Nutrition, Digestion, Urination

NUTRITION

Nutrients of the Body

A nutrient is defined as a chemical that an organism needs to live and grow, or a substance used in an organism's metabolism which must be taken in from its environment. There are six general types of nutrients that your body needs to produce energy:

  1. Water

  2. Carbohydrates

  3. Lipids

  4. Proteins

  5. Vitamins

  6. Minerals

WATER

Importance of Water

  • Water is considered the most important nutrient and is the most abundant substance in the human body, comprising about three-quarters of the human mass.

  • It is a major component in every cell and acts as a containing medium for electrolytes and all other ions throughout the human body.

  • Water plays a vital role in removing toxins from the body.

Water Requirements

  • The estimated water requirement for an average adult is two liters per day.

Water Toxicity

  • It is possible to suffer from water toxicity, which results from the intake of excess water. This condition can lead to dilution of important electrolytes (mineral salts), potentially causing erratic heart rhythms and even death.

CARBOHYDRATES

Definition and Types

  • Carbohydrates include sugars and starches. During digestion, complex carbohydrates are broken down into simple sugars that can cross the intestinal epithelium.

  • An exception to this is cellulose, which cannot be digested by humans and exits the gastrointestinal tract largely unchanged. However, cellulose is still important as a source of dietary fiber.

Metabolism

  • Sugars are typically converted into glucose by the liver, as glucose is the form of sugar that cells most often use for respiration.

  • When blood glucose levels are high, the hormone insulin is released, causing glucose to be stored as glycogen in cells. Conversely, when glucose levels are low, the hormone glucagon is released to promote the release of glucose from glycogen.

LIPIDS

Overview

  • Lipids are a broad group of naturally occurring molecules that include fats, waxes, and sterols. Sources of lipids vary, and they come in types such as phospholipids, cholesterol, and triglycerides (fats).

  • Lipids serve important functions such as the construction of cellular structures and acting as an energy source when necessary.

Triglycerides

  • Triglycerides are the most common dietary lipids.

Role of the Liver

  • The liver plays a major role in lipid metabolism and usage, capable of converting one form of fatty acid into another, except for linoleic acid which is an essential fatty acid.

  • Linoleic acid cannot be synthesized by the liver and must be included in the diet, obtainable from vegetable oils and nuts. All other fatty acids are termed nonessential because the liver can produce them.

Cholesterol

  • Cholesterol is not used as an energy source by the body but is necessary for the formation of cell membranes and membranous organelles, and for the construction of steroid hormones.

Lipoproteins

  • Lipoproteins are clusters of proteins and lipids that act as carriers for lipids, including cholesterol, in the bloodstream. There are two main categories of lipoproteins:

    • LDL (low-density lipoprotein): Often referred to as the “bad” cholesterol, it is associated with the deposition of cholesterol on arterial walls.

    • HDL (high-density lipoprotein): Known as the “good” cholesterol, it helps carry cholesterol out of the bloodstream and is denser than LDL.

PROTEINS

Amino Acid Structure

Structure of Amino Acids

  • Amino acids consist of:

    • An amino group (NH2)

    • A carboxylic acid group (COOH)

    • An alpha-carbon

    • A side chain (R)

  • The following visual representation is of an amino acid structure:

  H   H   O
  |   |   ||
  H-N-C-C-OH
      R

Protein Digestion

  • Proteins must be broken down into their constituent amino acids before they can cross the intestinal epithelium. Due to their large size, amino acids require cellular energy to transport them across.

  • The liver can convert one type of amino acid into another type.

Essential and Nonessential Amino Acids

  • There are 20 standard amino acids that comprise most proteins. Amino acids that can be synthesized by the liver are termed nonessential amino acids and do not need to be included in the diet.

  • Eight amino acids cannot be synthesized by the liver and are classified as essential amino acids, meaning they must be included in your diet.

VITAMINS

Definition

  • Vitamins are organic compounds (excluding macromolecules such as proteins, carbohydrates, lipids, or nucleic acids) required by the body in small amounts and cannot be synthesized by the body. They are often referred to as vital amines.

Classes of Vitamins

  • Vitamins are divided into two classes based on their solubility:

    1. Fat-soluble vitamins: These are soluble in fats and can be stored in the body, particularly in the liver. Since they are storable, they are needed in lower quantities. Fat-soluble vitamins include A, D, E, and K.

    2. Water-soluble vitamins: These vitamins are soluble in water; hence, they cannot be stored in the body. Any excess above the recommended daily allowance (RDA) is excreted. Overconsumption can lead to hypervitaminosis (vitamin overdose). Water-soluble vitamins include vitamin C and the B group.

MINERALS

Definition and Importance

  • Minerals are inorganic compounds required by the body in small amounts, making up about 4% of the body mass, mainly concentrated in teeth and bones.

Classes of Minerals

  • Minerals can be divided into two classes:

    1. Major Minerals (Macrominerals): Required by the body in relatively large amounts. They include potassium, sulfur, sodium, chloride, calcium, and phosphorus (with calcium and phosphorus accounting for 75% of the total minerals).

