Capillaries are the smallest blood vessels. They are responsible for diffusion of gasses and other materials across their walls. For example, alveoli in the lungs are surrounded by capillaries. There, carbon dioxide from the blood diffuses through the capillary walls, into the alveoli, to be exhaled. The opposite is true for oxygen. All of these other blood vessels have walls which are too think to allow any exchange of materials.

The name of this wall is the interventricular septum. It keeps the right and left sides of the heart separate. Remember the importance of keeping these chambers separate; the circulatory system is actually two circuits: pulmonary and systemic. Pulmonary circulation involves blood traveling from the right side of the heart to the lungs to become oxygenated, then returning to the left side of the heart where it is pumped to the body, where the oxygen is taken up by cells. Lastly, the oxygen-poor blood returns from the body to the right side of the heart and the circuit starts again. Mixing blood between these two circuits would result in compromised circulatory system function. 

Pulmonary arteries carry oxygen-poor blood from the heart to the lungs so that it can become oxygenated. Pulmonary veins carry oxygen-rich blood from the lungs back to the heart. The aorta carries oxygen-rich blood from the heart to the rest of the body and brain. The superior and inferior vena cavae carry oxygen-poor blood from the body back to the heart so that it can travel to the lungs and become oxygenated again.

Blood flows back to the heart through the pulmonary veins after being oxygenated in the lungs and re-enters the heart through the left atrium. The blood then passes into the left ventricle, which pumps an enormous volume of blood via the aorta to the rest of the body. This chamber of the heart is under a lot of pressure since the heart must pump blood through the many miles of vessels and arteries; therefore, the left ventricle wall must be thicker to withstand the force of these contractions. 

The left ventricle pumps blood into the aorta, through which oxygen-rich blood circulates throughout the body. 

The sympathetic nervous system is a branch of the autonomic nervous system and provides the "fight or flight" response. For this, muscles will need an increased amount of blood, so the heart increases its force of contraction and rate of contraction (heart rate). Additionally, more oxygen needs to be supplied to the muscles, so the respiratory rate also increases. The increase in cardiac output leads to an increase in blood pressure in the sympathetic response. 

During the sympathetic response, blood flow is shunted from other organs and sent to the heart and muscles instead. Therefore, digestion and urine production is decreased. Less blood to the kidneys means less urine gets produced.

Epithelial tissue forms membranes that cover the body, as well as lines body cavities and glands. Simple squamous epithelium consists of thin, flattened cells, that are one layer thick and allow for the easy movement of substances. This is especially important for gas exchange in the alveoli of the lungs and in the capillaries of the body. The minimal barrier provided by simple squamous epithelium allows molecules to easily diffuse across the membranes.

Stratified squamous epithelium is many layers thick and forms the skin and lining of mucous membranes. Simple cuboidal epithelium functions in absorption and secretion in the kidney tubules and salivary glands. Simple columnar epithelium forms a thick layer that lines the esophagus, stomach, intestines, and uterus. Pseudostratified columnar epithelium possess beating cilia and transport particles in the respiratory tract and reporductive system. 

Ciliated epithelium are located in the respiratory tract. There are hair-like processes, cilia, located on the surface that wave and function to sweep materials, such as mucous and dust, across the surface. Cuboidal epithelial calls are as high as they are wide. Cuboidal cells line the ducts of glands and organs and function in the absorption and transportation of substances. Squamous epithelial cells are flat and allow substances to freely move across their surface. These cells are located in the lungs and the kidneys. Stratified epithelial cells form layers, with mitosis taking place in the lowest layer. Skin is formed from stratified epithelial cells. Transitional epithelial cells adjust to mechanical changes such as stretching. This type of tissue is found only in the urinary system.

Keratinized stratified squamous epithelium makes up the outer layer of skin. Other types of epithelial tissues are found throughout the digestive and reproductive tracts, and other areas of the body.

Since capillaries are the site of diffusion of substances (oxygen and carbon dioxide, for example), they need to have extremely thin walls. Their walls are made of simple (single-layered) epithelium. The other options would be too thick to facilitate diffusion of substances.