37-1). The kidneys receive 20% to 25% of cardiac output, yet they represent a mere 1% of live weight. This disproportionately enormous blood flow comes in through a Figure 37-1. Organs of the urinary system. The mammalian urinary system consists of paired kidneys (RK and LK) and ureters (RU and LU) supplying the bladder (B), from which a single urethra (U) drains to the exterior. This ventral view shows the vascular arrangement at the major trunk vessels (aorta, A; vena cava, VC; left renal vein, LRV; right renal artery, RRA), the relative position of the adrenals (RA and LA), and the origin of the ureters. short system of renal arteries that branch acutely off the aorta almost at right angles (Figure 37-1). Hemodynamic principles cause the concentration of blood cells to be greatest in the center of a vessel where velocity of flow is highest (the phenomenon 2007 version – page 320 is called laminar flow ). Towards the walls of blood vessels, flow is lower because of friction and the blood is relatively cell-poor (or plasma-rich). Blood that is "skimmed- off” into acutely angled branching vessels has a relatively lower packed cell volume. The viscosity of this blood is less because of the fewer cells, and vascular resistance is low because of the short, wide blood vessels. All of these factors contribute to the exceedingly high rate of renal blood flow, as explained in detail in Chapter 18, Section D....
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- Fall '08
- interstitial fluid, DCT, apical membrane