This chapter will cover general physiologic considerations concerning the renal system as they pertain to the practice of anesthesia.
GENERAL PHYSIOLOGIC CONSIDERATIONS
The renal or nephric system plays several vital roles, including regulating the volume and composition of the extracellular fluid (Table 37-1).1 The primary function of the kidney is to produce urine, while several associated organs act to store or to conduct the urine on its pathway to elimination from the body. The kidneys also participate in adjusting the pH of the blood, eliminating toxic wastes, activating (and responding to) vitamin D, and producing the hormone erythropoietin. Multiple mechanisms act to regulate systemic arterial blood pressure with the kidneys playing a major role. While we consider the metabolic machinery of the liver to play the main role in gluconeogenesis for the body, the kidneys’ active metabolism can provide a substantial fraction of new glucose production, especially in times of prolonged fasting.
Table 37-1Functions of the Renal System |Favorite Table|Download (.pdf) Table 37-1 Functions of the Renal System
Regulation of volume and composition of the extracellular fluid
Regulation of acid-base balance
Elimination of metabolic by-products and drugs
Regulation of red blood cell production
Activation of vitamin D
Regulation of systemic blood pressure
Gluconeogenesis (during prolonged fasting)
The two kidneys normally reside on each side of the vertebral column, retroperitoneally in the abdominal cavity with their uppermost portions just below the ribs.2 The left kidney occupies a slightly more cephalad position than the right, which is lower due to the presence of the liver. The ureters drain the urine from kidneys into the bladder for storage until urination drains the bladder via the urethra Fig. 37-1. The complicated embryologic development of the closely related urinary and reproductive systems cause them to be frequent sites for congenital malformations, often requiring corrective surgery.
Urinary system in a woman. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology, 14th ed. New York, NY: McGraw Hill; 2016.)
Each bean-shaped kidney is about the size of a fist with the concave central portion facing toward the midline of the body. This indented central hilum is the area where the nerves, blood vessels, and ureters enter or leave the kidney. About 90% of the kidney’s tissue is composed of blood vessels and the tubules responsible for forming urine. The renal outer surface is encased in a capsule. Under the capsule is the granular-appearing cortex, where urine formation begins in the nephrons. Underlying the cortex is the medulla, which is arranged in pyramid-like structures pointed toward the hollow center of the kidney. The medulla is composed of urine-carrying tubules and blood ...