Sunday, December 28, 2008

Science Form 3 Blood Circulation and Transport

Introduction to function of Heart

The role of the heart is to pump oxygen-rich blood to every living cell in the body. In order to achieve its goal, it must continuously beat for a person’s entire lifespan. Because of its vital role, a non-beating heart always results in death. The human heart beats approximately 80,000 to 100,000 a day and pumps almost 2,000 gallons of blood. This means that in a person’s life lasting 70 to 90 years, the heart beats approximately two to three billion times and pumps 50 to 65 million gallons of blood. Because the heart is so essential for human sustenance, it is made up of a muscle different from skeletal muscle that allows it to constantly beat.

In order for the heart to deliver oxygenated blood to all cells, blood is pumped through arteries. Veins bring deoxygenated blood cells to the lungs, which then are oxygenated, and then sent back to heart. In this way, a continuous cycle is formed of the heart pumping oxygenated blood and deoxygenated blood out to their designated destinations, and therefore the heart maintains the circulatory system.

Systole: Stage when the ventricles of heart are contracting resulting in blood being pumped out to the lungs and the rest of the body.
- Thick, muscular walls of both ventricles contract.
- Pressure rises in both ventricles, causing the bicuspid and tricuspid valves to close. Therefore, blood is forced up the aorta and the pulmonary artery.
- The atria relax during this time. The left atrium receives blood from the pulmonary vein, and the right atrium from the vena cava.

Diastole: Stage when the ventricles of the heart are relaxed and not contracting. During this stage, the atria are filled with blood and pump blood into the ventricles.
- Thick, muscular walls of both ventricles relax.
- Pressure in both ventricles falls low enough for bicuspid valves to open.
- The atria contract, and blood is forced into the ventricles, expanding them. The blood pressure in the aorta is decreased, therefore the semi-lunar valves close.

Blood Vessels

What are the arteries?
The heart pumps blood into the blood vessels, a series of pipes that take blood from the heart to the organs of the body (via the arteries) and send the blood back into the heart (via the veins). The arteries are muscular and elastic (like a rubber band) and are able to send the blood in a pulsatile form to all organs. As the arteries enter an organ, they branch and narrow significantly to reach all parts of this organ. Arteries expand as they receive blood from the heart and recoil back like a rubber band to aid in pushing the blood forward to the organs. Arteries carry oxygenated blood to all the organs except the lung. The pulmonary artery arises from the right ventricle and carries blood deficient in oxygen into the lung.

What are the veins?
Once the nutrients and oxygen are delivered to an organ, the blood needs to return to the heart to start the cycle over again. The capillary branches rejoin, forming larger, flexible vessels capable of holding blood. These collecting blood vessels are called veins. The small veins link up to become larger veins.The larger veins eventually lead back to the atria of the heart via the inferior vena cava (the main vein carrying blood to the heart from the lower body) or the superior vena cava (the main vein carrying blood from the upper body, especially the head). Unlike the artery,the vein has very little muscle layer. Also,1-way valves help the blood to flow in 1 direction toward the heart through the veins. Incompetence of these valves can lead to varicose veins and accumulation of fluid in the legs upon sitting or standing for long periods of time.

What are the capillaries?
In the organs, the vessels become small and their walls very thin. These tiny vessels are called capillaries. Across the capillaries, oxygen is exchanged between the blood and the organs. Also, these capillaries facilitate nutrient delivery and waste pickup. The capillaries are so thin and small that blood elements are forced to line up single file to pass though this vessel. At this scale, the individual blood cells pass into the capillaries like cars passing through a tollgate. Having regulators at the openings of the capillaries means that blood can be diverted and delivered to the organs in quantities proportionate to the needs of these organs. For instance, when someone is jogging, more blood is delivered to the muscles of the legs to meet the higher oxygen demands.

Blood Vessel Comparisons

Blood Type

Comparisons between artery, veins and capillary
LumenNarrowFairly WideVery Narrow
Structure of WallArteryVeinsOne cell thick and gaps in between allowing the exchange of materials
Direction of blood flowAway from heartTowards the heartFrom artery to veins
Blood pressureHighLowVery Low
Rate of blood flowRapid and irregularSlow and regularVery slow
Presence of valveNoYesNo
Type of bllod carriedOxygenated(pulmonary artery)Deoxygenated(pulmonary vein)Both

Human Blood
Blood is made up of
a) Fluid component called plasma
b) Blood cells namely the red and white blood cells
c) Platelets

Red Blood Cells: Riding on the Red Road

Red blood cells perform the most important blood duty. A single drop of blood contains millions of red blood cells which are constantly traveling through your body delivering oxygen and removing waste. If they weren't, your body would slowly die.

Red blood cells are red only because they contain a protein chemical called hemoglobin which is bright red in color. Hemoglobin contains the element Iron, making it an excellent vehicle for transporting oxygen and carbon dioxide. As blood passes through the lungs, oxygen molecules attach to the hemoglobin. As the blood passes through the body's tissue, the hemoglobin releases the oxygen to the cells. The empty hemoglobin molecules then bond with the tissue's carbon dioxide or other waste gases, transporting it away.

Over time, the red blood cells get worn out and eventually die. The average life cycle of a red blood cell is 120 days. Your bones are continually producing new blood cells, replenishing your supply. The blood itself, however, is re-circulated throughout your body, not being remade all of the time.

Since the human body is continually making more blood, it is safe for healthy adults to donate blood. The blood is then stored for use in emergency situations. Initially after giving blood, the donor may feel some momentary lightheadedness due to the loss of oxygen-rich red blood cells and blood sugar. The body quickly stabilizes itself.

