Eukaryotic cell of the human body

The human body contains about 100 trillion cells, which are the basic unit of life. Everything in the body is made up of cells, including tissues and organs.

Each cell is highly organised and contains many organelles which enable the cell to carry out its function.

Here is an example of how the organelles of eukaryotic cells work together to enable the cell to carry out its functions – protein synthesis:

DNA carries instructions for making hormones and they are found in the nucleus. The nucleus then copies these instructions into molecules called mRNA. These molecules then leave the nucleus through a nuclear pore before attaching themselves to the ribosomes on the rough endoplasmic reticulum. The ribosomes then read the instructions for making hormones before the molecules are pinched off into vesicles. These vesicles travel to the Golgi apparatus where they fuse together, enabling the Golgi apparatus to process and package the hormone molecule before again being pinched off into vesicles and carried to the cell surface membrane. Once at the cells surface, the vesicles fuse with the membrane which opens up, releasing the hormone molecule outside the cell.

Tissues of the human body

Tissues are made up of many cells that are grouped together to perform similar functions.

There are 4 main tissues in the human body.

(1) Epithelial tissue

Epithelial tissues are found throughout the body and cover the body’s surface. This type of tissue is specialized to cover or line (cavities and some organs) all surfaces of the body. They are also the major tissues found in glands.

Epithelial tissues may be one or more than one-cell thick and may be made up of layers. Their cells are packed very closely together. If they are one-cell thick they are called simple epithelium. If they are more than one-cell thick they are known as stratified epithelium, an example of which is skin.

These are some of the types of cells in epithelial tissue:

  • Simple cuboidal – found in glandular tissue
  • Simple squamous – form the linings of cavities and make up the outer layer of skin
  • Simple columnar – lines the stomach
  • Transitional – found in the bladder, consist of many layers
  • Stratified squamous – made up of flattened cells on top of a basement membrane
  • Stratified cuboidal – found in glands such as sweat glands

Simple epithelium

(2) Connective tissue

Connective tissues support the body, binding structures together. It is a fibrous tissue which consists of relatively few cells and has a lot of intercellular space and substance, leading to the cells being wide apart from each other. They are found throughout the body.

These are some of the cell types found in connective tissue:

  • Fibroblast – secretes proteins
  • Macrophage – protects the body from infections
  • Mast cell – found in the inner layer of the skin and release substances into the tissue following an allergic response.
  • Adipose cells – synthesize and contain large globules of fat

Specialised types of connective tissue are:

  • Adipose – for fat storage
  • Cartilage – a form of fibrous tissue that gives support to various parts of the body such as the nose
  • Bone – contains collagen and calcium and is the main form of support and structure in the body; makes up the skeleton
  • Elastic fibres – they deform and recoil, this helps to widen the airways
  • Areola – holds organs in place, found beneath the skin

(3) Muscle tissue

This type of tissue consists of cells that can relax and contract, allowing movement of internal and external body parts. It is found in bundles and rests within connective tissues.

There are three types of muscle cells, each of which is found in various different parts of the body:

  • Smooth muscle tissue – spindle shaped and have a large nucleus in the centre. They form a layer in the walls of hollow organs. They are a specialised tissue type and may be found in the lungs, as well as hormone receptors. It is an involuntary type of tissue meaning it is not controlled by the brain.
  • Skeletal – brings movement to bones of the skeleton such as limbs or the jaw. These movements are voluntary. They are also involved in the breathing process; they contract, constricting the airways.
  • Cardiac – found in the walls of the heart they have a nucleus found in the centre of the cells. They play an important role in the circulation of blood throughout the body, and its motion is under involuntary control.

Smooth muscle tissue

(4) Nerve tissues

Found in the brain, spinal cord and nerves. This tissue controls the functions of the body by reacting to stimuli and conducting impulses to organs in the body. This tissue plays a role in memory, communication and emotion. It consists of neurons that connect to different neurons and to other parts of the central nervous system. Types of neurons are: sensory neurons, connector neurons, somatic neurons and motor neurons. These are grouped into three types: Unipolar (sensory), Multipolar (motor) and Bipolar. Nerve tissue is also made up of support cells called neuroglia or glia cells.

An example of how tissues in the human body may work together:

Some tissues work in tandem with each other, such as the elastic fibres and the smooth muscles which work together in the airways.

As the smooth muscle contracts, it constricts the airways, deforming the elastic fibres. This helps the control of airflow. When the smooth muscle then relaxes, the elastic fibres recoil to their original shape, helping to dilate the airway.

Organs of the human body

An organ is the next level of organization in the human body and is made up of a group of tissues (at least two different types of tissues) that work together performing a similar function.

Examples of organs in the human body: Heart, Spleen, Kidneys, Large Intestine, Small intestine, Skin, Bladder, Appendix, Male/Female Genitals, Pancreas, Stomach, Liver, Voice Box and the Brain.

