Unit 4: The circulatory and excretory systems                        1   2   3   4

2.3. The blood vessels

Blood vessels are the ducts through which blood circulates.

- Arteries

They are the vessels from where blood exits to the heart.

Their walls are very thick, elastic and muscular.

Arteries are divided progressively in more and more thin vessels (arterioles) as they separate from the heart.

- Veins

They are the vessels from where blood enters to the heart.

Their walls are thin, deformable and less muscular.

They have semilunar valves to prevent blood flows back.

Veins are divided progressively in more and more thin vessels (venules) as they separate from the heart.

- Capillaries

     They are the thinnest vessels.

     Their wall is formed only by a layer of cells (endothelium).

     The exchange of substances with tissues is made through them.

Video: Types of blood vessels

a) Blood flow (hemodynamic) 

Arteries are very elastic. This characteristic allows them:

         - Expand when receives the blood from the heart during systole.

         - Return to their natural diameter during diastole to force blood to flow

            continuously.

The force with which blood punches the arteries wall provokes a pressure on them. This pressure is stronger during systole, when it is provoked by heart, and lower when it is provoked by expanded arteries during diastole. So there are two values for arterial pressure: systolic pressure (maximum) and diastolic pressure (minimal)

The pulse is the regular expansion and contraction of the arteries when blood passes through.

Arterioles can regulate the amount of blood which arrives to organs. They contract or relax to reduce or increase, the blood flow, to adequate it to the demands of the organ.

When blood arrives to capillaries the blood pressure is very low and the speed of circulation very slow. This makes easier the exchange of substances between capillaries and tissues.

Blood have to return to heart through venules and veins. But there is not any pump to impulse it. The circulation within the veins is due to:

- The suction which heart makes during diastole.

- The pressure makes over the veins by the skeletal muscles that

   surround them.

The backward flow of blood in veins is avoided by the semilunar valves.

b) Blood circulation

     Human blood circulation has three important characteristics:

     - It is closed: This means that blood always circulates within blood

       vessels.

     - It is double, because blood goes through two circuits:

       - Pulmonary circuit (from heart to lungs and from lungs to heart)

       - Systemic circuit (from heart to body and from body to heart)

     - It is complete: This means that oxygenated and deoxygenated blood

       do not mix within the heart.

         Pulmonary circulation

It is also known as minor circulation because the circuit that blood follows is shorter and only involves lungs.

Deoxygenated blood exits from the heart by Pulmonary artery to the lungs where it is recharged of oxygen and then the oxygenated blood returns to the heart by Pulmonary veins.

         Systemic circulation

It is also known as major or general circulation because the circuit is longer and involves all body organs.

Oxygenated blood exits from the heart by Aorta artery to be distributed by the tissues. Cells take the oxygen and the deoxygenated blood returns to the heart by Cava veins.

When deoxygenated blood arrives to the heart goes through its right side, meanwhile the oxygenated blood goes through its left side. The interventricular septum avoid that they mix.

Each organ has an artery that provides it of nutrients and oxygen and a vein that collects the waste substances and carbon dioxide. These artery and vein are called like the organ (ej: gastric artery and gastric vein). Every one of these arteries and veins are branches of the Aorta artery and of one of the Cava veins.

2.11. Why people that pass long time standing up, has usually their feet

          swollen?

2.12. The exchange of substances is made in capillaries but not in arteries

          or veins, why?

2.13. Indicate why:

        a. The ventricular wall is thicker and stronger than the atrial wall

        b. The left ventricular wall is thicker and stronger than the right one.

        c. Sometimes it is used the expressions: “left heart” and “right heart”

2.14. How can we know if we have cut a vein or an artery?

2.15. Is it the arterial blood always oxygenated blood

          and the venous blood, deoxygenated blood? Why?

2.16. Listen and indicate what type of blood vessel is described: