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Transport in Living Organisms: CSEC Biology

Transport is all about getting materials to where they need to go. In every multi-cellular organism, there exists a set structure of physical mechanisms by which materials or supplies are moved to satisfy the needs and sustain the life of the organism.


Why do multi-cellular organisms need transport systems? It all comes down to the square-cube law, which states that as a shape grows in size, it's volume increases far faster than it's surface area. This applies to multi-cellular organisms in that they have far lower surface area to volume ratios compared to unicellular organisms. This means that while a unicellular organism (like the amoeba) can use simple diffusion (passive transport) to effectively transport food supplies, waste, and other materials to sustain its life, a multi-cellular organism (like a human) cannot. Because of the number of cells a human has (about 37.2 trillion for the average human), passive transport methods could not supply enough resources or perform quickly enough for every cell to get what it needs to be sustained. Oxygen and water would get used up by outer cells before even reaching the underlying cells. Hence, multi-cellular organisms require mass transport systems to accommodate the sheer number of cells they have.


What is being transported? In every organism, there are a few specific compounds that it transports depending on its nature and the structure of its transport system. All mass transport systems, however, carry water.

For the purpose of the CSEC syllabus, you are required to know the compounds transported in plants and humans.

Plants, in which the transport system consists of xylem vessels, phloem sieve tubes and companion cells (which will be looked at later in greater detail), are responsible for moving around water and dissolved minerals (in the xylem) and sucrose and amino acids (in the phloem).

On the other hand, the human circulatory system transports several other substances, including water, oxygen, carbon dioxide, hormones, amino acids and glucose, which are all dissolved in human juice, correctly known as blood.


The Human Circulatory System

The circulatory system in humans is composed of blood, blood vessels and the heart.


The Heart

The human heart is a fist-sized muscular organ that single-heart-edly moves blood all around the body. The right side pumps deoxygenated blood coming from the rest of the body while the left side pumps oxygenated blood coming from the lungs.

These two sides are each divided into two chambers, the atria and the ventricles- bringing the total chamber count to four.



On the right side of the heart, deoxygenated blood from the head enters through the superior vena cava and blood from the body enters through the inferior vena cava into the right atrium. The blood then moves past the tricuspid valve (which prevents backflow during the contraction of the heart) and into the right ventricle. The deoxygenated blood is then pushed up past the pulmonary valve (which, unsurprisingly, also prevents backflow) into the right and left pulmonary arteries, which lead to the right and left lungs respectively. There, the blood exchanges its carbon dioxide for oxygen so that it can supply the body's cells with the sweet oxygen it needs.

Coming from the alveoli in the lungs, the newly refreshed and oxygenated blood enters through the pulmonary veins into the left atrium of the heart. It is then pushed past the mitral or bicuspid valve and into the left ventricle. The blood then moves up past the aortic valve and into the aorta, where it can then travel to the rest of the body.

What you will notice in the heart is that the left ventricle has a thicker myocardium layer. This is because it needs to make stronger contracts to propel the blood further distances all around the body.


An important thing to note is the fact that humans have a double circulation as opposed to a single circulation in animals such as fish. This means that as blood enters the heart it is pumped to the lungs, exchanges carbon dioxide for oxygen, and returns to the heart where further pumping propels it through the rest of the body. The blood moves through the heart twice during each cardiac cycle.  In a single circulation, the blood has to pass through two capillary beds, one after the other. This makes blood flow more slowly compared with a double circulation.


The heart beats around 115,200 times per day. However, each individual beat of the heart is made up of two distinct phases, systole and diastole.

Systole is the contraction of the heart, and diastole is the relaxation of the heart.


The body also consists of several circulation pathways from the heart, including:

1) Pulmonary Circuit- allows deoxygenated blood to be transported into the lungs for gas exchange , so that oxygenated blood can now flow into the left heart.

2) Coronary Circuit- allows oxygenated blood to be delivered to cardiac muscle cells in the heart wall , and its deoxygenated blood is drained back to the right atrium.

3) Systemic Circuit- allows oxygenated blood from the left heart to be delivered to tissue cells through arteries and arterioles , and deoxygenated blood is transported back to the right heart through veins and venules.



The Blood Vessels

The blood vessels in the human body are veins, arteries and capillaries. In all, the blood vessels in the average adult human stretch for about 100,000 km throughout the body.

Arteries and veins have walls composed of three layers: the tunica intima, the tunica media, and the tunica adventitia, which surround the inner blood-containing space, the lumen.

The tunica intima is composed of endothelial cells on a basement membrane and a subendothelial layer of collagen and elastic fibers.

The tunica media consists of smooth muscle cells, elastic fibers, and collagen.

The tunica adventitia is composed of collagen and elastic fibers. The thickness and proportions of each layer differ between the vessels due the difference in their function.



As shown in the picture to the left, the lumen (that is, the space in the middle where the blood flows) is far smaller in the artery than in the vein. This is because the artery needs to maintain the higher blood pressure coming from the heart.

