Human Organ Systems

Overview

Nervous System 

The nervous system is composed of the brain, spinal cord, nerves, and sensory organs.  It's main function is to coordination the body's activities, detect all stimuli and form responses to them.  The brain is the control center of the the nervous system.

Human Brain

The Central Nervous System is made up of the brain and spinal cord.  The spinal cord transports sensory and motor  information from other parts of the body to the brain.

The brain only weighs 3 pounds, but is composed of more than 12 billion neurons and 50 billion supporting glial cells. With the spinal cord, the brain monitors and regulates many unconscious bodily processes such as heart rate and breathing, and coordinates most voluntary movements. Most important, it is the site of consciousness and of all the intellectual functions that allow humans to think and create.
 

The brain produces electrical signals, which, together with chemical reactions, let the parts of the body communicate. Nerves send these signals throughout the body.

The Nerve Cell

Each neuron consists of a nucleus situated in the cell body, where outgrowths called processes originate from. The main one of these processes is the axon, which is responsible for carrying outgoing messages from the cell. This axon can originate from the central nervous system (CNS) and extend all the way to the body's extremities,  providing an efficient highway for messages.  Dendrites are smaller secondary processes that grow from the cell body and axon. On the end of these dendrites lie the axon terminals, which plug into a cell where the electrical signal from a nerve cell to the target cell can be made. This 'plug' (the axon terminal) connects into a receptor on the target cell and can transmit information between cells.

• 7 cervical (neck) segments

• 12 thoracic segments

• 5 lumbar segments

• 5 sacral segments

• 4 fused coccygeal segment

Skeletal  System

The skeletal system consists of the bones, tendons, ligaments, and cartilage.  It's main function is to support the body and provide protection for the internal organs. 

Muscular System

The muscular system  consists of skeletal muscles.  These muscles attach to the skeleton and provide movement and locomotion.  There are 3 main types of muscle tissue as you will see listed below.

Integumentary System

Hair, Skin, and Nails

The integumentary system consists of the skin and its derivatives; hair and nails. This system is the body's first line of defense against foreign invaders and  protects against mechanical injury, infection (disease) and also prevents  the inner organs from drying out by retaining body fluids. Your skin also plays an important role to eliminate waste products, regulate body temperature and protects the body from harmful ultraviolet rays from the sun.

Everyone, young and old, should minimize exposure to the sun and protect themselves from  Ultraviolet Radiation. UVA and UVB rays damage the DNA in skin cells and can lead to deadly forms of skin cancer (Melanoma Carcinoma). Sun exposure increases the fine lines on skin, known as wrinkles, and accelerates the timeline of the aging process.  Prevention is easily achieved by limiting the amount of time you are exposed to UV rays and by using products that help prevent exposure to UVA and UVB rays.  With this said, it is understandable why it is important to take care of your skin! 

Secretions from the sebaceous and sweat glands maintain the pH on the skin's surface at the pH of about 3 to 5. This acidic surface keeps many invaders from making a home on your skin.  Your body's natural flora of bacteria have adapted to this acidic surface.

Digestive System

The digestive system begins at the mouth and ends at the anus.   Consists of mouth, pharynx, esophagus, stomach, intestines, liver, pancreas, and anus.  The main function of the digestive system is to process food through ingestion through the mouth, digestion, in the stomach,  absorption of water and nutrients in the intestines, and elimination through the anus. 

Larynx

Anterior                              Posterior 

Gallbladder

Excretory System

The excretory system is responsible for the disposal of metabolic wastes and the regulation of osmotic balance of the blood.  This organ system consists of the kidneys, ureters, urinary bladder,  and urethra.  The kidneys are bean shaped and lie in the superior lumbar region of the posterior abdominal wall.  The right kidney is crowded by the liver and is slightly inferior to the left kidney.  The average kidney is about the size of a large bar of soap.  The convex portion  of the kidney shown in the photograph is called the lateral portion.  The concave portion is  called the renal hilus.  This is where the renal artery, renal vein and renal pelvis leave the kidney.  The smaller image displays the kidneys, adrenal gland (superior to kidney) which is part of the endocrine system, the inferior vena cava, aorta, ureter, and urinary bladder.  The kidneys filer  waste particles from the blood and send toxins, excess water, and metabolic wastes out of the body in the form of urine, while the useful substances from the filtrate are sent back into the blood.  The three nitrogenous compounds that are eliminated from the body are urea, uric acid and creatinine.  Eliminating wastes is only one job of the kidneys, they also regulate the volume and chemical makeup of the blood by maintaining equilibrium between acids and bases and  between water and salts.

 

Excretory System
Kidney

Respiratory System

The respiratory system consists of the lungs, trachea, and other breathing tubes.  It main function is gas exchange; simply put, the uptake of oxygen and the disposal or removal of carbon dioxide.  When you breath in you inhale the oxygen that the lungs need to re-supply red blood cells with oxygen, which is then transported to the rest of the body.  When you breath out or exhale, you are getting rid of the carbon dioxide that the body is trying to eliminate.  The respiratory system work in conjunction with the circulatory system.

Circulatory  System

The circulatory system consists of the heart, blood vessels and blood.  It's main function is to distribute materials throughout the body.

