**Renal Physiological Engineering – Optimization Aspects**

David Chee-Eng Ng1 and Dhanjoo N. Ghista2

*1Department of Nuclear Medicine and PET, Singapore General Hospital, 2Department of Graduate and Continuing Education, Framingham State University, Framingham, Massachusetts, 1Singapore 2USA* 

## **1. Introduction**

814 Biomedical Science, Engineering and Technology

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Renal overall functional performance is characterized by excretory function of major end products of protein metabolism, regulation of ionic processes, maintenance of fluid balance and blood volume regulation. Minor functions include hormonal regulation of red cell production and stabilization of blood pressure. Although the kidneys comprise less than 0.5% of total body weight, they receive approximately 20% of the total cardiac output [1]. This consideration underscore the important role played by the kidneys.

The renal circulation has a unique sequence of vascular elements: a high-resistance afferent arteriole, a high-pressure glomerular filtration capillary structure, another high-resistance efferent arteriole and a series of tubular structures with unique absorption/excretion properties. The basic functional unit is the nephron. The nephron consists of a glomerular filtration structure and a tubular system, with its associated vascular elements.

Fig. 1. Diagram of a nephron unit (adapted from [1]).

Structurally, there are 500,000 – 1,000,000 nephron units in the kidney [1,2]. Each nephron has a length of 40 mm and tubular diameter of 50 um. Microscopic puncture and perfusion techniques make it possible to measure single-nephron filtration rates, absorption and secretion rates. The single-nephron glomerular filtration rate (SNGFR) is approximately 30 nl/min.

Approximately 1200 ml/min of blood flow to the kidneys which represents 20% of total cardiac output. In the filtration mechanism, a total filtration surface of 1 m2 is found. A global renal fitrate of 120 ml/min (180 L/day) is produced by both kidneys, of which 99.4% of water is reabsorbed to yield about 1 liter of urine/day. The concentration ability of the renal system is largely resides in the tubular system embedded within the renal medulla. Figure 1 demonstrates the disparate functionality of the renal nephron unit.

In this paper we will analyse the operating characteristics of the renal system and examine the optimal features of several aspects of renal physiological engineering mechanisms, particularly, the countercurrent multiplier mechanism for urine concentration and how optimal renal function in terms of renal clearance is maintained.
