Circulatory and Lymphatic Systems: Functions & Components

Posted on Mar 10, 2025 in Biology

The International Environment and the Circulatory System

The internal human environment consists of:

  • International fluid (plasma): It surrounds and bathes the cells.
  • Blood and lymph: They collect and distribute substances the cells need and eliminate waste from the cells through the circulatory system.

The circulatory system consists of two systems:

  • The Blood Circulatory System: It carries nutrients and oxygen throughout the body and collects metabolic waste for elimination. The circulatory fluid is blood.
  • The Lymphatic System: It has three main functions:
    1. Return excess interstitial fluid to the blood.
    2. Transport absorbed fat from the small intestine to the blood.
    3. Defend the body in the immune system.

The Lymphatic System

It is a network of vessels and structures distributed throughout the body.

  • Lymph Capillaries: Very thin vessels closed at one end, found in all tissues. Their function is to collect intestinal fluid.
  • Lymph Nodes (Glands): Small bean-shaped organs, located in major arteries and veins but are most abundant around the ears, neck, and armpits. Their function is to contain different cells that locate and fight microorganisms that could cause infections. During an infection, the nodes become larger (artery, vein, lymphatic vessels).
  • Lymphatic Vessels: Larger vessels with valves that prevent the lymph from flowing backward. Their function is to return excess lymph to the circulatory system.

Blood

Blood is a thick, red liquid that flows inside the blood vessels.

  • Blood Plasma: A yellowish liquid that makes up 55% of blood. It consists of water and dissolved substances, including proteins involved in functions like the immune system.
  • Blood Cells: Red blood cells, white blood cells, and platelets make up the remaining 45% of blood. These are formed in the red bone marrow (femur).

White Blood Cells

  • Size: Largest blood cells.
  • Nucleus: Present.
  • Types: Neutrophils, basophils, eosinophils, lymphocytes, monocytes.
  • Function: Protect the body against infection and foreign substances.
  • Lifespan: Variable.
  • Count: 5,000-10,000 per mm3 of blood.

Red Blood Cells

  • Most abundant.
  • Nucleus: Absent.
  • Shape: Biconcave disc.
  • Essential Component: Hemoglobin, responsible for its red color.
  • Function: Transport oxygen; pick up carbon dioxide for elimination.
  • Lifespan: About four months.
  • Count: 4.5-5.5 million red blood cells per mm3 of blood.

Platelets

  • Cell fragments.
  • Nucleus: Absent.
  • Function: Help blood to clot by forming a plug to close small breaks in the blood vessels.
  • Lifespan: 8-12 days.
  • Count: 150,000-450,000 platelets per mm3 of blood.

Blood has four main functions: transport of solid and gaseous substances, defense, control of bleeding, and maintaining body temperature.

Blood Vessels

The blood circulatory system includes the blood vessels and the heart.

  • Arteries:
    • They carry blood away from the heart.
    • Size: Larger near the heart, thinner and smaller arterioles farther from the heart.
    • Arterial Walls: Thick and elastic.
  • Veins:
    • They carry blood from the organs to the heart.
    • Size: Venules (thin veins) farther from the heart; larger and thicker veins close to the heart.
    • Vein Wall: Thin and not very elastic.
  • Capillaries:
    • They connect arterioles and venules.
    • Size: Tiny.
    • Capillary Walls: One layer of flat cells, the capillary endothelium. It facilitates the exchange of gases, nutrients, and waste between the blood and the cells.