Why does venous blood still have oxygen?

blood flow

Nutrients, messenger substances and oxygen are brought to the places where they are needed via the bloodstream.

In addition, metabolic end products are transported from the cells to the excretory organs via the blood vessels.

Together with the heart, the bloodstream forms the cardiovascular system.

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Messenger substances and components of the blood coagulation system and immune system are also brought to the places where they are needed via the arteries and veins.

In addition to their main function as a transport system for the blood, the vessels also perform various other tasks - such as controlling blood pressure. The blood circulating in the vasculature also plays an important role in maintaining body temperature and water and electrolyte balance.

Like any other organ, blood vessels can become sick. The consequences are, for example, the dreaded atherosclerosis, which can lead to heart attacks and strokes, or - much more harmless - varicose veins.

How are the vascular systems divided?

Basically, the human blood vessel system can be divided into two large areas - the arterial and the venous system. Differentiating criteria are on the one hand the type of blood vessels and on the other hand their function in the body. To put it simply, arteries are all vessels through which blood is transported from the heart to the various organs.

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The blood then flows back to the heart via the veins.

++ More on the topic: Veins ++

In order to understand the blood vessel system, one must also be familiar with the human bloodstream. This is also divided into two areas, each of which has both arteries and veins: the pulmonary circulation and the body circulation. In both cases, the starting point is the heart. This hollow organ, which is divided into two separate heart chambers, is the driving force in both the pulmonary circulation and the body's circulation.

Pulmonary circulation

The starting point of the pulmonary circulation is the right ventricle. It pumps deoxygenated blood from the heart through the pulmonary artery into the lungs. In the lungs, the blood vessels then continue to branch out until they become the finest hair vessels, the so-called capillaries.

These surround the more than 100 million air sacs (alveoli, see Fig. 1) in the human lungs in a dense network. A vital process takes place between the air we breathe in the alveoli and the blood in the pulmonary capillaries: gas exchange.

On the one hand, carbon dioxide (CO2) is released from the blood of the pulmonary capillaries into the alveoli through the extremely thin wall between the alveoli and pulmonary capillaries, and on the other hand, the previously oxygen-poor blood absorbs the vital oxygen from the inhaled air. After the gas exchange has taken place, the blood vessels reunite to form increasingly larger veins and finally to the pulmonary veins, which transport the oxygen-enriched blood back to the heart - more precisely to the left atrium, which opens into the left ventricle.

Body circulation

Exactly there, in the left ventricle, the body's circulation begins. After passing through the lungs, it pumps the oxygen-rich blood into the main artery, the aorta. This divides into several large arteries, which then lead to the various organ systems and parts of the body. For example, blood reaches the kidney via the renal artery and the leg via the femoral artery. The body's circulation is also called the great circulation because it supplies our entire organism with blood, from the brain to the internal organs such as stomach and intestines to muscles and skin.

The branches of the arteries continue to branch until they become arterioles (small arterial branches) and finally capillaries. These permeate all organs and tissues and release oxygen and nutrients to the body cells and absorb carbon dioxide and metabolic end products.

The capillaries then change into small vein branches (venules) that collect the oxygen-poor blood flowing back from the body cells. They connect to form increasingly larger veins, which eventually flow into the superior vena cava (blood from the head, neck and arms) and the inferior vena cava (blood from the rest of the body). These two large veins lead the blood back to the heart, more precisely to the right atrium, which opens into the right ventricle - this in turn is the starting point for the pulmonary circulation.

Portal vein system

The portal vein system occupies a special position within the vascular system. The portal vein is a large vein in which the blood from the stomach, intestines, pancreas and spleen collects. Your blood is low in oxygen and rich in nutrients after you eat. The portal vein joins the liver. There, the nutrient-rich blood from the portal vein mixes with oxygen-rich blood from the hepatic artery. The liver cells metabolize the nutrients supplied from the intestine - they are broken down, converted or stored. Possible toxins are also removed from the blood in this way before it flows from the liver into the inferior vena cava and from there back to the heart.

Likewise - as an undesirable effect - orally ingested drugs are broken down in the liver to varying degrees before they can reach the target organ via the bloodstream.

Arterial and Venous Vascular System

Although the arterial and venous vascular systems essentially perform the same task - transporting blood through the body - there are numerous differences: For example, the arteries downstream of the "pump" heart have to withstand a much higher pressure than the veins and are therefore structured differently. The problems that can occur with the two types of vessels also differ.

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Ulrich Kraft (doctor and medical journalist)
Editorial editing:
Dr. med. Matthias Thalhammer, Mag. (FH) Silvia Hecher, MSc

Status of medical information:

Piper, Wolfgang: Internal medicine. Springer, Berlin (2007)
Herbert Lippert: Textbook anatomy. Elsevier-Verlag, 7th edition 2006

Malte Ludwig: Angiology in clinic and practice. Thieme 1998