5.2 Regulation of Blood Sugar

• sugar is in form of glucose
• blood sugar: amount of glucose in blood
• cells need constant supply of glucose as it is a source of energy
• Energy is released from glucose molecules by cellular respiration:
  • Glucose + oxygen → Carbon dioxide + Water + Energy in form of heat and ATP
  • Increased rates of cellular respiration (in mitochondria) increases use of glucose by body cells and decrease blood glucose levels
  • increased rate is influenced by thyroxine
• body’s source of glucose is food we eat
  • carbohydrates broken down into glucose during digestion and absorbed into blood through walls of small intestine
• after a meal, blood glucose concentration can rise sharply
  • homeostasis maintain levels
  • any excess in blood must be removed and stored ready for use in cellular activities between meals

Glucose and Glycogen

• glucose is stored as glycogen (polysaccharide made up of thousands of glucose molecules bonded together in branching chains; functions as a store of glucose in muscles and liver cells)
  • able to store 500g (100 in liver and remainder in skeletal muscle cells)
  • when there is not enough glucose in blood, some glycogen can be converted

Role of Liver

• below diaphragm
• converts glucose into glycogen for storage or glycogen to glucose for release in the blood
• liver’s blood supply comes through hepatic portal vein (brings blood directly from stomach, spleen, pancreas and small and large intestines)
  • has first chance to absorb nutrients
• when high proportion of carbohydrates are consumed, the breakdown of products, mainly glucose, are absorbed into blood capillaries of villi of small intestine
• hepatic portal vein carries glucose to liver where glucose may:
  • be removed from blood by liver to provide energy for functioning
  • removed by liver and/or muscles and converted into glycogen for storage
  • continue to circulate in blood, becoming available for body cells to absorb and use as energy
  • converted into fat for long-term storage
• glycogenesis (genesis means to make something and glycogen refers to glycogen thus, making glycogen): process whereby glucose molecules are chemically combined in long chains to form glycogen molecules
  • stimulated by pancreatic hormone, insulin
• glycogen can’t be used by cells, must be converted back into glucose
• glycogen stored in liver is available for conversion of glucose to maintain blood sugar levels and supply energy for liver activity
• glycogen in muscle cells provide glucose required for muscle activity
• levels of glucose drop below normal, glycogenolysis occurs
• glycogenolysis (lysis means to break down and glycogen refers to glycogen thus, breaking down glycogen): process of converting glycogen back to glucose
  • occurs between meals
  • stimulated by pancreatic hormone, glucagon
• glycogen stored in liver is short-term energy
  • only provide glucose for body cells for 6 hours
  • if more energy is required, body uses energy reserves in fat

Role of pancreas

• lies in the curve of duodenum
• the cells in the islets of Langerhans:
  • ==alpha==: secretes ==glucagon==
  • $beta$: secretes $insulin$
• $insulin$ causes a ⬇️ in blood glucose levels by:
  • enables entry of glucose into cells
  • accelerating transport of glucose from blood into body cells (especially skeletal muscles)
  • promotes conversion of glucose into glycogen in liver and muscles
  • accelerate glycogenesis in liver and skeletal muscle
  • promotes fat storage
  • stimulate lipogenesis (conversion of glucose into fat (lipids))
  • promotes protein synthesis
  • stimulate protein synthesis (conversion of glucose into protein)
• levels of blood glucose regulates secretion of insulin via negative feedback loop
  • when blood glucose levels rise above normal, chemoreceptors in $beta$ cells of islets of Langerhans stimulate cells to secrete $insulin$
  • as the level of blood glucose decreases, cells are no longer stimulated and production reduces
• ==glucagon== ⬆️ blood glucose levels by:
  • converts liver glycogen into glucose
  • stimulating glycogenolysis
  • stimulating gluconeogensis ((neo means new, genesis means make, thus making glucose from another source) process of producing glucose molecules from lipids and amino acids) - involves breakdown of lipids (fats) in a process called lipolysis
  • mild stimulating effect on protein breakdown
• glucose formed is then released into blood
• also controlled by a negative feedback system
  • when blood glucose levels fall below normal, chemoreceptors in alpha cells of islets of Langerhans stimulate cells to secrete glucagon
  • as the level of blood glucose increases, cells are no longer stimulated and production is reduced

Role of adrenal glands

• contains outer part, the cortex, and the inner part, medulla

Adrenal Cortex

• secretes glucocorticoid hormones (such as cortisol: hormone that promotes normal metabolism) in response to low blood glucose levels
  • cortisol regulates carbohydrate metabolism, stimulates glycogenolysis and the transfer rate of amino acids to the liver for gluconeogensis

Adrenal Medulla

• secretes adrenaline and noradrenaline
  • effects mimics sympathetic nervous system and elevates blood glucose levels
  • specifically, adrenaline promotes glycogenolysis and counteracts effect of insulin (as glycogenolysis is stimulated by glucagon which is the opposite of insulins effects)
  • adrenaline also stimulates the production of lactic acid (in muscle cells) which can be used by the liver to manufacture glucose as gluconeogensis

Blood glucose homeostasis

• normal level of glucose in blood is between 4 and 6 millimoles per litre (5 mmol/L is equivalent to 90 mg/100mL)

Feedback loop due to strenuous activity

Stimulus: Reduced blood glucose levels (using muscles burns glucose)

Receptor: Chemoreceptors in the alpha cells of the Islets of Langerhans

Modulator: Alpha cells secrete glucagon into bloodstream

Effector: Liver, skeletal muscle cells and adipose tissue (body fat)

Response: Liver → glycogenolysis and gluconeogensis, skeletal muscle cells → glycogenolysis, adipose tissue → lipolysis

Feedback: Blood glucose levels increase = negative feedback