Explanation of Urine Volume and Concentration in DCT

When the time fil­trate reach­es the dis­tal con­vo­lut­ed tubule of the ascend­ing Limb DCT(distal con­vo­lut­ed tubule) most of the solute have been reab­sorbed. The fil­trate enter­ing DCT does has a high con­cen­tra­tion of Urea and oth­er wastages through it is hypoos­mot­ic to the inter­sti­tial flu­id as NaCl has been trans­port­ed out of the Tubu­lar lumen. 

distal convoluted tubule
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This rea­son is impor­tant for trans­porta­tion of sodi­um hydro­gen and NH3 into the tubule from the inter­sti­tial flu­id and sodi­um chlo­rine out of the tubule into the inter­sti­tial fluid. 

As the salts are pumped out of the lumen water also moves out passively. 

If the body requires addi­tion­al water it is reab­sorbed here. 

The activ­i­ty of the cells of DCT depends on the con­di­tions of osmot­ic stress faced by the body and is under nor­mal hor­mon­al control. 

Under con­di­tions of diure­sis there is if no water has to be con­served in the body then the flu­id pass­es an unchanged from the DCT into the col­lect­ing duct. 

When water needs to be con­served then the hypo­thal­a­mus secrets the antid­i­uret­ic hor­mone ADH which changes the per­me­abil­i­ty of the DCT was to allow water to move out of the tube use into the inter­sti­tial flu­id of the kid­ney in order to be retained by the body. 

When a DH titres are high in the body more aqua­por­ins are required in the epithe­lial cells of the tubule water dif­fus­es out into the body and the urine flu­id becomes is-osmot­ic to blood when it enters the col­lect­ing duct.

In amphib­ians and rep­tiles there is no loop of hen­le so the flu­id from the prox­i­mal tubule pass­es into the dis­tal tubule direct­ly and the final urine may be hypoos­mot­ic or at best ISO osmot­ic to blood. 

Urine in these ani­mals is made hypoos­mot­ic by the option of source and under dry con­di­tions the vol­ume is also reduced by these option of water in the dis­tal tubule under the influ­ence of ADH. 

In low­er ver­te­brates ADH is referred to as vasotocin. 

Collecting duct

The flu­id that reach­es the col­lect­ing duct may be hypoos­mot­ic or isos­mot­ic depend­ing on the water sta­tus of the animal. 

Final adjust­ments in urine com­po­si­tion are made in the col­lect­ing ducts who’s was become per­me­able only under the influ­ence of ADH. 

In the absence of Ada secre­tion the hyper­os­mot­ic urine that enters the col­lect­ing ducts is excret­ed unchanged and sew a rel­a­tive­ly large vol­ume of urine is excreted. 

Under sit­u­a­tions demand­ing water con­ser­va­tion ADH is secret­ed and the walls of the col­lect­ing ducts become per­me­able to water. 

The urine flu­id pass­ing through the col­lect­ing ducts become pro­gres­sive­ly con­cen­trat­ed and a urine hyper­os­mot­ic to blood and isos­mot­ic to inter­sti­tial flu­id is excreted. 

Tubular secretion

Apart from potas­si­um hydro­gen and H3 some organ­ic acids and organ­ic bases that are not bound to the plas­ma pro­teins are secret­ed from the plas­ma into the Tubu­lar lumen or excretion. 

Organ­ic and ions are sit­u­at­ed along the sodi­um gra­di­ent that is estab­lished by Sodi­um Potas­si­um pump. 

These sec­re­tary mech­a­nism are not spe­cif­ic and as a result many new sub­stance includ­ing drug mol­e­cules and tox­ins can be secret­ed cou­pled to the trans­port­ing molecules. 

Most of the Tubu­lar secre­tion hap­pens in the DCT but some occurs in the ear­ly part of the col­lect­ing duct. Kid­neys also main­tained and acid base bal­ance by secret­ing excess h + ions

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