The tunica media is also thicker in the artery than in the vein, since it has to sustain the higher pressure of the oxygenated blood moving around the body.

You will also notice the valves present in the vein. These are a specific adaptation of the vein due to the fact that it carries blood at a far lower pressure than the arteries do. The valves in the vein, just like the valves in the heart, prevent the backflow of blood.

Arteries carry blood away from the heart while veins carry blood towards the heart.

Another notable difference between arteries and veins is that arteries get smaller as they move away from the heart and towards the capillary beds, while the veins get larger as they move towards the heart from the capillary beds.


Capillaries are the smallest blood vessels (being microscopic in size). The only consist of a thin tunica intima (endothelial cells layer).


Capillaries do not function independently. Instead they tend to form interweaving networks called capillary beds.


The flow of blood from an arteriole to a venule through a capillary bed is called the microcirculation.




Blood vessels are also responsible for the activities of vasoconstriction (reduction in the lumen's diameter by the contraction of the smooth muscle cells in the tunica media) and vasodilation (increase in lumen diameter as the smooth muscle relaxes).

Vasodilation and vasoconstriction occur in response to different changes based on the body's needs. Vasodilation increases blood flow and lowers blood pressure while vasoconstriction decreases blood flow and increases blood pressure. Based on this, you can probably imagine some of the causes and purposes of each, such as:

Vasodilation would be caused by: low oxygen levels, a decrease in available nutrients, increases in temperature

Vasoconstriction serves the purpose of: stabilizing blood pressure or raising blood pressure, reducing loss of body heat in cold temperatures, controlling how blood is distributed throughout the body, sending more nutrients and oxygen to organs that need them and protecting the body against blood and fluid loss.


The Blood

Blood is a connective tissue consisting of plasma, platelets and the blood cells.

The blood has several functions:

1) Transport- Blood carries oxygen and carbon dioxide between the lungs and other organs, nutrients from the digestive system and storage organs, wastes to the liver and kidneys for detoxification or removal, hormones from endocrine glands to target organs, and heat to the skin for removal.

2) Protection- Assists in inflammation, leukocytes destroy foreign microorganisms and cancer cells, antibodies and other proteins neutralize and destroy pathogens, and platelets help in clotting to prevent blood loss.

3) Regulation- Transfers water to and from tissues, helps to stabilize water balance, buffers acids and bases, and helps to stabilize pH.


Plasma

Blood plasma is a straw-colored, sticky fluid, which constitutes about 55% of blood by volume. Although it is mostly water (about 90%), plasma contains over 100 different dissolved solutes, including nutrients, gases, hormones, wastes and products of cell activity, ions, and proteins.


Blood Cells

Blood cells are divided into the erythrocytes and the leukocytes.


Erythrocytes, or red blood cells, would not be considered to be true cells as they have neither a nucleus nor organelles. Erythrocytes normally constitute about 45% of the total volume of a blood sample, a percentage known as the hematocrit. Normal hematocrit values vary. In healthy males the norm is about 47%; in females it is about 42%.


They are shaped like biconcave discs—(flattened discs with depressed centers). This biconcave shape is maintained by a structure of proteins called spectrin attached to the plasma membrane. They are flexible, and this allows them to twist and squeeze as they are carried through the microscopic capillaries (smaller than the erythrocyte itslef) and return to their previous shape.

Erythrocytes have plasma (cell) membranes but have no nuclei (they are anucleate) and essentially no organelles. Red blood cells have a useful life span of 100 to 120 days, after which they become trapped in the spleen or are consumed by macrophages. Being anucleate, they have some important limitations, such as being unable to synthesize new proteins, to grow, or to divide.

Erythrocytes' structures are specifically adapted to their function:

1. Their small size and biconcave shape provides a larger surface area relative to volume . The biconcave disc shape is ideally suited for gas exchange because no point within the cytoplasm is far from the surface.

2. Apart from water, an erythrocyte is over 97% hemoglobin, the molecule that binds to and transports respiratory gases.

3. Because erythrocytes lack mitochondria and generate ATP anaerobically (see more here), they do not consume any of the oxygen they are transporting, making them very efficient oxygen transporters.


Red blood cells' main function is to transport oxygen from the lungs to organs around the body. They do this using hemoglobin, which binds easily and reversibly with oxygen.

You might be wondering why hemoglobin doesn't just exist freely in plasma, since it is what is really driving the gaseous exchange- and if you weren't, you might be wondering now.

Having hemoglobin contained in erythrocytes prevents it (1) from breaking into fragments that would leak out of the bloodstream (through the rather porous capillary membranes) and (2) from contributing to blood viscosity and osmotic pressure.

You can read more about osmotic pressure here, but in short, it is the difference in water potential between the blood in the capillary and the surrounding tissue created by proteins and other dissolved materials in the blood.