When we need to collect blood from patients we typically draw blood from peripheral veins that are superficial. These are the veins that you can see in some individuals hands and forearms. There are superficial leg veins that are visible. Drugs like nitroglycerin or other vasodilators do not have significant effects on the superficial veins that you'd be trying to withdrawal blood from in the extremities (i.e. arms and legs).
If you wanted to really engorge someone's veins, you'd likely have to load them up with intravenous fluids. However, this would not be safe in patients needing routine blood draws; especially elderly people who might become fluid-overloaded to the point at which the develop congestive heart failure. Younger patients might temporarily have minimal increases in their circulating blood volume, but their kidneys would very quickly eliminate the excess fluid as urine.
Veins have several advantages over arteries. From a purely practical standpoint, veins are easier to access due to their superficial location compared to the arteries which are located deeper under the skin. They have thinner walls (much less smooth muscle surrounding them) than arteries, and have less innervation, so piercing them with a needle requires less force and doesn't hurt as much. Venous pressure is also lower than arterial pressure, so there is less of a chance of blood seeping back out through the puncture point before it heals. Because of their thinner walls, veins tend to be larger than the corresponding artery in the area, so they hold more blood, making collection easier and faster.
Finally, it is somewhat safer if a small embolism (bubble in the blood) is introduced into a vein rather than an artery. Blood flow in veins always goes to larger and larger vessels, so there is very little chance of a vessel being blocked by the embolism before the bubble reaches the heart/lungs and is hopefully destroyed. Blood flow in an artery, on the other hand, always moves into smaller and smaller vessels, eventually ending in capilllaries, and there is a chance that a bubble introduced by a blood draw (generally rare) or more commonly an intravenous line (IV) could block a small blood vessel, potentially leading to hypoxia in the affected tissues.
Primarily because nerves do not contain blood.
Veins and arteries do, and some people just have smaller or more difficult veins to locate with a needle.
I believe you mean to ask “Why do some people have very small veins to extract blood from for a test?” (You cannot get blood from nerves.)
The reasons are many for small veins and arteries. If the person is small then their veins and arteries are small. If they do not exercise that can allo te veins to shrink somewhat.
Perhaps they just inherited that from their paents who also had small veins and arteries.
1. The veins are more readily accessible for venipuncture, as several are superficial. Arteries are generally more deeply buried. 2. Veins are not under the same pressure as arteries, so blood flows slowly and steadily from veins. From arteries, blood would pulse out and may actually spurt out. 3. There are occasions when arterial blood oxygen needs to be measured, and blood is actually taken directly from the artery. It’s a bit more complicated.
Veins are never blue, purple, or green. They’re white, actually. Here’s why they look blue when we view them through the skin surface.
Blood in the veins is relatively low in oxygen (typically about 79% saturated as compared to 98–100% saturated arterial blood). Deoxygenated hemoglobin has a deeper red to red-violet color, as shown in these samples of arterial and venous blood.
The veins themselves are composed largely of collagen, a white protein. So is the dermis of the skin. That’s the same kind of tissue and same color as you see in this Achilles’ tendon.
When light reflected from deoxygenated blood filters through the collagenous wall of the vein and collagen of the skin on its way to your eye, it can come out with the bluish color that we see in some of our surface veins. In significant part, this comes about through a physical phenomenon called Tyndall scattering of light [1]—the same thing that makes the sky or a mountain lake look blue. It’s not because the blood is blue or the veins are blue.
1. Tyndall effect - Wikipedia [ https://en.wikipedia.org/wiki/Tyndall_effect ]
All veins roll. However, depending on the person who is drawing your blood they can keep them from rolling. A good phlebotomist should use both hands, the left hand goes around your arm with the left thumb anchoring the vein. Then they should use the right hand to go above the left thumb and hit the anchored vein. Quick and easy. Clinics, hospitals and urgent care think anyone can be a phlebotomist. Not so. Sure it’s easy to dig around for a vein 3–4 times. Maybe for the phlebotomist, but it’s agonizing for you. So they blame you for their lack of skill. They say, oops, your veins roll, you have tough veins and my overall favorite, you have no veins. Everyone has veins and they’re located in the same spot. If you know basic anatomy you can find veins that you can’t see. I was manager of the hospital IV team for 12 years and taught IV therapy to newly hired nurses. I could tell the minute a nurse handled an angiocath needle whether or not she would ever, and I mean ever, be able to start an IV. I was sitting in my doctors office for an appointment and heard a patient back in the lab begging for the phlebotomist to stop. Then I hear my favorite excuse “you have no veins”. I left the waiting room and went to the lab, passing my doctor on the way. He asked me if I would try to draw the gentleman’s blood. Both anticubes ( the crook of the arms) were blown. This man was a farmer. Tough skin, but once you push through you need to pull back or go through the vein. I went to the deeper vein and had blood drawn in less than a few minutes. I told my doctor the phlebotomist had to go. He asked if I would work in his lab? I’m retired after working for 45 years. Anyway, what he pays a phlebotomist and what I would charge is different. But I offered to teach this phlebotomist how to access veins. 2 weeks later she knew enough about drawing blood, I allowed her to draw mine. So, your veins roll and it’s up to the skill of the person drawing your blood to keep them from rolling. If they blame anything on you, complain. If they dig around for a vein, complain. It’s painful on a normal day and don’t let them blame you for their lack of skill.
