Major functions

The kidneys are often thought of as the body's filters, removing toxins and metabolic waste products from the body. The kidneys certainly perform this task; however they have a few more responsibilities, without which we would not be able to survive.
 
 

Ultrafiltration

Of the renal blood flow, 125 ml/min is filtered by glomerulus. In one day, the kidney filters approximately 180 liters of blood and produces 1.5 liters of urine. Thus, it is evident that the kidneys possess extraordinary mechanisms to reabsorb water while removing metabolic waste by-products and toxins. [3] Kidney function is measured by the glomerular filtration rate (ml/min) which is defined as the filtration of a solute that is not reabsorbed nor secreted. The clearance (mass removal of a solute/concentration of solute) of a solute can also be used to quantify renal function. 

Endocrine Function

The kidneys secrete the following hormones to initiate processes that occur in other parts of the body: 

1. Erythropoietin to stimulate erythrocyte production in bone marrow. 

2. The active form of Vitamin D, 1,25-dihydroxyvitamin D3, to aid in gut absorption of calcium for bone deposition. 

3. Renin to help regulate blood volume and potassium balance (described in Volume Regulation). 
 
 

Osmolarity Regulation

Antidiuretic hormone (ADH or vasopressin) is synthesized in the hypothalamus and released in the posterior pituitary in response to an increase in osmolarity as sensed by osmoreceptors in the anterior hypothalamus. The presence of ADH increases the water permeability of the collecting tubule permitting water reabsorption and a shift towards normal plasma osmolarity. In the process, concentrated urine is formed. In the absence of ADH, the water permeability of the collecting tubules is low, more water is excreted in diluted urine, and blood osmolarity returns to normal. 
 
 

Volume Regulation

In addition to the kidney being anatomically designed to create concentrated urine in order to conserve fluid volume, the kidney is also designed to regulate extracellular fluid volume through the Renin-Angiotensin-Aldosterone (RAA) pathway and Atrial Natriuretic Factor (ANF). 

The kidneys initiate the RAA pathway by secreting renin within the lumen of the afferent arteriole. Renin initiates the cascade of reactions that releases aldosterone from adrenal cortex; thereby stimulating the reabsorption of Na+ in the collecting tubule lumen. Since sodium is primarily an extracellular solute, a change in its concentration will lead to a change in extracellular volume (plasma and interstitial volume). 

Atrial Natriuretic Factor (ANF) is secreted by cells in the atria of the heart to inhibit Na+ reabsorption in the kidneys when there is an excess of Na+ and fluid in plasma. It also inhibits secretion of aldosterone, which also inhibits Na+ reabsorption. 
 
 

Acid-Base Regulation

Everyday metabolism of proteins and phospholipids generates sulfuric and phosphoric acids, respectively. In order to maintain a normal physiologic pH, the body maintains a buffer reserve of bicarbonate ions. The kidneys regenerate this buffer reserve and excrete the acidic metabolic waste products.