ISSA Nutritionist Chapter 2: Cells, Organ Systems, And Digestion 1

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    Chapter Goals:

    • Know the structure and function of the animal cell.
    • Be able to tell the differences between the various types of cells in the body. 
    • Know the differences between the various cell types in the body.
    • Be able to discuss the aspects of the human digestive system and their functions.


    The human cell is considered to be the building block of life, and in the human body, we have trillions of these cells.

    The cells are going to be used for structure, absorption of nutrients, generation of energy, movement of waste products, and performance of specialized functions essential for life.

    The function and makeup will be dependent on the location of the cells.

    Cell Composition

    Cells are made up of water, along with inorganic and organic molecules, with water being around 70% of the mass of the cell. 

    Fats, carbs, and proteins (the three main macronutrients) are all present in some form within cells. 

    A basic animal cell is composed of a cell wall, cytoplasm, and a nucleus. 

    The cytoplasm hosts the organelles, which are the structures that perform specialized metabolic tasks. 

    The organelles include:

    • Nucleolus
    • Nucleus
    • Ribosome
    • Vesicle
    • Rough ER
    • Smooth ER
    • Golgi apparatus
    • Centriole
    • Mitochondria
    • Lysosome
    • Peroxisome
    • Microtubule

    Structural Composition

    The outside structure of the cell membrane is needed for life as we know it. It serves for the protection of the cell, and it also allows them to interact with other cells. 

    A membrane encloses each cell, and it is embedded with proteins allowing things to cross this lipid bilayer.

    The proteins make up around 60% of the cell membrane, with the rest being fats. 

    Human cell types

    The body is made up of many cell types, and they all have quite specific functions. In total, we have around 200 types of cells in an adult human. 

    Shortly following the fertilization of human eggs by sperm cells, a 70 – 100 cell organism known as a blastocyst is made. This takes place around five days from conception, and the cells are, for the most part, undifferentiated. From that point on, they will change their roles and functions.

    Some of the most prevalent human cell types are:

    • Stem cell
    • Red blood cell
    • White blood cell
    • Platelet
    • Nerve cell
    • Neurological cell
    • Muscle cell
    • Cartilage cell
    • Bone cell
    • Skin cell
    • Endothelial cell
    • Epithelial cell 
    • Adipose cell
    • Sex cell


    Human tissues are groups of cells that function together to perform some grander task. We break this down into four categories:

    Epithelial tissue

    The epithelial tissue cells line the cavities of the body, so this is where the epithelial tissues are. 

    Sheets of these cells form the epidermis layer of skin, and they also line the various tracts throughout the body 

    These tissues may be defined by the types of epithelial cells within. They can be squamous, cuboidal, or columnar. 

    In addition, the epithelial cells may be transitional, glandular, simple, or stratified. 

    Connective tissue

    This is any tissue that serves to support, connect, or bind another tissue in the body. 

    We can divide these connective tissues as being loose, dense, or specialized. 

    The loose connective tissue is made out of collagen, elastin, and reticular fibers, and it holds the organs in their place. 

    Dense connective tissues are made of the same components as the last, and they make up the tendons and ligaments that serve to connect muscle to bone and other bones to bones. 

    Specialized connective tissue serves its own specific purposes. This can be anything from fatty cartilage to bone and lymph fluid. 

    Muscle tissue

    We have three types of muscle tissue within our body. The most common of which is skeletal muscle tissue. 

    Skeletal muscle tissue is responsible for our voluntary contraction, and it represents around 40% of the human body mass. 

    Smooth muscle tissue is not as abundant as the skeletal variety, but it is in charge of a larger role, which is the involuntary contraction of every single organ system. 

    Cardiac muscle tissue is the most unique of the types. It is involuntary and found in the heart. It contains branched and striated fibers allowing for the propagation of signals through the individual cells. 

    Nervous tissue

    This broad category encompasses the cells of the nervous system that control the movement of the body and the function of the body. 

    The cells in these tissues are going to be nerve cells and neuroglial cells. 

