Circulatory System

Circulatory System

All animals must exchange materials with their environment, including nutrients and wastes, O₂, CO₂, etc., thus need a system that will do this. The more complex the organism, the more complex this system must be. Arthropods, like insects and spiders, have an open circulatory system, in which the blood is pumped forward by the heart, but then flows through the body cavity, directly bathing the internal organs. Vertebrates, like humans, have a closed circulatory system in which the blood stays in the circulatory system as it circulates, and chemicals are exchanged by diffusion. Our system is also called our cardiovascular system, and is composed of our heart plus our arteries and veins. In a person’s heart, the atria(plural of atrium) receive blood from the veins and the ventricles send blood to the arteries. As the arteries become more finely divided, they are called arterioles. The finest divisions of our vascular system are called capillaries. As the vessels get larger again, the smallest are called venules which join and enlarge to form veins. Note that the distinction between arteries and veins is by direction of blood flow, not oxygen content. Veins carry blood toward the heart and arteries carry it away from the heart. Because of this, not all arteries carry oxygenated blood. The two major exceptions, in which arteries are carrying deoxygenated blood are the pulmonary artery which carries deoxygenated blood from the heart to the lungs (to pick up oxygen there) and the umbilical arteries which carry deoxygenated blood away from the baby’s body to the placenta (to pick up oxygen there). We have double circulation: we have a separate pulmonary circuit to the lungs and a systemic circuit to the body.
This illustration is orientated as thought you were looking at this heart in another person standing in front of you. The path of blood flow in a human, then, is as follows:

1. The superior (a) and inferior (b) vena cava are the main veins that receive blood from the body. The superior vena cava drains the head and arms, and the inferior vena cava drains the lower body.
2. The right atrium receives blood from the body via the vena cavae. The atria are on the top in the heart.
3. The blood then passes through the right atrioventricular valve, which is forced shut when the ventricles contract, preventing blood from reentering the atrium.
4. The blood goes into the right ventricle (note that it has a thinner wall; it only pumps to lungs). The ventricles are on the bottom of the heart.
5. The right semilunar valve marks the beginning of the artery. Again, it is supposed to close to prevent blood from flowing back into the ventricle.
6. The pulmonary artery or pulmonary trunk is the main artery taking deoxygenated blood to the lungs.
7. Blood goes to the right and left lungs, where capillaries are in close contact with the thin-walled alveoli so the blood can release CO₂ and pick up O₂.
8. From the lungs, the pulmonary vein carries oxygenated blood back into the heart.
9. The left atrium receives oxygenated blood from the lungs.
10. The blood passes through the left atrioventricular valve.
11. The blood enters the left ventricle. Note the thickened wall; the left ventricle must pump blood throughout the whole body.
12. The blood passes through the left semilunar valve at the beginning of the aorta.
13. The aorta is the main artery to the body. One of the first arteries to branch off is the coronary artery, which supplies blood to the heart muscle itself so it can pump. The coronary artery goes around the heart like a crown. A blockage of the coronary artery or one of its branches is very serious because this can cause portions of the heart to die if they don’t get nutrients and oxygen. This is a coronary heart attack. From the capillaries in the heart muscle, the blood flows back through the coronary vein, which lies on top of the artery.
14. The aorta divides into arteries to distribute blood to the body.
15. Small arteries are called arterioles.
16. The smallest vessels are the capillaries.
17. These join again to form venules, the smallest of the veins.
18. These, in turn, join to form the larger veins, which carry the blood back to the superior and inferior vena cava.

The atrioventricular and semilunar valves prevent backflow as the heart contracts. Defects in any of these that allow some blood to leak backwards cause distinctive sounds through astethoscope, thus are called heart murmurs.
The sinoatrial node controls the heart beat. This natural pacemaker is located in the upper wall of the right atrium, and is composed of muscle tissue that sends electrical impulses to the rest of both atria to contract. The impulse then spreads to the ventricles, causing them to contract. The heart cycle involves three phases:

1. The atria contract and force blood into the ventricles. If the atria don’t contract, this is called atrial fibrillation and pooled blood in the atria can begin to clot. When the atria start beating normally again, these clots may be sent throughout the person’s system. If one of these clots lodges in an arteriole somewhere, it could cause a stroke, heart attack, or similar problem. As blood is pushed into the ventricles, when the A-V valves close, the ventricular walls vibrate a little casuing the first sound of the heart beat, the “lub” sound.
2.  The ventricles contract and force blood into the arteries. This is called systole and the systolic blood pressure (BP) is the higher of the two numbers, when the heart is actively contracting and putting pressure on the blood. When the semilunar valves snap shut, this causes the second sound of the heart beat, the “dup.”
3.  The heart relaxes and blood flows into the atria and ventricles. This is called diastole. The diastolic BP is the lower of the two numbers, when the heart is relaxed, and so, is a measure of how much pressure the arteries, themselves, are putting on the blood. Clogged arteries are less elastic, so the blood is under more pressure, thus more likely to cause the arteries to burst.

