Nutrition in Humans - Assimilation, Liver Roles & Alcohol Effects

4.4 What is assimilation?

  • Assimilation: The process where absorbed nutrients are incorporated into new constituents or used to provide energy.

  • After absorption, blood in the villi (blood capillaries and lacteals) is rich in nutrients.

  • The main organ of assimilation is the liver.

Transport of Glucose and Amino Acids

  • Blood capillaries combine to form larger blood vessels.

  • These vessels unite to form a large vein called the hepatic portal vein.

Blood Vessels in Liver

  1. Hepatic Portal Vein: Transports nutrients (mainly sugars and amino acids) from the small intestine to the liver.

  2. Hepatic Artery: Transports oxygenated blood from the heart to the liver.

  3. Hepatic Vein: Transports:

    • Deoxygenated blood from the liver to the heart.

    • Some glucose and amino acids from the liver to other parts of the body.

Glucose Assimilation

  • All cells use glucose as a source of energy.

    • Glucose is broken down during tissue respiration to release energy for vital cell activities.

  • Excess glucose is converted to glycogen for storage in the liver under the stimulation of insulin, a hormone secreted by the pancreas (see 4.5 roles of liver).

Amino Acids Assimilation

  • Amino acids taken up by cells are:

    • Converted into new proteins used for growth and repair of worn-out parts of the body.

    • Used to form enzymes and hormones.

  • Excess amino acids are deaminated in the liver (see 4.5 roles of liver).

Transport of Fats

  • Fats are absorbed into lymphatic capillaries (lacteals), which combine to form larger lymphatic vessels.

  • Fats are transported by the lymphatic system directly to the heart and then re-enter blood circulation.

  • Blood transports fats to all parts of the body, especially the liver.

Fats Assimilation

  • When there is sufficient supply of glucose (e.g., under normal conditions):

    • Fats are not broken down.

    • Used to build protoplasm like cell membranes.

  • When glucose is in short supply (e.g., fasting):

    • Fats are broken down.

    • Provide energy needed for vital activities in the body.

  • Excess fats are stored in adipose tissues found:

    • Beneath the skin.

    • Around the heart and kidneys.

  • This protects organs by acting as insulation.

4.5 What are the roles of the liver?

  1. Regulation of Blood Glucose Concentration

    • Hormones (insulin/glucagon) are secreted by a special group of cells in the pancreas called islets of Langerhans.

    • These hormones stimulate the liver to carry out metabolic processes to regulate blood glucose level.

    • After a heavy meal:

      • Blood glucose concentration rises above normal.

      • Insulin (a hormone) is secreted.

      • The liver is stimulated to convert glucose into glycogen for storage.

      • Blood glucose concentration decreases back to normal.

    • During fasting:

      • Blood glucose concentration falls below normal.

      • Glucagon (a hormone) is secreted.

      • The liver is stimulated to convert glycogen into glucose to be released into the bloodstream.

      • Blood glucose concentration increases back to normal.

  2. Deamination

    • Deamination: The process by which amino groups are removed from amino acids and converted to urea.

    • Urea is then removed from the body as urine.

    • Remains of deaminated amino acids are converted into glucose in the liver.

    • Excess glucose formed in this way is converted into glycogen.

  3. Detoxification

    • The process of converting poisonous substances into harmless ones.

    • E.g., liver cells contain an enzyme that breaks down alcohol to compounds that can be used in respiration to provide energy for cell activities.

  4. Production of bile.

  5. Breakdown of hormones.

  6. Breakdown of hemoglobin in worn-out red blood cells to produce iron (for storage) and bile pigments.

  7. Protein synthesis (e.g., plasma protein for blood clotting).

4.6 What are the Effects of Alcohol Consumption?

  • Harmful effects on the digestive system

    • Alcohol stimulates acid secretion in the stomach.

      • Excess stomach acid increases the risk of gastritis.

    • Prolonged alcohol use leads to cirrhosis.

      • A disease in which liver cells are destroyed and replaced with scar tissue, making the liver less able to function.

      • May lead to hemorrhage (heavy bleeding) in the liver, liver failure, and even death.

  • Harmful effects on the nervous system

    • Short-term effects

      • Slows down brain function as alcohol is a depressant.

      • Reduced self-control.

      • Increased reaction time.

      • Slurred speech, blurred vision, and poor muscular coordination.

      • Drunk drivers may drive faster with less caution, leading to a higher chance of traffic accidents.

    • Long-term effects

      • Shrinkage of brain volume.

        • The brain becomes smaller than normal.

        • Especially the region associated with memory and reasoning.

      • Heavy consumption of alcohol during pregnancy interferes with the development of the fetus’s brain.

        • May cause lifelong physical, mental, and behavioral problems.

  • Social implications

    • May become addicted to alcohol (alcoholics) and may:

      • Neglect their work and families.

      • Exhibit violent behaviors.

      • Be at a higher risk of committing crimes.