    The nerve cells are often known as neurons, and they allow for signals from the brain to reach the right place in the body. they can be afferent or efferent neurons. 

    Neuroglial cells are found in the central nervous system, so this means both the brain and the spinal cord. 

    The peripheral nervous system also includes nerve cells. These are any cells outside of the CNS.

    Organs and Organ Systems

    There are 11 major organ systems in the human body that work together to make the body move and work. They can be separated, but each of their actions and functions will often involve one another working together.

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    Integumentary system

    This is the largest organ system. It covers the whole body, which means it contains skin, hair, and nails. Its purpose is to protect the internal organ systems from damage, prevent the loss of fluids, and to regulate the temperature of the body. 

    The epidermis, dermis, and hypodermis are the layers of skin that we have. 

    Muscular system

    This is a collection of muscle fibers throughout the human body, with the main function being contraction. 

    Muscles are responsible for the movement of the body, along with posture, joint stability, and the production of heat. 

    Skeletal system

    Humans are vertebrates, or animals that have a vertebral column or spine. The skeletal system is made up of bones, ligaments, and tendons, which are all pieces of connective tissue, and this accounts for about 20 percent of the mass of the human body. 

    The purpose here is to provide a framework of protection for the soft organs inside the body and the components of the nervous system like the brain and the spinal cord. 

    Bones contain more calcium than anything else in the body, and they are the main framework for the skeletal system.

    We break down the skeletal system into the axial and appendicular skeleton. 

    The axial skeleton is made up of 80 bones in an adult human and includes things like the vertical axis, which is made of the sternum, cranium, and vertebral column. 

    The appendicular skeleton is made up of 126 bones, and it includes the bones of the appendages attaching to the axial skeleton. 

    Nervous system

    The nervous system allows us to communicate with, control, and to regulate the other organ systems for the proper functioning of the body. 

    The CNS and PNS provide all of the sensory and motor neurons that are needed for the transmission of electric signals throughout the body and then translated to movement.

    The motor neurons in the PNS are broken down to the somatic nervous system and the autonomic nervous system. 

    The somatic nervous system works to serve the skeletal muscle, and the autonomic nervous system works to serve the involuntary things in the body. 

    Circulatory system

    This system circulates blood through the vascular system around the body and is made up of the heart, arteries, veins, capillaries, and blood. 

    This system is considered to be a closed system, as the fluid stays within the organ system. 

    The circulatory system is very important for the transport of nutrients from the digestive system to the tissues of the body, and also for clearance of waste from physical activity like when we are exercising. 

    The arteries work to carry blood away from the heart. 

    The veins work to carry the blood back to the heart.

    The capillaries work to transport nutrients and oxygen or carbon dioxide on a microscopic scale.

    Lymphatic system

    This is often not considered by people, but it works to play many vital roles in the body. 

    The lymph nodes are some of the main organs in the system, and they work to remove the foreign particles and filter stuff in circulation within the body.

    Lymphocytes are what consume and destroy the foreign bacteria and viruses we have invading our system.

    The lymph system also filters the excess fluid from the spaces in between the cells, known as the interstitial space.

    The lymphatic system also works to absorb the fats and fat-soluble vitamins from the digestive system with the aid of their special vessels in the lining of our intestines. 

    Respiratory system

    This is often given the same name as the circulatory system, but it is actually separate from it. 

    The system is responsible for breathing, supplying oxygen, ad exchanging the gases in our body. 

    Gas exchange is vital for cellular metabolism.

    Both inhalation and exhalation are controlled by the autonomic nervous system and occur every 3 – 5 seconds when an adult human is at resting levels.

    Ventilation processes will become quicker when the energy needs for the body are increased.

    Endocrine system

    This system works really close with the nervous system in order to produce, release, and to regulate the chemical messengers known as hormones throughout the body. 

    Our hormones will affect our growth, development, and metabolic activity of the tissues found throughout the body. 

    We place our endocrine glands into two different categories: exocrine and endocrine. 

