The absorption of some of the components of the glomerular filtrate back into the blood as the filtrate flows through the nephrons of the kidney. Glucose, amino acids, and salts can be reabsorbed against a concentration gradient and their transport across the nephron into the capillaries requires energy (see active transport). Other components, such as ammonia and urea, are secreted rather than absorbed (see secretion), while certain ions, including potassium and bicarbonate, can either leave passively (see proximal convoluted tubule) or be actively taken up (see distal convoluted tubule) by the tubules according to the omolarity of the interstitial fluid surrounding the tubules, which reflects the overall ionic balance throughout the body.