Transport and Defense

The Human Body’s Organization
“You might recall that all organisms have hierarchical organization in the body. For all
living organisms, it starts at the cellular level. Let’s think about the human body and how
it's organized from a cell to the whole organism. 

Humans are multicellular organisms, made of several systems that work together. The
human body has many different types of cells that work together in groups. When these
cells work together to carry out specific functions, they are called tissues. When groups
of tissues work together to perform a function, this group of tissues is called an organ.
When groups of organs work together to perform a function, they are called an organ
system. All the organs systems work together to make up the whole organism. 

Organ systems provide movement, transport substances, and perform many other
functions. Organ systems work together to maintain homeostasis. Homeostasis is an
organism's ability to maintain steady internal conditions when outside conditions
change.

Digestion and Excretion
Humans need food, water, and oxygen to survive. Food contains the energy that is
processed by the body. The process by which food is broken down is called digestion.
After digestion, substances that are not used by the body are removed through
Elimination.

The Digestive System
A human digestive system is the group of organs that work together to break down
foods into substances that your cells can use. The function of the digestive system is to
ingest food, digest it, absorb the nutrients, and eliminate any waste products through
excretion.

Ingestion
The first step of the process is called ingestion, where food or liquids are brought into
the mouth and swallowed. Ingestion and digestion in the mouth occur at the same time.
After swallowing, foods digested in the organs of the body.

Digestion
When food enters the mouth, digestion begins. Before your body can absorb nutrients
from food, the food must be broken down into small molecules by digestion. There are
two types of digestion—mechanical and chemical. In mechanical digestion, food is
physically broken into smaller pieces by chewing or grinding. In chemical digestion,
chemical reactions through enzymes break down pieces of food into smaller molecules. 

Digestion begins in the mouth. Enzymes aid in the digestion process. An enzyme is
protein that speeds up chemical reactions. It helps to break down large molecules into
smaller molecules. With the action of your teeth and enzymes, digestion begins shortly
after food enters the mouth. Food then moves from the mouth into the esophagus (ih
SAH fuh gus), with the action of muscles, which connects the mouth to the stomach. The
stomach is a large, hollow organ. It temporarily stores food. The stomach also contains
an acidic fluid called gastric juice which breaks down foods that we eat. 

After leaving the stomach, food moves into the small intestine. This is where most
digestion occurs. Since the small intestine is so long (approximately three and half times
your body length), this is where food is processed the longest.

Absorption
The small intestine is also the site of nutrient absorption. This long tube connects to the
stomach and has tiny folds that look like fingerlike projections called villi (VIH Ii; singular,
villus). Each villus contains small blood vessels. When digested food is ready for
absorption, nutrients and water are absorbed in the villi. Nutrients are transported to the
rest of the body in the blood of the circulatory system. Any leftover food is moved into
the large intestine, also known as the colon (KOH Iun), where more water from waste
materials is absorbed. 

Elimination of Waste

The final function of the digestive system is to remove waste from the body. Any food

that cannot be digested or absorbed is passed through the large intestine. The solid

undigested materials move into the rectum, then exits from the body as solid waste.

Nutrition
One of the important functions of the digestive system is the absorption of nutrients. A
nutrient is a part of food used by the body to grow and survive. There are several types
of nutrients that your body needs. Proteins, fats, carbohydrates, vitamins, and minerals
are just a few examples. Ensuring that you choose foods with the right nutrients is
important for healthy development and body function.

Excretion
The excretory system removes solid, liquid, and gas waste materials from the body. The
lungs, skin, liver, kidneys, bladder, and rectum are parts of the excretory system. Lungs
remove carbon dioxide and excess water as water vapor. The skin removes water and
salt when you sweat.

Filtering Blood
The urinary system includes the body organs that filter your blood, removing waste and
excess water. The urinary system’s primary function is to remove liquid wastes from the
blood, creating urine as a waste product. This system consists of the kidneys, ureter,
bladder, and urethra. The kidneys are two important bean-shaped organs that act as the
filtration system for the blood. The pair filters approximately 140 liters of blood each day,
leaving behind liquid waste. Once waste and excess water are filtered from the blood,
they travel through the ureters to the bladder for storage. They exit from the body
through the urethra in the form of urine.

Liquid Waste Removal
Urine is produced by both the urinary system and liver. When the liver breaks down
proteins, urea forms. Urea is toxic if it stays in the body. The kidneys remove urea from
the body by making urine. Urine contains water, urea, and other waste chemicals.

Kidney failure impacts 2.4% of adults aged 18 and over in the United States.  Those with

acute kidney failure can be treated using dialysis.  Patients are hooked up to a machine

for hours while blood is pumped out and filtered with a fluid called dialysate.  The clean

blood is then pumped back into the body.  

Solid Waste Removal
Like the liver, the rectum is a part of both the excretory system and digestive system.
Food substances that are not absorbed by the small intestine are mixed with other
wastes and form feces. The rectum stores feces until it moves out of the body in the form
of solid waste.

Respiration and Circulation

Take a moment and exhale as much air as you can. Did you notice that you took a big breath in afterward? That big breath is a response from your body making sure it gets the oxygen it needs.

Respiration
The respiratory system is the organs that supply your blood with oxygen. This system includes the nose, mouth, pharynx, larynx, trachea, bronchi, and lungs. The function of the respiratory system is to supply the body with oxygen and help eliminate gaseous wastes like carbon dioxide.