    2. Trace Elements (Microminerals): Required by the body in relatively small amounts. They include iron, copper, iodine, manganese, zinc, cobalt, and selenium.

DIGESTIVE AND URINARY SYSTEMS

Overview of Systems

  • The digestive system breaks down nutrients in food and eliminates solid waste as feces.

  • The urinary system removes water-soluble waste and determines the volume and composition of body fluids.

Homeostasis Maintenance

  • The digestive system maintains homeostasis of nutrients by breaking down nutrients from food and delivering them to the blood.

  • The circulatory system plays a role in delivering these nutrients to cells throughout the body.

  • The urinary system maintains homeostasis of body fluids by producing urine to excrete nitrogenous wastes (mainly urea) and regulating water and electrolyte levels.

Organs of the Digestive System

  • The human digestive system consists of the gastrointestinal tract and accessory structures. Some key organs and their functions include:

    • Salivary glands: Secrete saliva containing enzymes that initiate the breakdown of carbohydrates.

    • Mouth: Responsible for the mechanical breakdown of food and the beginning of carbohydrate chemical digestion.

    • Pharynx: Connects the mouth with the esophagus and routes air to the trachea.

    • Liver: Produces bile, which emulsifies fat.

    • Gallbladder: Stores and releases bile.

    • Pancreas: Produces and releases digestive enzymes and bicarbonate ions into the small intestine.

    • Esophagus: Uses peristalsis to push food into the stomach.

    • Stomach: Mixes food and carries out enzymatic digestion of proteins.

    • Small intestine: Site of final enzymatic breakdown of food molecules and the primary site for food and water absorption.

    • Large intestine: Absorbs remaining water and minerals, regulates fecal elimination.

    • Appendix, Rectum, and Anus: Final components for waste elimination.

Phases of Digestion

Digestion in the Mouth

  • Enzymes in saliva begin the digestion of starch into sugar monomers. Teeth mechanically break food into smaller pieces while the tongue pushes food to the back of the mouth, swallowing it into the pharynx. The trachea closes as food passes through the esophagus to reach the stomach.

Digestion in the Stomach

  • The stomach serves as a muscular container that receives food from the esophagus. Its primary functions include mechanical and chemical digestion of food, including both churning (mechanical) and gastric juices rich in enzymes (chemical).

Digestion in the Small Intestine

  • The small intestine is a long, tubular organ lined with highly folded epithelial cells that maximize surface area for nutrient absorption.

Pancreatic Enzymes

  • The pancreas delivers pancreatic juices containing:

    • Trypsin and chymotrypsin: Break down polypeptides.

    • Amylase: Digests starch.

    • Lipase: Breaks down fats.

    • Alkaline sodium bicarbonate: Neutralizes stomach acid entering the small intestine.

Bile Function

  • The liver produces bile, which emulsifies fats into small globules that can mix with water. The gallbladder stores bile and releases it into the small intestine, allowing lipid-digesting enzymes (lipases) to function efficiently.

Absorption in the Large Intestine

  • The large intestine receives chyme from the small intestine. Its lining absorbs water, ions, and minerals from this chyme, while any remaining waste is eliminated as feces.

Excretion of Nitrogenous Waste

  • The digestive system is crucial for providing energy and raw materials necessary for metabolism. As metabolic reactions occur in cells, they release waste into the bloodstream which must be eliminated.

Types of Nitrogenous Waste by Organism

  • Metabolic waste varies among different organisms:

    • Aquatic animals (bony fish): Excrete ammonia.

    • Mammals, most amphibians, and some cartilaginous fish (e.g., sharks): Excrete urea.

    • Birds, insects, many reptiles, and land snails: Excrete uric acid.

Functions of the Urinary System

  • The urinary system performs several functions:

    1. Filters blood.

    2. Eliminates nitrogenous wastes.

    3. Regulates ion concentrations in body fluids.

    4. Produces, stores, and eliminates urine.

Cleansing Blood in the Kidneys

  • Kidneys serve as the major excretory organs, cleansing blood and forming urine that travels through ureters to the urinary bladder, ultimately released through the urethra.

Nephron Structure

  • Each kidney is composed of blood vessels and millions of nephrons, which are the functional units responsible for filtering and cleansing the blood.

Hormonal Regulation of Kidney Function

  • Hormones such as antidiuretic hormone (ADH) and aldosterone regulate kidney functions. High levels of ADH signal kidneys to reduce water loss in urine, while aldosterone promotes sodium reabsorption into the bloodstream from nephrons, with water following by osmosis.

Dialysis as a Substitute Filtration System

  • A dialysis machine serves as an external filtration system where blood is pumped out of the patient's body through a semipermeable membrane, allowing wastes and toxins along with excess water to diffuse out; however, unlike nephrons, a dialysis machine cannot help the body reabsorb useful substances.