An example of an organ – the Kidney

The main function of a kidney is to remove waste products such as urea and toxins from the blood. The kidneys also conserve minerals such as water and salts.

Kidneys contain muscle tissues – smooth muscle which is found in blood vessels of the kidney. They also contain epithelial cells that make up the tubules of the filtration system, and are surrounded by fibrous tissue. The kidneys also contain nervous tissue, which enables the kidney and the nervous system to communicate via the renal plexus , helping to reduce the volume of blood being delivered to the kidneys per unit time.

Organ systems in the human body

An organ system is a group of organs that work together to perform certain tasks.

There are 11 major organ systems in the human body:

Reproductive system – produces sex cells and hormones and allows for the process of birth to take place

Integumentary system – helps to regulate the body’s temperature

Urinary system – cleans waste products from the blood some of which gets excreted from the body as urine

Skeletal system – needed for movement of the body

Digestive system – Breaks down food, separating the waste from the nutrients

Muscular system – also needed for body movement

Respiratory system – ensures the body gets the supply of oxygen that is needed, while removing carbon dioxide from the blood stream

Nervous system – controls the body, with the brain being the centre point

Lymphatic system – contains the lymph and helps with the movement of liquids. Also plays an important role as a defense mechanism for the body against disease

Endocrine system – secretes hormones into the blood

Circulatory system – transports substances around the body

‘The organ systems work together to maintain a constant internal environment called homeostasis within the body to ensure survival of the organism’.

Example of how organ systems work together – when sleeping

While asleep, the circulatory systems are ensuring cells have the sufficient nutrients so they can function. The endocrine and nervous systems regulate heart rate, body temperature and blood pressure. As dawn approaches and the body begins to awake, our hormone levels change and our heart rate increases. The body also feels hunger, a feeling brought on by the hypothalamus region of the brain, which ‘works through an intricate feedback system involving hormones, nerves and sensory organs’ to communicate to the rest of the body that we need to eat. As we begin to eat, our sense organs – eyes, nose, and tongue begin to ready the body for food.

The Human Digestive System

The human digestive system’s function is to process food and involves a number of glands and organs, such as the mouth, stomach, intestines, liver and pancreas.


Food is broken down into smaller pieces by chewing and by the action of enzymes which are formed in the salivary glands.

  • The tissues found in the mouth are epithelial tissues, which form the linings of cavities (i.e., the mouth), and they protect and line the inner surface of the mouth. They are mainly stratified squamous tissue which protect against abrasions.


The food is then forced into the esophagus (a tube that runs from the mouth to the stomach) by muscular contractions. These contractions and muscular movements is what enables us to eat solid food.

  • Contains a well-developed layer of smooth muscle. There are also glands called submucosal glands which are found scattered within the connective tissue. The top of the Esophagus is made up of skeletal muscle and the bottom consists of smooth muscle and then also a mixture of connective tissue and smooth muscle.


This organ bathes the food in strong stomach acids such as gastric acid.

  • This organ has a very strong muscular wall, and is lined by epithelial which is simple columnar and secrets mucus. The stomach has an outer fibrous layer of tissue. Beneath this is smooth muscle which is arranged in different types of layers (horizontal, circular etc). Under the smooth muscle is the submucosa, which is made up of a loose connective tissue. In this connective tissue there is blood and lymph vessels and also nerves which make up the autonomic nervous system. The innermost layer of the stomach is called the mucosa which contains different kinds of cells that secrete hydrochloric acid and other digestive juices, as well as the mucus that protects the stomach from being digested by its own juices.

Small intestine

Digestive enzymes (produced by the inner walls of the intestines), pancreatic enzymes and bile which is produced in the liver help to break down the food even further.

  • Also contains the pancreas, an organ that secretes digestive enzymes. The inner walls of the small intestines are lined with simple columnar epithelial tissue.

Large intestine

‘After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like: Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon’.

  • The large intestine resembles a hollow tube that has three distinct layers: The mucosa has an inner surface consisting of a single layer of epithelial cells with a basal membrane of connective tissue which is supplied by blood and lymph vessels, and it also has a narrow band of smooth muscle. The submucosa is another layer which has a loose matrix of connective tissue containing blood and lymph vessels, and the ganglia of enteric neurons. The muscularis externa, which consists of two layers of smooth muscle which run at right angles to one another; the inner layer encircles the wall of the intestine, and the outer layer runs lengthways.


The solid waste is stored in the rectum and then excreted by the anus.

  • Contains a lot of muscular tissue which contract and expand, allowing the anus to open and the stool to pass through.









8. <a href=””>Organs and Organ Systems</a>

9. Article from The Columbia Univ. Coll. of physicians and surgeons complete home medical guide, published: January 1st 1995, written by: Lewy Robert
10. Col, Jeananda. Enchanted Learning. 2001

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