A double 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)
- 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.
- 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
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.
A few key terms
- Volume blood/heartbeat
- Approx. 70 ml
- Volume blood/time
- Approx. 5 L/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.
Systole (main contraction)
- Ventricles contract.
- Tricuspid & mitral valves close.
- Blood pumped out to the body.
- Venctricles relax
- Pulmonary and aortic valves close.
- The atria contract.
- Ventricles fill with blood.
The blood vessels
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.
Arteries branch into capillaries.
- 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.
Blood flow in the veins
- Low blood pressure.
- Valves prevent the blood from flowing backwards.
- Muscle contractions push the blood forward.