- Week 2, the Transport System

The Transport System, which is also known as Circulatory system, is made up of vessels and muscles that help and control the flow of blood around the body. The transport system is mainly consisted of blood, blood vessels and the heart, which is responsible for the pumping and spreading of blood throughout the body. Although it is only one transport system, there are two types of circulation; pulmonary and systemic.

Systemic circulation is a process where oxygenated blood is carried away from the heart, towards all the parts of the body and returns with deoxygenated blood back to the heart. As the blood begins to flow, it leaves the heart through the left ventricle and goes through the aorta.

The artery aorta is the largest artery in the body, therefore, the pressure inside it is greater than the pressure within other arteries. Also, when leaving the heart, the aorta gets segmented into three branches, allowing blood to reach capillaries from all over the body. When blood finally reaches its destination, delivering oxygen to

all the cells, it comes back to the heart through the vena cava and it is, obviously, deoxygenated.

The pulmonary circulation is the one following the order where blood goes through the heart, followed by lungs and back to the heart again. In other words, as its name mentions, "pulmonary" (from Latin, pulmonarius) refers to lungs, therefore, it is the process involving such organs. As the systemic circulation ends, the pulmonary one starts by leaving the heart with deoxygenated blood through the pulmonary artery and towards the lungs, where an exchange will happen between oxygen and carbon dioxide.

An essential element of the circulatory system is the presence of blood.

Blood itself is composed essentially of three parts; plasma, which is the fluid; red blood cells that transport O2 and CO2; white blood cells belonging to the immune system; and platelets, cell fragments that help blood clotting.

Additionally, the transport system is also highly dependent on the heart. Having four different chambers, the heart is mostly divided into left and right; left side being the one responsible for the systemic transport, and the right one for the pulmonary circulation. As seen in the right diagram, the heart has many vessels, those of which receive a different name.

However, although they serve different purposes, they can also be generally ca

tegorized into three main segments; arteries, veins and capillaries. The first one, arteries, carry blood away from the heart through their strong thick walls, smooth muscle and small lumen, which increase the pressure. Veins on the other hand, carry blood to the heart, thing of which I personally relate to the portuguese word "vem" (comes), through their thin wall and large lumen. Also, given that the blood is coming to the heart, pressure is lost during the process (it loses through the segmentations of arteries/veins, distance traveled...), resulting in a smaller pressure within the vein itself. Finally, capillaries are what connect arteries and veins. They also have a wall of one-cell thickness, which allows a faster diffusion of materials across the membrane.

Finally, in order for the blood to reach every cell in the body, it has to be pumped by the heart with enough pressure and force that will allow its travel. In other words, the heart must beat. The pacemaker is located in the wall of the right atrium at the Sinoatrial node (SAN). Each time the pacemaker sends out a signal, the heart carries out a contraction or a beat. The heart beats by itself (myogenic). Nerves and hormones can transmit messages to the pacemaker: Sympathetic nerve secretes adrenaline (also known as "epinephrine") which carries messages from the brain to the pacemaker telling the pacemaker to speed up the beating of the heart. Another nerve tells it to slow it down, and Adrenalin, carried to the pacemaker by the bloodstream tells it to speed up the beating of the heart.

- Week 1, Digestion

This week in Biology class we learned about the process of digestion and all the "aspects" involved in such. To digest something is to break down nutrients such as proteins, carbohydrates and lipids. The process of digestion is a series of chemical reactions where ingested food is broken down into smaller and smaller forms. For example, proteins, carbohydrates and lipids are broken down into amino acids, monosaccharides and fatty acids, respectively, with the help of enzymes.
When enzymes are involved in the digestion process of nutrients, it is called a chemical digestion. On the other hand, nutrients such as vitamins, minerals and water don't need to be digested, once they are absorbed by the human body.

The digestion process starts with ingestion, which takes place in the mouth. When food is eaten, teeth break it into smaller molecules, process also known as mechanical digestion. Mechanical digestion may also occur inside the mouth due to the enzyme salivary amylase (present on the saliva produced by the salivary glands), which can break down starch. Nevertheless, after this first breakage of food, it moves to the esophagus and forward with peristalsis; involuntary contractions that push food down. Peristalsis can also be considered a part of mechanical digestion, once it squishes and breaks food.

Passing through the muscular tube which is the esophagus, food goes to the stomach, where both mechanical and chemical digestions will occur. Such digestions are reflected on the mashing and mixing of food and in the pepsin digestion of proteins. Additionally, the stomach is filled with HCl, a gastric acid which helps the breakage of nutrients, activates the protease pepsin and acts as a barrier against germs. In order to avoid the leaking of such acid, the walls of the stomach are filled with mucus. After food passes through the stomach, it goes to the liver, where the gallbladder is responsible for the storage of bile, which separates fat into smaller parts, hence, allowing its future absorption. Followed by the liver, there is the pancreas, the most active site for chemical digestion. The pancreas produces all types of enzymes, such as amylase, lipase, trypsin, and dipeptidase, and it also produces insulin, that controls the amount of sugar in the blood. The rich juices from both the pancreas and the gallbladder are later received by the deodenum located in the small intestine. The small intestine has villi, finger-like extensions, which greatly increase the surface area for absorption of molecules such as glucose, amino acids and fatty acids. When such nutrients are absorbed, they are distributed to the cells through the blood, also due to villi. Villi are made of lacteal, small vessels of lymphatic system, and capillary beds, small vessels of circulatory system. In this way, nutrients that are absorbed easily go to the blood, circulatory system, and are directed to a wide variety of body cells.

Finally, whatever was not absorbed by the small intestine is taken to the large intestine. As mentioned before, water was not digested, however, it is only absorbed at the end of the process, in the large intestine. This second intestine also allows bacteria to produce vitamins B and K, and acts in pre

paration for undigested, or unabsorbed, food to be eliminated. In the large intestine such waste material is compacted and is sent to the rectum, which will later release it through the anus.

A nice and explanatory video on digestion can be seen if clicked here.