11.7.4. The Human Circulatory System. Heart, Blood Vessels and Blood Pressure


The Human Circulatory System.


A double circulatory system

The human circulatory system.The human circulatory system.The human circulatory system is double:

  • A pulmonary circulation (to the lungs, blood is oxygenated)
  • A systemic circulation (to the rest of the body, where oxygen is consumed)

Pulmonary circulation

  • Oxygen-poor blood in the vena cava blood enters the right atrium, and then the right ventricle.
  • Pumped to the lungs via the pulmonary artery.
  • Oxygen-rich blood via the pulmonary vein to the left atrium.

Systemic circulation

  • From the left atrium via the aorta to the system ("the body"). 
  • Oxygen & nutrients consumed, carbon dioxide & waste products taken up
  • Back to the heart via the vena cava

The heart

Schematic diagram of the heart.Schematic diagram of the heart.

Cross section of the human heart.Cross section of the human heart.Several valves prevent the blood from flowing backward, back into the heart.

Coronary arteries provide blood to the heart.

The entire heart is enclosed by the pericardium:

  • Protects the heart from dilating too much.
  • Protects the heart from infections from surrounding organs.

The heartbeat

A few key terms

Stroke volume

  • Volume blood/heartbeat
  • Approx. 70 ml

Cardiac output

  • Volume blood/time
  • Approx. 5 L/min

Heart rate

  • Beats/min
  • Resting = about 60 beats/min

How the heart beats

Sinoatrial node (SA node)

  • At the right atrium.
  • Nerve impulses that coordinate the beats.
  • Affected by hormones and the nervous system.

Movements during the heartbeat.Movements during the heartbeat.Systole (main contraction)

  • Ventricles contract.
  • Tricuspid & mitral valves close.
  • Blood pumped out to the body.

Diastole (”expansion”)

  • Venctricles relax
  • Pulmonary and aortic valves close.
  • The atria contract.
  • Ventricles fill with blood.

The blood vessels

Blood pressure

The blood exerts a pressure against the arterial walls.

  • Systolic pressure = maximum during a heartbeat.
  • Diastolic pressure = minimum between heartbeats.
  • Normal is approx. 115/70.


Arteries lead blood from the heart.

  • Aorta = the main and largest artery.
  • Relatively thick to be able to withstand the pressure.
Cross section of an artery.Cross section of an artery. Diagram of an artery.Diagram of an artery.


Arteries branch into capillaries.

Very narrow:

  • Enough for a red blood cell to squeeze through.

Only a thin layer of epithelial cells.

Gases and nutrients may diffuse.

The capillaries form a tight web:

  • No cell is more than 130 μm from a capillary.

Large surface area ⇒ lower blood pressure.


Capillaries join, form veins.

  • Veins lead blood to the heart.
  • Low blood pressure.
  • Not as thick as the arteries.
Diagram of a vein.Diagram of a vein. When the surrounding muscle contracts, the vein is compressed, the valve opens, and the blood is pushed forward.When the surrounding muscle contracts, the vein is compressed, the valve opens, and the blood is pushed forward.

Blood flow in the veins

  • Low blood pressure.
  • Valves prevent the blood from flowing backwards.
  • Muscle contractions push the blood forward.

Learning check

[Open learning check in a new window.]