The rate of contraction is the heart rate. A baby’s heart starts beating when it is about four weeks old (the mother’s period is two weeks late, and she’s just beginning to suspect she might be pregnant). A newborn’s heart rate is around 135 to 140 beats per minute (bpm). By age 15 to 30, the rate decreases to about 65-75 bpm, then speeds up slightly as the person ages. The pulse is a wave of contraction of the artery walls (which roughly corresponds to the heart rate) as blood is forced into the arteries. Pulse is usually measured using the radial artery (the one along the radius). To find your pulse, rest your right arm in the palm of your left hand. Curl the fingers of your left hand up around the thumb side of your right wrist. Place several fingers of your left hand along and just to the outside (thumb side) of the tendon that runs along your wrist. With gentle pressure, you should be able to feel your pulse.
Blood pressure is maximum during systole, when the heart is pushing, and minimum during diastole, when the heart is relaxed. In a living person, the blood pressure doesn’t go to zero because the thick, elastic artery walls exert pressure on the blood. A sphygmomanometer is the instrument used to determine BP. The artery used to determine BP is the brachial artery, which runs down the upper arm, splitting into the radial and ulnar arteries near the elbow. The cuff of the sphygmomanometer is wrapped around the arm just above the elbow and pumped up to block off blood flow (the pressure exerted by the cuff is higher than the systolic pressure). The pressure in the cuff is gradually decreased, and when it equals the person’s systolic pressure, the heart can force blood under the cuff, and a sound is heard as the pulses of blood surge under the cuff. As the pressure in the cuff is lowered, when it equals the diastolic pressure, blood can flow freely, so the sound disappears (not enough pressure is exerted by cuff to restrict blood flow). Thus, by listening for the first sound, and when the sound becomes faint, while watching the pressure indicator on the sphygomomanometer, it is possible to determine someone’s blood pressure. Typically, when you go to the doctor’s office, one of the first things that is done to you is that someone (a nurse?) takes your blood pressure. I have frequently had the experience that when I ask what the results were, I initially get the answer “It’s OK.” Here’s a tip: you, not they, are in charge of your health. The only way you can educate yourself to how your body works is to keep re-asking the question until you get a real answer. You need to know the actual numbers to be able to evaluate if things have changed or are good or bad. Be persistent and eventually they’ll tell you what your BP is.
(clipart edited from Corel Presentations 8)
A neonate’s BP is around 80/45 mm Hg meaning that the systolic pressure is equivalent to air pressure that will support a column of mercury 80 millimeters high in a barometer, and the diastolic is equivalent to the air pressure that will support a column of mercury 45 millimeters high. For adults in their 20s, 120/80 mm Hg is considered average for a male and 115/75 mm Hg for a female, thus the accepted average is said to be 120/80 mm Hg. With age, the arteries become less elastic (due in part to undesirable lipid deposits in their walls), so the BP rises. Hypertension is when the BP is too high. There are two ways this could happen: either the systolic pressure is greater than 145 to 160 mm Hg and/or the diastolic is greater than 90 to 100. Major contributing factors include the amounts of salt, cholesterol (and other lipids), and sugar in one’s diet and the amount of exercise the person gets. Frequently, diureticsare prescribed to try to remove water from the person’s blood, thus lowering the blood volume and hopefully thereby, the BP. However, many diuretics also remove potassium (and other beneficial minerals?) from the person’s system, and if serum potassium levels are not carefully monitored and go to low, this could cause a heart attack!
A thrombus is a blood clot (platelets and fibrin) which forms within a vessel and blocks the blood flow. These can result from surgery or from conditions like atrial fibrillation. An embolus is a moving thrombus which may “get stuck” somewhere. If thrombi or emboli lodge in an artery supplying blood to the heart, this can cause a coronary embolism or heart attack or myocardial infarction. If one of these becomes lodged in an artery in the lungs, it is also a life-threatening pulmonary embolism, and if in the brain, a cerebral (or cerebellar) embolism or stroke or cerebrovascular accident (CVA).
A hemorrhage is bleeding, especially profuse, and can be severe if internal.
A hematoma is a local swelling or tumor filled with blood; a bruise, especially a large one. Sometimes, if the injury is extensive, it can calcify as it heals, leading to a hard lump (which may need to be surgically removed).


