Question

The hormone that helps increase the reabsorption of water from the kidney tubules.

Answer 1

Renal tubule is the portion of nephron which contains the fluid filtered through the glomerulus. The nephron has three parts which helps in renal excretion: the proximal tubule, the distal tubule and the glomerulus.

Complete answer:
Antidiuretic hormone/Vasopressin which is secreted by the posterior gland of pituitary (at the base of brain) targets the functional unit nephrons to increase water reabsorption from renal tubules which in turn decrease the urine content. Antidiuretic hormone binds to receptors on the cells in collecting ducts of kidney and is responsible for reabsorption of water back into the circulation. Absence of this hormone can lead to collecting ducts virtually impermeable to water and increase the urine content.
The antidiuretic hormone synthesized as a peptide in neurons in Hypothalamus and converted to AVP. High amounts of this hormone are dangerous as they cause fluid imbalance which can be responsible for cerebral edema or seizures.
Action of Antidiuretic Hormone-

In the kidney it regulates the osmolarity as well the volume.
It acts specifically in distal convoluted tubules (DCT) and collecting ducts (CD).
Regulates permeability of water in the distal tubule and collecting duct.

Note: Reabsorption can be done by passive diffusion, active transport and cotransport. Mostly water is reabsorbed by glucose and sodium cotransport.Vasopressin is a vasoconstrictor which also acts upon heart by decreasing the cardiac output and heart rate. It also acts on the CNS and increases baroreceptor reflex sensitivity.