Suggestions
Use up and down arrows to review and enter to select.Please wait while we process your payment
If you don't see it, please check your spam folder. Sometimes it can end up there.
If you don't see it, please check your spam folder. Sometimes it can end up there.
Please wait while we process your payment
By signing up you agree to our terms and privacy policy.
Don’t have an account? Subscribe now
Create Your Account
Sign up for your FREE 7-day trial
Already have an account? Log in
Your Email
Choose Your Plan
Individual
Group Discount
Save over 50% with a SparkNotes PLUS Annual Plan!
Purchasing SparkNotes PLUS for a group?
Get Annual Plans at a discount when you buy 2 or more!
Price
$24.99 $18.74 /subscription + tax
Subtotal $37.48 + tax
Save 25% on 2-49 accounts
Save 30% on 50-99 accounts
Want 100 or more? Contact us for a customized plan.
Your Plan
Payment Details
Payment Summary
SparkNotes Plus
You'll be billed after your free trial ends.
7-Day Free Trial
Not Applicable
Renews May 3, 2024 April 26, 2024
Discounts (applied to next billing)
DUE NOW
US $0.00
SNPLUSROCKS20 | 20% Discount
This is not a valid promo code.
Discount Code (one code per order)
SparkNotes PLUS Annual Plan - Group Discount
Qty: 00
SparkNotes Plus subscription is $4.99/month or $24.99/year as selected above. The free trial period is the first 7 days of your subscription. TO CANCEL YOUR SUBSCRIPTION AND AVOID BEING CHARGED, YOU MUST CANCEL BEFORE THE END OF THE FREE TRIAL PERIOD. You may cancel your subscription on your Subscription and Billing page or contact Customer Support at custserv@bn.com. Your subscription will continue automatically once the free trial period is over. Free trial is available to new customers only.
Choose Your Plan
For the next 7 days, you'll have access to awesome PLUS stuff like AP English test prep, No Fear Shakespeare translations and audio, a note-taking tool, personalized dashboard, & much more!
You’ve successfully purchased a group discount. Your group members can use the joining link below to redeem their group membership. You'll also receive an email with the link.
Members will be prompted to log in or create an account to redeem their group membership.
Thanks for creating a SparkNotes account! Continue to start your free trial.
We're sorry, we could not create your account. SparkNotes PLUS is not available in your country. See what countries we’re in.
There was an error creating your account. Please check your payment details and try again.
Please wait while we process your payment
Your PLUS subscription has expired
Please wait while we process your payment
Please wait while we process your payment
Vitamin D is produced in the skin when exposed to radiation from the sun. Vitamin D attaches to vitamin D-binding protein in the blood and carried to the liver, where it undergoes hydroxylation into 25-hydroxyvitamin D. It is again hydroxylated in the kidney to 1, 25 dihydroxyvitamin D. Vitamin D enhances the efficiency of the small intestine to absorb dietary calcium and phosphorous, and mobilizes calcium and phosphorous stores from the bone.
The vitamin-D metabolite 25-hydroxyvitamin D (25-OH-D) can be measured in the serum as an indication of liver stores. Concentrations are measured by a competitive protein-binding assay. The half-life of 25-OH-D is three weeks; serum levels can reflect vitamin D from diet and sunlight production over a period of several weeks to several months. Normal levels in circulation fall between 8 and 60 ng/mL (20-150 nmol/L). Values below 10 ng/mL (25 nmol/L) are considered to indicate vitamin-D deficiency. Toxicity of vitamin D can be seen at concentrations greater than 150 ng/mL (375 nmol/L).
Serum vitamin-D levels may increase in summer (with increased exposure to sun), and may decrease with old age, or as a result of malabsorption diseases.
The metabolite 1, 25-dihydroxyvitamin D (1,25-OH2D) is measured with a competitive receptor-binding assay. The assay provides a direct measure of the biological activity of 1,25 OH2D. Normal serum values fall between 16 and 60 picograms (pg) per ml (38-144 pmol/L).
This functional measure of vitamin D is an indirect measure of vitamin-D status. A high level of activity of alkaline phosphatase indicates a vitamin-D deficient state.
Measurement of 24-hour urinary calcium excretion can be used as an estimate of calcium absorption, but is not a very specific test of vitamin-D status.
Please wait while we process your payment