Arteries, Veins, Capillaries, and the Circulatory System

Posted on Feb 18, 2025 in Biology

Arteries, Veins, and Capillaries

Arteries: Blood vessels through which blood flows, leaving the heart and distributing throughout the body. They are highly elastic and have three layers: tunica externa (connective tissue), internal layer (fine epithelial cells), and tunica media (smooth muscle fibers and elastic fibers).

Veins: Blood vessels where blood circulates from the capillaries towards the heart. Semilunar valves prevent backflow of blood.

Capillaries: Blood vessels between veins and arteries. They have only one layer to facilitate the exchange of substances (water, nutrients, CO2, urea, oxygen).

Similarities and Differences

Similarities:

• Both are ducts/vessels.
• Both carry blood.
• Arteries and veins have three layers.

Differences:

• Arteries are more elastic and withstand higher pressure.
• The tunica media is thinner in veins, and they have more elastic fibers.
• Capillaries have only one layer of cells.
• Veins have semilunar valves that prevent blood from flowing backward.
• Arteries carry blood from the heart, and veins carry blood to the heart.

Circulatory System Function

Function: To distribute food and oxygen throughout the body and collect waste materials produced by cells.

Components:

• Blood and lymph (fluid)
• Heart: A pump that drives blood
• A series of conduits (veins = blood vessels, arteries, capillaries) and lymph vessels.

Blood

Blood is a tissue with cells suspended in a liquid medium.

Composition:

• Plasma: A slightly viscous, salty, yellow liquid.
• Blood cells:
  1. Red blood cells (Erythrocytes)
  2. White blood cells (Leukocytes)
  3. Platelets

Plasma

Plasma is composed of 90% water and other substances such as nutrients (amino acids, lipids, glucose, mineral salts), waste products (urea (kidney), CO2 (lungs)), hormones, and other substances like fibrinogen (clotting), antibodies, heparin, and toxic substances.

Red Blood Cells (Erythrocytes)

These cells have lost their nucleus and contain a large amount of hemoglobin (a red protein). Their function is to carry oxygen from the lungs to all cells of the body for respiration. There are approximately 5 million red blood cells per mm³. Anemia is caused by a lack of iron or vitamin B12.

White Blood Cells (Leukocytes)

These cells have a nucleus. Their function is defense and cleaning. There are approximately 8,000 leukocytes per mm³. Lymphocytes (antibodies) destroy microbes. Pus is composed of microbes and white blood cells; it is the result of particles that have died from the fight between the two.

Platelets

These are cell fragments without a nucleus. Their function is blood clotting. There are between 150,000 and 300,000 platelets per mm³. They produce thrombin, which converts fibrinogen into fibrin, forming fibers that trap red blood cells and create a blood clot.

Heart

The heart is a hollow, muscular organ about the size of a fist, with a conical tip pointing down and to the left. It is located in the thorax, between the lungs. Its function is to pump blood. The atria are above the ventricles.

Arteries

• Carotid: Ascends the neck to the head.
• Subclavian: Extends from the heart to the shoulders and then to the arms.
• Pulmonary: Located in the area of the lungs.
• Aorta: Descends towards the legs.
• Femoral: Passes through the legs.

Veins

• Jugular: Ascends the neck to the head.
• Subclavian: Descends the arms.
• Superior Vena Cava: Comes from the heart and is in the chest area.
• Inferior Vena Cava: Descends towards the legs.
• Pulmonary: Located in the area of the lungs.
• Iliac: Located in the legs.

Note: Remember that in diagrams, left is right and right is left (opposite).

Veins: Blood vessels that carry blood to the heart.

Arteries: Blood vessels that carry blood from the heart.

Bicuspid (Mitral) and Tricuspid Valves: Control blood flow within the heart.