BASIC RENAL PHYSIOLOGY: Introduction

INTRODUCTION

A. Role of Kidneys: primarily regulation or homeostasis (rather than excretion)

1. Regulation of blood plasma and interstitial fluid composition (homeostasis), especially

a. inorganic ions (e.g. Na⁺, K⁺, Cl^-, Ca²⁺)
b. body fluid volume
c. osmolality (total dissolved particle concentration) and fluid distibutioin
d. H⁺ (pH) -- acid-base balance

2. Excretion of (non-volatile) metabolic end products (e.g. urea, uric acid, creatinine, NH4⁺) and "foreign" solutes (e.g. some drugs)

Note: The above are functions are vital; loss of renal function leads to debilitation beginning in about one day and terminating in death in one-two weeks (end stage renal/kidney failure/disease)

3. Endocrine organ, secreting

a. renin, for regulation of Na⁺, ECF (extracellular fluid) volume, vascular resistance
b. erythropoietin, for regulation of erythrocyte production
c. calcitriol, related to calcium regulation

4. Metabolic functions: e.g. peptide degradation, synthesis of NH₃ and H⁺

B. General Renal Mechanisms of Body Fluid Regulation

Selective withdrawal of substances from the blood plasma flowing through the kidneys and excretion these substances in the urine (main mechanism)

Note: When renal function is measured, the values generally refer to both kidneys taken together; e.g. renal blood flow refers to the total blood flow rate through both kidneys, renal sodium excretion rate refers to the total excretion of sodium ion by both kidneys

C. Composition of Blood Plasma and Urine (typical)

	Plasma (P)	Urine (U)	Clearance (C)
Na⁺	145	100 meq/L	0.7 ml/min
Cl^-	110	100	0.9
K⁺	5	50	10
HCO₃^-	24	1	0.04
glucose (fasting)	5 (90)	0 mM/L (0 mg/dl)	0 ml/min
plasma proteins	8	0 gm/dl	0
urea	7 (20)	300 mM/L (900 mg/dl)	45
uric acid	0.3 (5)	3 mM/L (45 mg/dl)	9
creatinine	0.01 (1)	1.2 mM/L (120 mg/dl)	120
pH	7.40	6.0
osmolal conc.	290	600 mOsm/kg	2 ml/min

Note on Units
mM/L = millimoles/liter
mg/dl = milligrams/deciliter (100 ml; sometimes written as mg%)
mOsm/kg = milliosmoles/kg fluid (or milliosmolal)

Plasma has a relatively constant composition (normally)
Urine composition is variable in order to maintain homeostasis

In other words, what is most important for homeostasis is the difference between the composition of renal arterial blood and renal venous blood

D. Urine Formation Rate (V)

Normal 1 ml/min 1.5 L/day

Normal range 0.4-2 ml/min 0.5-3 L/day

Oliguria < 0.4 ml/min < 0.5 L/day

Anuria < 0.04 ml/min < 50 ml/day

Polyuria > 2 ml/min > 3 L/day

Normal	1 ml/min	1.5 L/day
Normal range	0.4-2 ml/min	0.5-3 L/day
Oliguria	< 0.4 ml/min	< 0.5 L/day
Anuria	< 0.04 ml/min	< 50 ml/day
Polyuria	> 2 ml/min	> 3 L/day