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This course covers introduction to human body systems, organization of body, levels of organization, and homeostasis.

Have you ever wondered how we do our regular activities like yawning, running, catching a ball? Or how we yawn, cough, or sneeze? These are coordinated functions of body cells and tissues to keep our body function normal and fight against various diseases. Some of these functions are voluntary, while others are involuntary. For example, running, catching a ball are voluntary as we can regulate them.  Yawning, sneezing are involuntary responses that cannot be controlled.

It is important to understand the structure of human body before figuring out how it functions. The human body consists of trillions of cells of different shapes and sizes performing a wide variety of functions.

Organization of human body

Human body is organized by levels of a hierarchy. Atoms and molecules constitute cells, which together form tissues, tissues comprise organs, organs form organ system, and organ systems together form a whole organism. Hence, levels of organization ranges from smallest chemical building blocks to a unique human organism.

Figure 1. Organization of human body system
Figure 1. Organization of human body system

A cell is the smallest structural and functional unit of life. It is a basic unit of life. Even the smallest living organism consists of cell (Figure 2). For example, each living bacterium is a single cell. Although cells constituting different organs and organ systems vary from each other, they influence each other directly or indirectly to keep the body functioning as a single unit.

Figure 2. Anatomy of human cells
Figure 2. Anatomy of human cells

A tissue is a group of many similar cells that work together to perform a specific function. There are four basic types of human tissues: epithelial, connective, muscle, and nervous tissues. These tissues are described below.

  1. Epithelial tissue: An epithelium is a tissue composed of one or more layers of cells covering the external and internal surfaces of the body parts. Types of epithelial tissues are squamous, cuboidal, columnar, ciliated, pseudostratified.
  2. Connective tissue: Connective tissue connects and binds cells scattered throughout an extracellular matrix made of fibrous proteins and glycoproteins attached to a basement membrane. For example, adipose tissue, bone, blood, cartilage are connective tissues. They consist of ground substance, the matrix, fibers (white, yellow and reticular), and cells such as, fibroblasts, mast cells, macrophages, plasma cells etc.
  3. Muscle tissue: Muscle tissue is excitable. Muscles respond to stimulation and contracting to provide movement. Skeletal muscle, smooth muscle, and cardiac muscle tissue are the three types of muscle tissue.
  4. Nervous tissue: Nervous tissue is formed by nerve cells or neurons (Figure 3) that allows the propagation of electrochemical signals in the form of nerve impulses.
Figure 3. Structure of a neuron
Figure 3. Structure of a neuron

An organ is a structure of the body composed of two or more tissue types (Figure 4). Each organ performs specific physiological functions.

Figure 4. 3D Illustration of human body organs anatomy
Figure 4. 3D Illustration of human body organs anatomy

An organ system is a group of organs that work together to perform major functions to fulfill physiological needs of the body. There are 12 organ systems in human body that work in coordination with each other (Figure 5).

Figure 5. The human body organ systems
Figure 5. The human body organ systems

Table showing various organs, organ systems, and their functions

Organ SystemTissues and OrgansFunction
ImmuneBone marrow, spleen, white blood cellProtects body against disease
SkeletalBone, cartilage, ligamentSupports and protects soft tissues, stores minerals, produces blood cells
Circulatory/CardiovascularHeart, blood, blood vessels (artery and vein)Transports oxygen, hormones, and nutrients to the body cells. Removes carbon dioxide and other metabolic wastes away from cells
LymphaticLymph nodes and vesselProvides defense against disease and moves lymph between body tissues and blood
IntegumentarySkin, hair, nailTemperature control, protection against microorganism and physical injury as well as water loss
UrinaryKidney, urinary bladderControls pH, water, and salt balance of the body
MuscularSmooth, skeleton, and cardiac muscleMovement of body and heat production
DigestiveIntestine, stomach, esophagusDigests foods and absorbs nutrients, water
EndocrinePituitary, thyroid, testes, ovaryProduces hormone that regulates growth and development of body
RespiratoryLungs, trachea, bronchi, pharynxRegulates gaseous exchange between blood and lungs
NervousBrain, spinal cord, cranial and spinal nervesTransfer of electrical stimulus
ReproductiveMale: penis, testes, prostrate, seminal vesicles

Females: uterus, fallopian tube, ovaries, vagina

Produces gametes (sperm and ova) and sex hormones such as progesterone, testosterone

Homeostasis

Homeostasis, derived from Greek work that means “steady”, is the ability of an organism to maintain dynamic equilibrium of their internal environment in response to the environmental changes. Temperature control, pH, water and electrolyte balance, blood pressure, and respiration are examples of homeostasis process.

The goal of homeostasis is the maintenance of equilibrium of a body. A change in the internal and/or external environment is called a stimulus. Receptors on the surface of cells detect theses stimulus and send signal to the control center, i.e. brain.  Brain then generates a response and regulates things accordingly. For example, high blood glucose after every meal is regulated by dissipating the glucose to tissues and storage for future use.

Negative feedback loop

In mammals, nervous and endocrine systems regulate homeostasis by a feedback mechanism (Figure 6). Homeostasis is maintained by a negative feedback loop. Positive feedback loops keep organism further out of homeostasis, which might be necessary for the other functions including their survival.