White Blood Cells: Battling Blood Cells
Whenever a germ or infection enters the body, the white blood cells snap to attention and race toward the scene of the crime. The white blood cells are continually on the lookout for signs of disease. When a germ does appear, the white blood cells have a variety of ways by which they can attack. Some will produce protective antibodies that will overpower the germ. Others will surround and devour the bacteria.

The white blood cells have a rather short life cycle, living from a few days to a few weeks. A drop of blood can contain anywhere from 7,000 to 25,000 white blood cells at a time. If an invading infection fights back and persists, that number will significantly increase.

A consistently high number of white blood cells is a symptom of Leukemia, a cancer of the blood. A Leukemia patient may have as many as 50,000 white blood cells in a single drop of blood.

Platelets: Sticky Situations
The human body does not handle excessive blood loss well. Therefore, the body has ways of protecting itself. When, for some unexpected reason, sudden blood loss occurs, the blood platelets kick into action.

Platelets are irregularly-shaped, colorless bodies that are present in blood. Their sticky surface lets them, along with other substances, form clots to stop bleeding.

When bleeding from a wound suddenly occurs, the platelets gather at the wound and attempt to block the blood flow. The mineral calcium, vitamin K, and a protein called fibrinogen help the platelets form a clot.

A clot begins to form when the blood is exposed to air. The platelets sense the presence of air and begin to break apart. They react with the fibrinogen to begin forming fibrin, which resembles tiny threads. The fibrin threads then begin to form a web-like mesh that traps the blood cells within it. This mesh of blood cells hardens as it dries, forming a clot, or "scab."

Calcium and vitamin K must be present in blood to support the formation of clots. If your blood is lacking these nutrients, it will take longer than normal for your blood to clot. If these nutrients are missing, you could bleed to death. A healthy diet provides most people with enough vitamins and minerals, but vitamin supplements are sometimes needed.

A scab is an external blood clot that we can easily see, but there are also internal blood clots. A bruise, or black-and-blue mark, is the result of a blood clot. Both scabs and bruises are clots that lead to healing. Some clots can be extremely dangerous. A blood clot that forms inside of a blood vessel can be deadly because it blocks the flow of blood, cutting off the supply of oxygen. A stroke is the result of a clot in an artery of the brain. Without a steady supply of oxygen, the brain cannot function normally. If the oxygen flow is broken, paralysis, brain damage, loss of sensory perceptions, or even death may occur.

Plasma: The Importance of Plasma
It's a straw-colored, clear liquid that is 90 percent water, and it is an essential ingredient for human survival.

It might seem like plasma is less important than the blood cells it carries. But that would be like saying that the stream is less important than the fish that swims in it. You can't have one without the other.

Besides water, plasma also contains dissolved salts and minerals like calcium, sodium, magnesium, and potassium. Microbe-fighting antibodies travel to the battlefields of disease by hitching a ride in the plasma.

Without plasma, the life-giving blood cells would be left floundering without transportation. Never underestimate the importance of plasma.

Systemic Circulation: It's All Throughout the BodySystemic circulation supplies nourishment to all of the tissue located throughout your body, with the exception of the heart and lungs because they have their own systems. Systemic circulation is a major part of the overall circulatory system.

The blood vessels (arteries, veins, and capillaries) are responsible for the delivery of oxygen and nutrients to the tissue. Oxygen-rich blood enters the blood vessels through the heart's main artery called the aorta. The forceful contraction of the heart's left ventricle forces the blood into the aorta which then branches into many smaller arteries which run throughout the body. The inside layer of an artery is very smooth, allowing the blood to flow quickly. The outside layer of an artery is very strong, allowing the blood to flow forcefully. The oxygen-rich blood enters the capillaries where the oxygen and nutrients are released. The waste products are collected and the waste-rich blood flows into the veins in order to circulate back to the heart where pulmonary circulation will allow the exchange of gases in the lungs.

During systemic circulation, blood passes through the kidneys. This phase of systemic circulation is known as renal circulation. During this phase, the kidneys filter much of the waste from the blood. Blood also passes through the small intestine during systemic circulation. This phase is known as portal circulation. During this phase, the blood from the small intestine collects in the portal vein which passes through the liver. The liver filters sugars from the blood, storing them for later.

Circulatory System: The Circle of Blood
On average, your body has about 5 liters of blood continually traveling through it by way of the circulatory system. The heart, the lungs, and the blood vessels work together to form the circle part of the circulatory system. The pumping of the heart forces the blood on its journey.

The body's circulatory system really has three distinct parts: pulmonary circulation, coronary circulation, and systemic circulation. Or, the lungs (pulmonary), the heart (coronary), and the rest of the system (systemic). Each part must be working independently in order for them to all work together.

Pulmonary Circulation: It's All in the Lungs

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. This is just one phase of the overall circulatory system.

The veins bring waste-rich blood back to the heart, entering the right atrium throughout two large veins called vena cavae. The right atrium fills with the waste-rich blood and then contracts, pushing the blood through a one-way valve into the right ventricle. The right ventricle fills and then contracts, pushing the blood into the pulmonary artery which leads to the lungs. In the lung capillaries, the exchange of carbon dioxide and oxygen takes place. The fresh, oxygen-rich blood enters the pulmonary veins and then returns to the heart, re-entering through the left atrium. The oxygen-rich blood then passes through a one-way valve into the left ventricle where it will exit the heart through the main artery, called the aorta. The left ventricle's contraction forces the blood into the aorta and the blood begins its journey throughout the body.

The one-way valves are important for preventing any backward flow of blood. The circulatory system is a network of one-way streets. If blood started flowing the wrong way, the blood gases (oxygen and carbon dioxide) might mix, causing a serious threat to your body.

You can use a stethoscope to hear pulmonary circulation. The two sounds you hear, "lub" and "dub," are the ventricles contracting and the valves closing.

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