Blood sugar, commonly called blood glucose, is the amount of glucose circulating in the blood and available to body cells for metabolism. Glucose itself is a simple sugar and the primary fuel for many tissues; people often refer to it simply as glucose. The fluid that carries glucose throughout the body is the blood, and concentrations are broadly similar in humans and many animals, although exact levels vary by species, age and physiological state.

Characteristics and regulation

Blood sugar reflects a balance between intake, storage and use. After a meal carbohydrates are digested to sugars, absorbed into the bloodstream and taken up by cells. Hormones — principally insulin and glucagon — coordinate uptake, storage (as glycogen or fat) and release. Cells use glucose for immediate energy or convert it to other forms for later use. The liver, muscle and adipose tissue are central to this homeostasis.

Clinical measurement and interpretation

Clinically, blood glucose is measured from capillary or venous samples and reported as a concentration. Results are used to assess metabolic state, guide treatment and screen for disorders. Normal ranges depend on fasting status and timing after meals. Repeated abnormal results lead to further testing and management plans.

Disorders: low and high blood sugar

Two common clinical problems relate to abnormal levels. Low blood sugar, or hypoglycemia, can cause sweating, trembling, confusion and, in severe cases, loss of consciousness. High blood sugar, or hyperglycemia, may cause thirst, frequent urination and longer-term damage to nerves, eyes and blood vessels. Persistently elevated glucose is the hallmark of diabetes mellitus, a group of conditions that require medical management.

Uses, importance and management

Monitoring blood sugar is essential for people with diabetes and useful in many other medical contexts. Management strategies include diet, physical activity, medications and insulin when required. Education about recognizing symptoms of hypo- and hyperglycemia and how to respond is a routine part of care.

Notable facts and context

  • Short-term glucose levels change with meals, exercise and stress.
  • Long-term control is often assessed by tests that reflect average exposure over weeks to months.
  • Maintaining appropriate blood glucose supports organ function and reduces risk of chronic complications.