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Blood Circulation

12 min read

Single and double circulation

A fish has a single circulation: the blood passes through the heart once on each circuit. A mammal has a double circulation: the blood passes through the heart twice, once for the lungs (pulmonary circulation) and once for the body (systemic circulation). This keeps oxygenated and deoxygenated blood apart and lets the body receive blood at high pressure.

Single and double circulationOn the left, a fish has a single circulation: the heart pumps blood to the gills, then on to the body, and back to the heart. The blood passes through the heart once each circuit. On the right, a mammal has a double circulation: the right side of the heart pumps blood to the lungs, it returns to the left side, which pumps it to the body. The blood passes through the heart twice each circuit, so it reaches the body at high pressure.HeartGillsBodysingle: 1 pumpRight heartLeft heartLungsBodydouble: 2 pumps (mammals)
The human heartA simplified heart with four chambers. Blood without oxygen, shown blue, comes from the body through the vena cava into the right atrium, passes to the right ventricle and is pumped to the lungs through the pulmonary artery. Blood with oxygen, shown red, returns from the lungs through the pulmonary vein into the left atrium, passes to the left ventricle and is pumped to the body through the aorta.right atriumleft atriumright ventricleleft ventriclevena cava (from body)pulmonary artery (to lungs)aorta (to body)pulmonary vein (from lungs)blood without oxygenblood with oxygen
Blood goes to the lungs to pick up oxygen, returns to the heart, and is then pumped to the body.
Blood vessels comparedCross-sections of an artery with a thick muscular wall and narrow space, a vein with a thinner wall, a wider space and valves, and a capillary whose wall is only one cell thick.Arterythick muscular wallnarrow space, high pressureVeinthin wall, wide spacevalves stop backflowCapillarywall one cell thickexchange with the cells
VesselWallBlood pressureJob
ArteryThick, muscular, elasticHighCarry blood from the heart
VeinThin, has valvesLowCarry blood back to the heart
CapillaryOne cell thickFallingExchange with the tissues

Maternal–foetal circulation

The foetus cannot breathe or eat, so its blood passes through the placenta. The blood of the mother and the baby flow close together but do not mix. Oxygen, glucose, amino acids and antibodies diffuse into the foetus, and carbon dioxide and urea diffuse out. The foetus has special features: an opening between the two atria (foramen ovale) and a vessel from the pulmonary artery to the aorta (ductus arteriosus) that let blood skip the lungs, which are not working. They close at birth.

The placentaThe placenta joins the foetus to the wall of the uterus through the umbilical cord. The mother’s blood and the foetal blood come very close but do not mix. Oxygen and food pass from mother to foetus, and carbon dioxide and urea pass from foetus to mother. The amnion and amniotic fluid protect the foetus.amniotic fluidfoetusumbilical cordmother's blood inblood outto foetus: oxygen, food, antibodiesto mother: carbon dioxide, urea
At the same oxygen pressure foetal haemoglobin is more saturated: it takes oxygen from the mother.
Try itHeart output

The cardiac output is the volume of blood pumped each minute. Choose the heart rate and the stroke volume.

Check yourself In a double circulation the blood goes through the heart...
Check yourself Which vessels have valves?
Check yourself What crosses the placenta from foetus to mother?

Key terms

Pulmonary circulation
Blood flow between the heart and the lungs.
Systemic circulation
Blood flow between the heart and the body.
Foramen ovale
An opening between the atria of the foetal heart.
Placenta
The organ where the foetal and maternal blood exchange substances.

Summary

  • A fish has single circulation; a mammal has double circulation.
  • Arteries, veins and capillaries differ in structure and function.
  • The placenta lets the mother and foetus exchange substances without mixing blood.
  • Foetal haemoglobin takes oxygen from the mother.
Transport in Plants Growth in Plants