Hemorrhoids are dilated or varicose veins in the anal area. Typically, these are caused not enough fiber in diet causing the feces to be very hard so the person has to strain to pass them. Increasing the amount of fiber in one’s diet can help prevent hemorrhoids and possibly aid in healing mild cases. Because vitamin C is necessary for collagen synthesis, it is necessary for strong capillary walls (one of the first signs of a vitamin C deficiency is easy bruising), so that and the bioflavonoid rutin (found in buckwheat) have proven useful for strengthening blood vessels and preventing/treating hemorrhoids and other varicose veins.


Edema is an accumulation of fluid (plasma) within tissues and/or the lymph system. There are many possible causes of edema from injury, to too much salt, to improperly functioning kidneys, to lack of exercise, to female hormonal changes, to a number of other possible causes. If in doubt, see a doctor.
Much like a heat pump for your house or your refrigerator coils, your cardiovascular system is also involved in countercurrent heating/cooling of your body. Arteries and veins lying near each other in your extremities, but flowing in opposite directions can absorb heat from each other as needed. When your core temperature is too high, the arteries carry heat to the extremities to be dissipated. As the blood returns via the veins, any excess heat still in the blood is transferred to the arterial blood and sent to the extremities, again. When your core temperature is too low, as the blood flows out in the arteries to nourish the extremities, its heat is transferred to the venous blood and sent back into the body to keep it warm.


In Raynaud’s Phenomenon, when the person (more common in women than men) gets cold, spasms in the tiny arteriole muscles cause the circulation in portions of the fingers or toes to completely “turn off,” and that portion of the finger/toe turns completely white. As the person warms up and circulation is restored, initially these areas of the fingers/toes will be cyanotic (blue), then will be flushed and red, before returning to normal. The Merck Manual suggests that there may be a relationship between migraine headaches and Raynaud’s. Diagnosis is confirmed by testing the blood pressure in not only the brachial artery, but also the radial and ulnar arteries, and using tiny cuffs made of Velcro® and aquarium tubing, each finger, both when the person is comfortably warm and when the person’s hands have been soaking in ice water. People with Raynaud’s need to make sure to wear warm mittens and heavy socks in winter weather, and since much heat is lost from our heads, wearing a scarf or hat can actually help to keep the person’s whole body warm and lessen the chances of a Raynaud’s episode in the fingers/toes!
Hardening of the arteries is also called arteriosclerosis, a generic term for a number of diseases in which the artery walls become thickened and lose elasticity. One special form of this is atherosclerosis which is a build-up of lipids on the inside of blood vessels. Major risk factors for atherosclerosis include hypertension, elevated serum lipids, elevated LDL (low-density lipoproteins, the bad guys) and lowered HDL (high-density lipoproteins, the good guys), smoking, diabetes, obesity, male sex, and family history. Female hormones offer protection against accumulation of arterial plaque, so usually, premenopausal women do not have as many problems with this as men do. However, after menopause, lipids will start to accumulate. I once hear a statistic that the average 55-year-old woman has a build-up equivalent to the average 18-year-old man.
 arterial Plaque. Atherosclerosis

Urinary system

Urinary system of the body plays a crucial role in our sustenance. Also referred to as the genitourinary or excretory system of the body, this system is responsible for the elimination of various metabolic waste materials from the body, in the form of urine. Essential nutrients from food ingested, is absorbed and used for their respective uses in the body. The remaining waste in the bowel and blood needs to be thrown out, thus, the kidney in coordination with the lungs, intestines and skin evacuates all wastes in the body. If the elimination of these waste substances is not done, accumulation of the same can conduce to poisoning. The urinary system gets rid of urea and other waste materials from the blood and produces urine to throw them out. Moreover, this system also controls the amount of water and mineral salts to be absorbed back into the bloodstream.

Parts of the Urinary System

The urinary system comprises two kidneys, two ureters, one bladder and urethra. All these parts of the urinary system coordinate with each other and eliminate wastes. Let us have a closer look at these parts and their functioning to understand the working of the urinary system.

 Kidneys: We have two bean-shaped, purplish-brown organs placed just below the rib cage, near the mid portion of the back. Approximately the size of one's fist, the kidneys are made up of myriads of infinitesimal filters, called nephrons, which are the functional units of the kidneys. Blood containing metabolic wastes are passed through these nephrons, which filter the urea from the blood. The remaining 99% of the body fluid is sent back to the body minus the urea. The urea filtered out is combined with water and other waste materials like organic materials and excess salts to form urine. So kidneys are sites of urine production! About one to two liters of urine is produced on a daily basis, depending on the amount of water consumed each day. Besides this, kidneys are also responsible for maintaining fluid and salt balance in the body, thereby maintaining blood consistency. 


