Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport. Indirect reabsorption
In some cases, reabsorption is indirect. For example, bicarbonate (HCO3−) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion (H+) into the tubule fluid via a Na/H exchanger:

In the lumen
The H+ combines with HCO3− to form carbonic acid (H2CO3)
Luminal carbonic anhydrase enzymatically converts H2CO3 into H2O and CO2
CO2 freely diffuses into the cell
In the epithelial cell
Cytoplasmic carbonic anhydrase converts the CO2 and H2O (which is abundant in the cell) into H2CO3
H2CO3 readily dissociates into H+ and HCO3−
HCO3− is facilitated out of the cells basolateral membrane

Influence of hormones
Some key regulatory hormones for reabsorption include:

aldosterone, which stimulates active sodium reabsorption (and water as a result)
antidiuretic hormone, which stimulates passive water reabsorptionBoth hormones exert their effects principally on the collecting ducts. Tubular secretion occurs simultaneously during reabsorption of filtrate. Substances, generally produced by body or the by-products of cell metabolism that can become toxic in high concentration, and some drugs (if taken). These all are secreted into the lumen of renal tubule. Tubular secretion can be either active or passive or co-transport.
Renal physiology (Latin rēnēs, "kidneys") is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules;  regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
Much of renal physiology is studied at the level of the nephron, the smallest functional unit of the kidney. Each nephron begins with a filtration component that filters the blood entering the kidney. This filtrate then flows along the length of the nephron, which is a tubular structure lined by a single layer of specialized cells and surrounded by capillaries. The major functions of these lining cells are the reabsorption of water and small molecules from the filtrate into the blood, and the secretion of wastes from the blood into the urine. Proper function of the kidney requires that it receives and adequately filters blood. This is performed at the microscopic level by many hundreds of thousands of filtration units called renal corpuscles, each of which is composed of a glomerulus and a Bowmans capsule. A global assessment of renal function is often ascertained by estimating the rate of filtration, called the glomerular filtration rate (GFR). Formation of urine
The kidneys ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion, whose sum is called renal clearance or renal excretion.