10.2 - Circulatory System of Humans

The circulatory system has 3 major components:

• The blood is a type of connective tissue, made up of plasma, blood cells and platelets. Blood is a medium of transport.
• The heart is a muscular pump that circulates the blood throughout the body.
• The blood vessels carry blood to all body tissues. They consist of arteries, veins and capillaries.

Structure of the Heart

The heart is a cone-shaped, muscular organ that is as big as a clenched fist. It has 4 chambers – left atrium, right atrium, left ventricle and right ventricle. The left and right sides of the heart are separated by a muscular wall called septum. The atria have thinner walls which act as collection chambers, while the ventricles function by pumping blood out of the heart.

The left ventricle has a thicker wall compared to the right ventricle because the left side needs to generate greater pressure to pump blood to all parts of the body. Meanwhile, the right ventricle pumps blood to the lungs only.

Parts of the heart and Functions

• Aorta: Main artery that transports oxygenated blood to the rest of the body.
• Vena cava: Main vein that carries deoxygenated blood back to the heart.
• Pulmonary artery: Carries deoxygenated blood to the lungs to be oxygenated.
• Pulmonary vein: Transports oxygenated blood from the lungs to the heart.
• Semilunar valves: Prevent backflow of blood into the right ventricle when the ventricle relaxes.
• Tricuspid valve: Prevent backflow of blood into the right atrium when the right ventricle contracts.
• Bicuspid valve: Prevent backflow of blood into the left atrium when the left ventricle contracts.

Figure 6 - Major components of human blood. Plasma constitutes 55% of whole blood, erythrocytes constitute 45% and buffy coat only covers less than 1% of the whole blood.

Composition of Human Blood

Blood is the medium of transport in humans. The cellular components of blood consist of red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. 55% of the blood is plasma, while the remaining 45% consist of the cellular components.

The major functions of the constituents of the plasma are summarized in the list below.

Components and Major functions

• Water (90% of the plasma): Acts as solvent to transport dissolved substances.
• Plasma proteins: Albumin, Fibrinogen, Fibrinogen
  • Maintains osmotic balance of the blood
  • A clotting factor that helps to stop bleeding.
  • Antibody that helps in body's defense
• Hormones: Regulate physiological activities in the body
• Dissolved substances (nutrients, waste products and respiratory gases)
  • Nutrients are important for energy production, growth and health maintenance
  • Oxygen is required for cellular respiration

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1. Erythrocytes

2. Platelets

• They are fragments of large cells produced from the bone marrow.
• No nucleus.
• Lifespan is less than 7 days.
• Play crucial role in blood clotting.

3. Leucocytes

• Colorless, irregular shapes.
• Nuclei and mitochondria are present.
• Made in the bone marrow.
• General function is to fight infections.
• Can be grouped into granular or agranular.

4. Granulocytes (have granules in their cytoplasm with lobed nuclei)

• Neutrophils
• Basophils
• Eosinophils

5. Agranulocytes (clear cytoplasm, nuclei not lobed)

• Lymphocytes
• Monocytes

Figure 7 - Neutrophils engulf and digest bacteria and dead cells. Their nucleus has 2 to 5 lobes.

Figure 8 - Basophils are involved in fighting inflammatory and allergic reactions. They secrete heparin to prevent blood clotting. Basophils are the lowest number of leucocytes found in the blood.

Figure 9 - Eosinophils release enzymes during inflammation in allergic reactions. They also kill parasitic worms. Their nucleus has 2 lobes.

Figure 10 - Lymphocytes produce antibodies during immune response. They also release antitoxins to combat toxins. Their nucleus occupies majority of the cytoplasm.

Figure 11 - Monocytes engulf and digest bacteria and dead cells by phagocytosis. Their nucleus is almost spherical in shape.

Human Blood Vessels

There are 3 types of blood vessels in humans which are arteries, veins and capillaries.

1. Arteries

• Blood vessels that carry the blood away from the heart.
• Transport blood at high pressure to all body cells.
• High pressure in the arteries due to the blood pumped from the heart.
• Branches to smaller vessels called the arterioles. Arterioles branch further into a network of capillaries to reach the body tissues.

2. Aorta

• The main artery that leaves the heart.
• Thick and elastic wall to withstand the high pressure of blood flowing through.

3. Capillaries

• Thin wall (one-cell thick)
• Allow the exchange of respiratory gases between the blood and body cells via diffusion.
• Nutrients, waste products and hormones are also exchanged between the blood and interstitial fluid.

4. Veins

• Capillaries rejoin into blood vessels called venules.
• The venules join together to form veins.
• Vena cava is the main vein that carries deoxygenated blood to the heart to be pumped to the lungs.

Characteristics of Arteries, Capillaries and Veins

• Arteries

  • Wall: Thick, muscular, elastic
  • Lumen: Small
  • Valve: Absent
  • Blood pressure: High
  • Direction of blood flow: Away from the heart (to the organs)

• Capillaries

  • Wall: One-cell thick, no muscle or elastic tissue
  • Lumen: Very small
  • Valve: Absent
  • Blood pressure: Low
  • Direction of blood flow: From arteries to the veins

• Veins

  • Wall: Thin, less muscular, less elastic
  • Lumen: Large
  • Valve: Present
  • Blood pressure: Very low
  • Direction of blood flow: Back to the heart (from the organs)