Ureters: Two tube-like structures called ureters connect the kidneys to the urinary bladder. The ureters are pipelines through which the waste-rich urine passes from the kidney to the bladder. Ureters feature the presence of muscles in their walls which undergo continuous contraction and relaxation movements, so as to force the urine downward towards the bladder. Every 10-15 seconds urine trickles from the kidneys and collects in the bladder. 

 Urinary Bladder: This pear-shaped, hollow organ is situated in the pelvic region and is kept in place by ligaments that bind it to the pelvic bones. Like ureter muscles, these muscles also contract and relax constantly. The muscles relax to allow entry of urine from ureter and contract (when full) to send urine out of the body, via the urethra. On an average, the bladder can hold about two cups of urine, however, the urge to urinate will begin when the bladder is about a cup full. The circular sphincter muscles present around the bladder's opening prevents the urine from leaking out from the bladder. The bladder does not release urine until it is full. Thank goodness the bladder waits till it's full! 

Urethra: When the bladder is gorged with urine, the nerves in the bladder send signals to one's brain to urinate. The brain in response sends signals to bladder muscles to contract and release the stored urine. The sphincter muscles also receive instructions from the brain to allow the urine to pass out. Urine exits from the bladder through a small tube called urethra, out of the body. 

Interesting Urinary System Facts for Kids

•  In a healthy adult, almost 440 gallons of blood is passed through the kidneys on a daily basis, thereby resulting in formation of almost one to two liters of urine. However, this amount varies with the amount of water (or fluid) intake and the amount of sweat produced by the body.
•  Though we keep describing kidneys as bean-shaped organs, the beans were named after the organ and not vice versa! Moreover, one of the other urinary system facts is that, if at all one kidney fails to function, the other kidney takes up the entire load of filtration.
•  Urine is almost odorless when it leaves a healthy body. Surprised! Well, another surprising urine fact is that urine is also sterile when it leaves the body. It does contain metabolic wastes, salts and fluids, however, it is devoid of viruses, bacteria and fungi. Once the urine comes outside the body, bacteria in the air converts chemicals present in it to other forms of smelly chemicals like ammonia, etc. So this is how the foul smell of urine comes!
•  Kidneys of the urinary system maintain the amount of fluid balance in the body. Consumption of excess water results in diluted, pale-colored urine and consumption of inadequate amounts of water results in conservation of water by the kidneys, thereby conducing to dark yellow-colored urine.
•  Sometimes people have this urge to urinate shortly after visiting the restroom. Then when they try to urinate, only a little urine comes out. This mostly happens during a bladder or urinary tract infection.
•  Frequent urination is also a sign of pregnancy. The growing uterus applies pressure on the urinary bladder, which causes one to urinate frequently. Moreover, during pregnancy blood flow to the kidneys increases rapidly, thereby resulting in faster filtration and urine formation. About 2 to 4% pregnant women develop urinary infections, which can also be the reason for frequent urination sensation.
•  Urine is often diluted and added to potted plants and plants in gardens. This is because, the adequate urea content in urine is a wonderful source of nitrogen to plants. So if you have a garden, you now know what to do!
•  Have you heard of urine therapy? Well, urine therapy involves application of urine for medical or cosmetic purposes. People apply urine on the skin and even drink urine for medicinal benefits. Don't freak out, they drink their own urine only! Moreover, during wars in historic times, urine was applied on open wounds to destroy bacteria, due to its antiseptic qualities. According to them, the darker the urine, the more effective would be the urine!
•  Urine should normally contain only salts, metabolic wastes and fluids. However, at times sugar is seen to be present in the urine. This is an indication of diabetes. Some other problems associated with the urinary system are polyuria (excess urine production), oliguria (little urine production), dysuria (difficulty and pain in urination), kidney failure, kidney stones, bladder control problems, prostate enlargement and urinary tract infections.
• Most of us have found ourselves in situations where we have had to control our full bladders due to the unavailability of a restroom. However, there is a control limit which shouldn't be exceeded. In the year 2007, Jennifer Strange (age 28) of California participated in a radio station's contest that involved drinking the maximum water without having to visit the restroom. She won the contest, however, was found dead the next day in her house. Doctors confirmed she died of water intoxication. Thus, it is important not to take the importance of the urinary system lightly.

The urinary system is truly a fascinating body system. The way each organ functions, the coordination between different structures, etc. causes one to look at the human body with awe!