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Introduction to the circulatory system

The circulatory system

The main function of the circulatory system is a transportation system.

The system transports substances around the body.

  • It transports oxygen from our lungs to the cells of the body.
  • It transports carbon dioxide from the cells to the lungs
  • It transports waste products from the cells to the kidneys and lungs
  • It transports hormones from the endocrine glands to the cells
  • It transports enzymes to the appropriate cells
  • It transports white corpuscles to the site of infection which allow the body to fight disease.
  • It transports nutrients from the digestive tract to supply energy to the cells.
  • It helps in the regulation of the body temperature

Blood consists of:

Plasma: mainly water, foods and carbon dioxide.

White blood cells (leucocytes): their man role is to protect the body fighting and destroying foreign bodies and dead cells and carrying away debris.

Red blood cells (erythrocytes): these contain haemoglobin and carry oxygen from the lungs to the cells of the body.

Platelets (thrombocytes): the cells bond together to form a clot when the skin is injured and the blood is exposed to air.

Other chemicals: hormones are transported in the blood.

Blood circulates through these blood vessels:

  • Arteries
  • Capillaries
  • Veins

 

The blood collects oxygen from the lungs and delivers it to the cells of the body. Glucose is also carried in the blood to be used by the cells together with the oxygen to supply energy. Blood supplies other materials to build and maintain cells or to make products, for example secretions. Inside the tissues, some fluid leaks from the capillaries as blood passes through them. When this fluid leaves the capillaries to enter the tissues it becomes tissue fluid.

Blood circulation

  • The human circulatory system is comprised of the blood, the vascular system and the heart.
  • The heart is the muscular organ which pumps the blood around the body. The vascular system is made up of arteries, veins, and capillaries.
  • Arteries are blood vessels that carry blood away from the heart.
  • Veins are blood vessels that return blood to the heart.
  • Capillaries are the smallest blood vessels, and are where the exchange of nutrients and gases takes place between the red blood cells and the body tissues.
  • Smaller arteries branch off the aorta.

The arteries of the head and neck
As stated above, the blood leaves the heart in arteries. These are called carotid arteries and are on each side of the neck.

These arteries divide into smaller arteries called the internal carotid and external carotid arteries. The internal passes the temporal bone and enters the head, taking blood to the brain. The external carotid artery stays outside the skull and divides into branches:

  • The occipital branch – supplies the back of neck and scalp
  • The temporal branch – supplies the sides of the face, head, scalp and skin
  • The facial branch – supplies the muscles and tissues of the face

The arteries divide repeatedly into smaller and smaller vessels until they form tiny blood capillaries. These are only one cell thick which allows substances carried into the blood to pass through them into the tissue fluid which nourishes and bathes the cells of the different body tissue.

The capillaries begin to join up again to form first small vessels called venules then lager ones called veins. These return the blood to the heart

Veins are less elastic than arteries and are closer to the surface of the skin. They also contain valves that prevent back flow of the blood.

Veins
The main veins are the internal and external jugular veins:
  • Internal jugular and its main branch
  • Facial vein which carries blood from the face and head
  • External jugular vein which carries blood to the scalp – this has two branches; the occipital branch and the temporal branch

The jugular vein joins to enter the subclavian vein and this lies above the clavicle.

Blood returns to the heart which pumps it to the lungs; this is where red blood cells then take on fresh oxygen and carbon dioxide is expelled from the blood. The blood then returns to the heart.

Blood flow through the skin
Normal body temperature is 37°C. Blood circulating through the organs and muscles becomes warm. If the body temperature starts to rise then the blood passes nearer the surface of the skin to release some of the heat to the environment. The skin will appear red and may feel warm. The blood is heated by passing through the liver.