Anti Diuretic Hormone and its role in the control of H20 in the body in relation to osmoreceptors.

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University of Hertfordshire

Department of Allied Health Professionals – Paramedic Science, Faculty of Health & Human Sciences.

Pathophysiology for Paramedics

Module Code 2ANN0003 Semester B 2004

Laboratory Report

Anti Diuretic Hormone and its role in the control of H20 in the body in relation to osmoreceptors.

Submission date 25 May 2004       

1. Introduction

Roughly 60% of the mass of the body is water, and despite wide variation in the amount of water taken in each day, body water content remains incredibly stable. Such precise control of body water and solute concentrations is a function of several hormones acting on both the kidneys and vascular system, but there is no doubt that antidiuretic hormone is a key player in this process.

Antidiuretic hormone, also known as vasopressin, is a nine amino acid peptide secreted from the posterior pituitary. Within hypothalamic neurons, the hormone is packaged in secretory vesicles with a carrier protein called neurophysin, and both are released upon hormone secretion.

The single most important effect of antidiuretic hormone is to conserve body water by reducing the output of urine. A diuretic is an agent that increases the rate of urine formation. Injection of small amounts of antidiuretic hormone into a person or animal results in antidiuresis or decreased formation of urine, and the hormone was named for this effect.

The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood. Osmolarity is sensed in the hypothalamus by neurons known as an osmoreceptors, and those neurons, in turn, simulate secretion from the neurons that produce antidiuretic hormone.

When plasma osmolarity is below a certain threshold, the osmoreceptors are not activated and antidiuretic hormone secretion is suppressed. When osmolarity increases above the threshold, the ever-alert osmoreceptors recognize this  the cue to stimulate the neurons that secrete antidiuretic hormone. As seen the the figure below, antidiuretic hormone concentrations rise steeply and linearly with increasing plasma osmolarity.

Osmotic control of antidiuretic hormone secretion makes perfect sense. Imagine walking across a desert: the sun is beating down and you begin to lose a considerable amount of body water through sweating. Loss of water results in concentration of blood solutes - plasma osmolarity increases. Should you increase urine production in such a situation? Clearly not. Rather, antidiuretic hormone is secreted, allowing almost all the water that would be lost in urine to be reabsorbed and conserved.

Appendix 1

AdvancedTM Micro-Osmometer (MODEL - 3300)

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        Sample Volume        20μL

        Sample Capacity        Single Sample

        Readout        2-line digital display

        Units        mOsm/kg H2O

        Range        0 to 2000 mOsm/kg H2O

        Linearity        less than 1% from straight line

        Resolution        1 mOsm/kg H2O

        Repeatability        ±2 mOsm/kg H2O (1 S.D.)

                between 0 & 400 mOsm/kg H2O;

                ±0.5% (1 S.D.) between 400 & 2000

                mOsm/kg H2O

        Storage Temperature        -40 to +160oF or

                -40 to +70oC

        Operating Temp (ambient)        64 to 95oF or

                18 to 35oC

        Drift        < 1 digit per month + 1 digit for

                every 9oF or 5oC ambient temperature


        Room Humidity        5 80% (non-condensing)

        Start-up Time        30 seconds from power on

        Test Time        60 seconds per sample

        Dimensions        W - 27cm, ...

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