Negative feedback arises out of balances the factors that mutually influence each other. It either increases or decreases the value, thus stimulus is regulated too. In other words, if the value is too high or too low, it tries to come in equilibrium.

For example, body temperature regulation in hot blooded animals works through a negative feedback loop. If blood temperature rises too high, it is sensed by specialized neurons in the hypothalamus of the brain. They send signals to other neurons, which in turn send signals to the blood vessels of the skin. The blood vessels dilate in response, so more blood flows close to the body surface and excess heat radiates from the body. Brain activates sweating for a cooling effect if it is too hot outside.

On the other hand, if temperature of the environment is too low, it is sensed by hypothalamus, which sends signal to cutaneous arteries to constrict. Shivering enhances this effect and body tries to maintain equilibrium of temperature.

Similarly, maintenance of acid base balance, glucose concentration, calcium levels, fluid volumes in the body are excellent examples of homeostasis.

Figure 6. Positive and negative feedback of a thyroid gland
Figure 6. Positive and negative feedback of a thyroid gland

Overview of body system

The human body consists of various organs and organ systems that work with each other as a unit to perform vital body functions. The twelve major body systems are discussed below briefly including organ involved and functions they perform.

Integumentary system

Skin, hair, nail, and sweat glands form integumentary system (Figure 7). They act as external body cover and protects deeper tissues from injury, prevents dehydration, stores fat, and produces vitamins and hormones.

Figure 7. Human integumenatry system
Figure 7. Human integumenatry system

Skeletal system

The human skeletal system provides support and protection to the body. Human skeletal system consists of bones (206 in number), ligaments, tendons, cartilage, and joint (Figure 8). Skeletal and muscular system work together for movement.

Figure 8. Human skeletal system
Figure 8. Human skeletal system

Muscular System

Three types of muscles: heart muscle, smooth muscle, and skeletal muscles constitute the human muscular system (Figure 9). They maintain posture and help in movement.

Figure 9. Human muscular system
Figure 9. Human muscular system

Respiratory system

Respiratory system mainly consists of  lungs, nasal cavity, pharynx, larynx, trachea, and bronchi (Figure 10). It helps in gas exchange between blood and environment.

Figure 10. Human respiratory system
Figure 10. Human respiratory system

Digestive system

The digestive system breaks down food to smaller molecule to provide energy and eliminates undigested waste. Digestive juices and enzymes are secreted for the digestion of food such as carbohydrates, proteins, fat etc. Oral cavity, esophagus, liver, stomach, small intestine, large intestine, rectum, anus, liver, pancreas etc. for the digestive system (Figure 11).

Figure 11. Human digestive system
Figure 11. Human digestive system

Endocrine system

The endocrinal system  controls chemical signals that allow body systems to act in coordination to regulate growth, homeostasis, metabolism, and sexual development. Pituitary, pineal, and thyroid gland, thymus, ovaries, testes are part of this system (Figure 12).

Figure 12. Human endocrine system
Figure 12. Human endocrine system

Nervous system

Major structures of the nervous system include brain, spinal cord, and nerves (Figure 13). It monitors internal organ system and responds to environmental changes.

Figure 13. Human nervous system
Figure 13. Human nervous system

Cardiovascula/circulatory system

The main function of the circulatory system is to transport nutrients and gasses to cells and tissues and remove waste and carbon dioxide from them. This system consists of heart, blood and blood vessels (Figure 14).

Figure 14. Human circulatory system
Figure 14. Human circulatory system

Lymphatic system

The lymphatic system consists of network of tubules and ducts that collect, filter, and return lymph to blood circulation. It also produces lymphocytes that play important role in immune regulation. Spleen, thymus, lymph vessels and nodes form the lymphatic system (Figure 15).

Figure 15. Human lymphatic system
Figure 15. Human lymphatic system

Excretory system

The excretory system removes wastes, maintains water balance, electrolyte levels, fluid levels, and normal pH of the blood. Kidneys, ureter, urethra, urinary bladder are the main organs of the excretory system (Figure 16).

Figure 16. Human excretory system
Figure 16. Human excretory system

Immune system

The immune system protects body against disease. Bone marrow, spleen, white blood cell, macrophages etc. form important part of immune system (Figure 17).

Figure 17. Human immune system
Figure 17. Human immune system

Reproductive system

The main function of the reproductive system is to produce offspring. Prostate gland, ductus deferens, scrotum, testes, penis are organs of male reproductive system, whereas mammary glands, uterus, Fallopian tube, ovary vagina are female reproductive organs (Figure 18).

Figure 18. Human reproductive system
Figure 18. Human reproductive system

Summary

Human body is organized in a hierarchy of various levels. In multicellular organisms, all levels work independently but coordinate  with each other to form a body system. Unicellular organisms are made of one cell, which performs all functions necessary for growth and development of the organism.

Cells are the basic units of life and form the first level of organization. Cells together form tissues which in turn form organs. Several organs together form organ systems. All the organ systems work together and carry out all life process in an organism. There are twelve major organ systems and they communicate with each other.  This communication allows the body to adjust functions of organs according to the needs of the body.

Homeostasis defines how body maintains equilibrium with its environment. Nervous and endocrine systems play key roles in maintaining homeostasis.

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