Inhalation and Exhalation
Air is brought into the lungs when specific muscles, like the diaphragm and intercostal
muscles, contract. Air leaves the lungs when the same muscles relax. When you inhale,
air enters the nostrils or mouth, and passes through the pharynx. This is a tubelike
passageway at the top of the throat that receives air, food, and liquids from the mouth or
nose. Since the pharynx is part of the throat, it is a part of both the digestive system and
respiratory system. Food goes through the pharynx to the esophagus. Air travels through
the pharynx to the larynx, also known as the voicebox. It is called the voicebox because it
houses the vocal cords. From the larynx, air passes to the trachea. The trachea is also
called the windpipe. It s a long, tube-like organ with cartilage rings surrounding it, that
connects to the bronchi. The bronchi are two narrow tubes that lead into the lungs.

Lungs are the main organs of the respiratory system that surround the bronchi. Inside the
lungs, the bronchi continue to branch into smaller and narrower tubes called bronchioles.
The bronchioles end in the alveoli, where gas exchange of oxygen and carbon dioxide
occurs. When you exhale, air leaves the body through the same structures in the reverse
Order.

Gas Exchange

In the lungs, the bronchioles end in microscopic sacs, or pouches, called alveoli (al VEE

uh li; singular, alveolus), where gas exchange occurs. During gas exchange, oxygen from

the air you breathe in moves into the blood, and carbon dioxide from your blood moves

into the alveoli. Alveoli look like bunches of grapes at the ends of the bronchioles. The

alveoli fill with air when you breathe in. This is also where gas exchange occurs. Oxygen

enters the bloodstream while carbon dioxide is removed during exhalation.

Circulation and Transport
The circulatory system is the group of organs and tissues that carry needed materials to

cells and remove their waste products. The function of the circulatory system is to
transport materials around the body and to work with other body systems to eliminate
waste products, 

To transport materials like oxygen, nutrients, and wastes around the body, the main
organs of the circulatory system work together. The main organs of the circulatory
system include the heart, blood vessels, and blood. Let's explore each part to
understand how the circulatory system functions.

The Heart
Your heart is made up of muscle cells that are constantly contracting and relaxing. Contractions pump blood in your heart out of the heart and to the rest of your body. When your heart muscles relax, blood from the rest of your body enters the heart. The heart has four chambers. Blood enters the upper two chambers of the heart, called the atria (AY tree uh; singular, atrium). Blood leaves through the lower two chambers of the heart, called the ventricles, to be transported around the body.

Blood Vessels
Blood travels through your body in tubes called vessels. If all the vessels in your body
were laid end to end in a single line, they would be approximately 95,000 kilometers
long. The three main types of blood vessels are arteries, veins, and capillaries. Arteries
carry blood from your heart to smaller vessels called arterioles, which end in the smallest
vessels called capillaries. Capillaries are one-cell thick. Their thin size allows the
exchange of oxygen, nutrients, carbon dioxide, and other wastes between other body
systems. 

Recall that capillaries surround the alveoli in your lungs and the villi in your intestines.
They are also present in most tissues in the body. These vessels are important for the
absorption of nutrients and the release of waste. After gas exchange at the capillaries,
veins are used to transport blood from the body to the heart again, carrying wastes like
carbon dioxide from tissues. Before getting to the heart, gaseous wastes are dropped off
at the lungs. When blood gets back to the heart, it is pumped through the same pathway
Again.

Blood
Blood is made up of several parts. The liquid part s called plasma, and it contains
nutrients, water, and carbon dioxide. Blood also contains red blood cells, platelets, and
white blood cells. Red blood cells carry oxygen. Platelets contain a clotting agent that
helps the body heal after a cut. White blood cells help to defend the body from
pathogens or foreign substances.

The Body’s Defense System
If you have ever experienced any symptoms of an illness, such as a fever, then you have
experienced your body's defense system in action. The Immune system is a combination of body defenses made up of the cells, tissues, and organs that fight pathogens in the body. The immune system’s main function is to protect the body from pathogens. A pathogen is an agent that causes disease. A bacterium, virus, or another type of microorganisms are examples of types of pathogens. 

The human body has many ways of protecting itself from pathogens. There are three
lines of defense. The first line is the physical barriers between the environment and the
inside of the body, including mucus and skin. The second is the immune response which
involves fighting the pathogen directly. The third response is building immunity by
building antibodies.

First Line of Defense: Mucus and Skin
Mucus (MYEW kus) and skin are parts of the first line of defense. They prevent toxins and
other substances from entering the body. Mucus is a gel-like substance in the nostrils,
trachea, and lungs. Mucus traps harmful substances and prevents them from entering
the body.

Integumentary System
Skin is also a part of the first line of defense. It is part of the integumentary system. The integumentary system is the organ system consisting of the skin, hair, nails, and exocrine glands. Its primary function is to protect the body from foreign and environmental substances by acting as a physical barrier, protecting underlying cells and tissues. This skin has many functions including protection from the Sun, regulating body temperature, waste removal through sweat, and sensory reception.

Second Line of Defense: The Immune
Response
If a pathogen manages to get past the integumentary system, the lymphatic system is
responsible for protecting the body from harmful substances and infections. Organs
include the spleen, thymus, bone marrow, lymph vessels, and lymph nodes. It has four
main functions including removing excess fluid around the organs, producing white
blood cells, absorbing fats, and transporting fats. 

White blood cells defend the body against infection, Lymphocytes (LIMH fuh sites) are a type of white blood cell. They travel the body through the circulatory system to sites of infection. Once the pathogen is engulfed and broken down, wastes are sent through the vessels in the body to the lymph nodes where it is collected by the circulatory system for removal

from the body.