Transport in humans, at a glance

Arteries carry blood away from the heart; veins return it. The vessel name identifies the organ or route.

• Right heart -> pulmonary artery -> lungs -> pulmonary vein -> left heart.
• Left heart -> aorta -> body organs -> vena cava -> right heart.
• The hepatic portal vein takes blood from the small intestine to the liver.

Keep in mind: It brings blood from the small intestine into the liver. The hepatic vein carries blood out.

Arteries withstand pressure, veins support low-pressure return, and thin capillaries allow exchange.

• Arteries have thick muscular, elastic walls.
• Veins have relatively thin walls, wide lumens and often valves.
• Capillary walls are one cell thick, giving a short diffusion distance.

Keep in mind: Valves prevent backflow. Pressure differences and surrounding muscle contractions move the blood.

Haemoglobin in red blood cells carries most oxygen; plasma carries the cells and many dissolved substances.

• Blood contains plasma, red cells, white cells and platelets.
• Haemoglobin binds oxygen reversibly.
• Plasma transports nutrients, ions, hormones, carbon dioxide, urea, vitamins and plasma proteins.

Keep in mind: It contains less oxygen and is dark red. Blue in diagrams is a convention that makes routes easier to distinguish.

White cells recognise and respond to foreign material; platelets help form a fibrin clot at damaged vessels.

• Phagocytes engulf and digest pathogens.
• Lymphocytes can produce antibodies specific to antigens; immune recognition can also cause tissue rejection.
• Platelets help trigger conversion of soluble fibrinogen into insoluble fibrin.

Keep in mind: Different white cells have different roles. Phagocytes engulf and digest; some lymphocytes produce antibodies.

Atria receive blood, ventricles pump it out, and valves prevent backflow when pressure changes.

• Right atrium -> tricuspid valve -> right ventricle -> pulmonary artery.
• Left atrium -> bicuspid valve -> left ventricle -> aorta.
• The septum separates the two sides; the left ventricular wall is thicker.

Keep in mind: The septum separates them. Blood travels from the right ventricle through the lungs before reaching the left side.

Contraction raises chamber pressure; relaxation lowers it. Pressure differences determine flow and valve position.

• Systole means contraction; diastole means relaxation.
• Atrial systole tops up the ventricles through open atrioventricular valves.
• Ventricular ejection has closed atrioventricular valves and open semilunar valves.

Keep in mind: The atria contract together first; the ventricles then contract together. This sequence supports filling followed by ejection.

Heart muscle needs its own coronary blood supply. An obstruction can reduce oxygen delivery and aerobic energy release.

• Coronary arteries supply the heart muscle itself.
• Fatty deposits can narrow the lumen; a clot can block flow.
• Smoking, inactivity and an unhealthy diet increase risk; risk is not certainty.

Keep in mind: It restricts blood supplying part of the heart muscle. Lung gas exchange is a different step in the oxygen-delivery pathway.