    Exocrine glands are things like the sweat glands, and they have ducts that are used to carry secretions to the surface of the body. 

    Endocrine glands are without ducts and involve secretions that will move directly to the bloodstream and be carried around the body internally. 

    Some major endocrine glands are:

    • Adrenal gland
    • Pituitary gland
    • Thyroid
    • Pancreas
    • Ovaries
    • Testes

    Urinary system

    The urinary system is in charge of secreting waste products and maintaining our fluid levels in the normal range. The organs we have involved here will be the kidneys, ureters, bladder, and urethra. 

    Reproductive system

    This is going to be different in males and females, but they both serve the purpose of procreating. 

    The four major functions for this system are:

    • Production of sperm and ova
    • Transportation and sustenance of the sperm and ova
    • Growth and development of offspring in females
    • Production of the sex hormones

    The main organs we have here will be the ovaries and the testes.

    Digestive system

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    This system is in charge of breaking down food into smaller molecules for use on a cellular level.

    We often refer to the system as the gastrointestinal tract.

    The six main functions that are responsible for breaking down food into energy are going to be:

    Ingestion, or taking food into the body via the mouth.

    Mechanical digestion, or the mastication and chewing and churning of the mouth and mixing actions within the stomach to break down foods.

    Chemical digestion, or the breaking down of food by way of enzymes released for the stomach.

    Movements, or the movement of food through the entire digestive system with rhythmic contractions of smooth muscle in the digestive tract, also known as peristalsis. 

    Absorption, or absorbing the simpler molecules through the cell membranes in the small intestine lining and moving this to the blood or the lymphatic system.

    Elimination, or the removal of the waste products and indigestible parts of foods. 

    The digestive tract as a whole is between 4 – 6 meters long, and it is an open system since it has an opening on both ends. 

    The Digestive System

    This is the path through which all foods must pass in order for them to provide nutritional value to the cells of the body.

    The actual processing of the food and its breakdown will be determined by the type of nutrient in question, but they must all pass through the same route of this system.

    Alimentary Tract


    Ingestion and the starting stage of mechanical digestion begin here in the mouth. 

    The salivary glands are the accessory organs. 

    Saliva has water, mucous, and amylase to help start the breakdown, primarily of the starches. It also works to clean the teeth and wet the food for easier swallowing.


    This is also called the throat, and it is a passageway for the transport of food, water, and other materials. 


    The food travels from the pharynx to the esophagus when swallowing occurs.


    This is in the upper left of the abdominal cavity, and it works to aid for both mechanical and chemical digestion.

    Gastrin juices are the acids that will help with digesting foods, and this is controlled via the hormone known as gastrin.

    The digestion process is controlled by gastrin in three phases:

    Thoughts and smells of foods will start the cephalic phase of gastric excretion.

    The gastric phase starts with the food entering the stomach.

    Once the liquid and somewhat broken down food that we call chyme exits the stomach and goes to the small intestine, the intestinal phases of gastrin start.

    Small intestine

    Many nutrients from foods are absorbed into the body through the small intestine, which we divide into three sections: the duodenum, jejunum, and the ileum.

    Large intestine

    The large intestine comes after the small, and it divides up into the colon, rectum, and anus. Actual digestion does not really occur here; instead, it works to absorb water and the electrolytes that may be left and then push the chyme to be eliminated from the body. 

    Accessory Organs

    These are organs that are not directly considered to be part of the digestive system, but they do have some importance in working with it. 


    The largest gland in the body, and it is used to receive blood from the hepatic artery and blood with nutrients from the digestive tract via the hepatic portal vein.


    This organ is attached to the liver, and the main role is to store bile to use in digestion. Bile is formed from water, bile salts, bile pigments, and cholesterol, and this aids us in digesting and absorbing fats.


    This organ is found behind the stomach, and it is used for its endocrine and exocrine functions within the body. It has a major role in using many digestive enzymes. 

    ISSA Nutritionist Chapter 2: Cells, Organ Systems, And Digestion 2
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