Blood is a connective tissue,  the ground substance is plasma, and the blood cells are called formed elements. Plasma makes up 55% and cellular/formed elements make up 45%. The three formed elements are erythrocytes, leukocytes and platelets, which develop from a single population of pluripotent stem cells  in the red marrow of bones.  This marrow is found in the ribs, vertebrae, breastbone, and pelvis of the fetus. The pluripotent  stem cells are unique because they have the potential to develop into any type of blood cell or cells that produce platelets.

• Granulocytes include the neutrophils, eosinophils, and basophils (their cytoplasm is often filled with granules). These are the work horses of acute inflammation (and other processes). Make sure you learn the neutrophil. In pathology and immunology, you'll call this same cell the PMN (polymorphonuclear leukocyte). Eosinophils are involved in allergic reactions and parasitic infections.
  • Another cell to mention here (although it is NOT a granulocyte) is the Mast Cell. It's very similar to the basophil: both release histamine (and other mediators). Some think that the mast cell is derived from the basophil. Just remember that the basophil circulates and the mast cell is found in peripheral tissues. Other than that, they are quite similar.
• Monocytes include the monoctye and the macrophage. The monocytes, like all leukocytes, use the bloodstream to reach the connective tissues.  There, they transform into macrophages, phagocytic cells that possess pseudopods and ingest a wide variety of foreign cells, molecules, and tiny particles of debris.  Once in the tissue, it matures into the macrophage. It can also mature even further into other cells. The monocyte/macrophage is the work horse of chronic inflammation.
• Lymphoctyes are white blood cells. They originate in the bone marrow and are derived from the same stem cell as the rest of the erythrocytes and leukocytes. These are the T-cells and B-cells that direct the immune system and produce antibodies, respectively. They are the central cells in our cell-mediated and humeral (antibody) defense mechanisms. The B-cell can mature into the plasma cell.

*Know that as a rule of thumb, bacterial infections cause granulocytosis and viral infections cause lymphocytosis. There are exceptions to this, but this is a very basic (and important) concept.

BLOOD PATHWAY
 heart -> elastic arteries -> muscular arteries -> arterioles -> capillaries -> venules -> veins -> heart

BLOOD VESSELS
Arteries take blood AWAY from the heart to other tissues. They carry blood high in oxygen (exception: pulmonary arteries) through  thick, springy walls.  As a result the blood pressure is higher in arteries than in veins. Veins take blood back to the heart and  carry blood low in oxygen and high in carbon dioxide (exception: pulmonary veins).  Their walls are thinner, and lumen is much larger than arteries.  The blood pressure, therefore, is lower in veins.  Connecting arteries and veins are tiny vessels called capillaries where gas exchange occurs.

BLOOD VESSEL STRUCTURE
Tunica Externa (Tunica Adventitia):  outermost layer, areolar connective tissue and largest layer in veins.
Tunica Media: middle layer, circularly arranged smooth muscle fibers and largest layer in arteries.
Tunica Intima: innermost layer - endothelium (simple squamous epithelium) and areolar connective tissue

Vascular System -Three Basic Layers: Tunica Intima (with endothelial lining), Tunica Media (smooth muscle), Tunica Adventitia (outer connective tissue).

Elastic Arteries vessel wall contains large amounts of elastin (a protein fiber). Muscular Arteries diameter ranges from .3mm - 1cm, they have less elastin than elastic arteries, and more smooth muscle in the tunica media.   Arterioles diameter is less than .3 mm, and  tunica media consists of 5 or fewer layers of smooth muscle.  Capillaries have a diameter only slightly larger than the diameter of an erythrocyte, consisting  only of  a tunica intima.  They form capillary bed and are the site for gas and nutrient exchange. Venules collect blood from capillaries.  Veins are formed when venules merge together, composed of tunica intima,  and most veins have valves to prevent back flow.

 

Blood	Iron
Blood Pressure
Cardiovascular System	The Heart

Immune System

The immune and lymphatic system consist of bone marrow, lymph nodes, thymus, spleen, lymph vessels, and white blood cells.  It's main function is fighting infection and cancer. better known as the body's defense system.

The lymphatic and immune systems are so closely related that we will place them under the same heading.

The main components of the lymphatic system are the lymphatic vessels. Think of the these vessels as the highway in which the disease organisms travel.  It is through the lymphatic vessels that escaped fluid from blood vessels is returned to the blood stream.  These vessels run all through the human body collecting a fluid called lymph.  This fluid travels in a one-way direction toward the heart (due to flap-like mini-valves which prevent backflow)  and empties into the great veins in the base of the neck.  The lymphatic vessels also return leaked proteins from the surrounding blood capillaries back to the bloodstream.  The lymphatic system has no organ (like the heart) to pump lymph through the body, and therefore relies on the pulsing of surrounding blood vessels, body movement and muscle contractions to move or squeeze fluid through the vessels.

The main components of  the immune system are lymphocytes, lymphoid tissue, and lymphoid organs (spleen, lymph nodes, thymus, tonsils, lymphoid nodules in the small intestine, and the appendix).  Think of the lymphoid tissue and organs as the parking garage or battle ground where the disease organisms are detained and ultimately destroyed. 

Endocrine System

The endocrine system consists of many glands; the pituitary, thyroid, adrenal glands, pancreas, and ovaries/testicles (which also secrete hormones).

 The Ear