My stage 4 kidney cancer was detected because my blood calcium level was too high. This was an indication of a cancer somewhere. It could have been a cancer in my bones releasing calcium from them, but in my case, most of 1 kidney was cancerous, so the kidneys were unable to remove as much calcium as usual from the blood they were filtering. A CT scan showed where the cancer was.
Two things here:
1. My GP made a very good pick up to spot the high calcium level and get it checked with a CT scan. All the specialists I have since seen have said so, suggesting many doctors would have thought ‘let’s wait and see what it is next time’. There would have been NO next time - if they hadn’t done the operation then to take out the kidney, I was literally weeks (not months) from death. You bet I gave my GP a big hug! 2. Unfortunately, this symptom only occurred when the cancer was so advanced and serious. I was having blood tests every 3 months so we knew it had not been high 3 months prior. So you cannot rely on a blood test for early detection of this sort of cancer. My surgeon said this tumour had been growing for decades. In short, I was lucky … and I still am 3.5 years later!
Doctors can and do use arterial blood sampling but usually only when it is absolutely necessary ie. they need an arterial blood sample for arterial blood gas analysis (ABG) to find out how much oxygen and CO2 is in the arterial blood or when they cannot get venous access (for examples in trauma or otherwise seriously ill patients).
Venous blood is preferred for number of reasons:
- It is easier. Veins are superficial and there are lot of them. If you compress the arm proximally you can see and feel the veins running across the length of the upper limb. If you are taking arterial blood you usually go for the radial artery and you usually have to rely on the pulse you feel (or hear using doppler). Veins also tend to be more tethered if you pick the right spot ie the vein doesn't wiggle away when you try inserting the needle and you don't need to go fishing for it. Veins also have thinner walls which makes it easier to get a needle in. Novices (like myself) may need to use a bit more force to get into the artery and sometimes they push the needle right through the back wall in the process.
- Arterial blood samples are more painful for the patient. I haven't had one done on myself but I have heard that even in optimal situation it is more painful than venous sampling. And because it can be more difficult than venous access the doctor may need to do more poking around to get in, which again, hurts.
- When you are taking blood from the radial artery you are effectively occluding it. Thus the hand relies on alternative arteries for blood supply. You can do Allen test (compression of radial and ulnar artery to cut of the blood supply temporary followed by release of ulnar artery to see if circulation to return) to ensure that there will be sufficient blood supply while taking blood. In theory you can cause ischemic damage to the hand if the collateral supply is poor.
- The arteries have much higher pressure than veins and once you have access the blood will be squirting out (hopefully to your syringe). Once you have withdrawn the needle you should apply pressure to the site for several minutes to reduce bruising. While the blood sample is clotting in your syringe becoming increasing useless. So you compromise between bruising and getting the job done.
- When you are having bloods taken it is usually not done by the doctor. Phlebotomists, health care assistants and nurses take large proportion of bloods in both primary care and hospital settings. They generally have not been trained to do (or their job description doesn't include) arterial blood sampling (with perhaps the exception of ICU nurses or specialist respiratory nurses). Thus it requires a doctor, one you may or may not to spare to do arterial sampling when it is not necessary.
- When you are having bloods taken at hospital the doctor or nurse usually uses it as an opportunity to put a line in for fluids and medications. Generally speaking drugs don't go into arteries. Unless you want to lose a limb.
Venous sampling is preferred mostly because it is easier for the doctor (+ other staff) and the patient and an arterial sample is not usually required. Getting a line in is always a bonus. The practical considerations are also there but usually if you need an arterial sample you will get it but it probably won't be a textbook performance.
There are two tests that could require fasting:
1. Fasting blood sugar 2. Triglycerides However, the doctor can order a random blood sugar, which does not require an 8 hour fast.
Triglycerides are usually not ordered alone, they are ordered as part of lipid panel. This requires a 12 hour fast.
Water is ok. Nothing else.
Veins carry blood inward toward the heart.
Your circulatory system has three main types of blood vessels: arteries, veins and capillaries. Each type does something different for the body.
Arteries are outward blood vessels. The largest artery is the aorta, and these arteries carry oxygenated blood around the body.
Veins are inward blood vessels. They carry blood that has had its oxygen used back to the heart for oxygenation.
Capillaries are the middle man. They are responsible for the actual exchange of oxygen and nutrients. They are the connection between local arterioles (small arteries) and venules (small veins).
Hope this helps!
It would depend on which veins you sample from and what tests you perform, but blood taken anywhere from any limb would be pretty much identical, certainly as far as common tests are concerned.
If you could get blood from the renal or hepatic veins (which would need a scanner for you to locate) they would be slightly different, hepatic vein would have lower oxygen level and high fat, sugar and protein content, as it would have picked up ‘food’ form the liver. Renal vein would have lower oxygen but lower urea levels as it would have been ‘cleaned’ by the kidney. Pulmonary vein would have higher oxygen level, equivalent to arterial blood as it is returning form the lungs.
