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.

Tissue fluid surrounds cells and forms the link between their membranes and nearby capillaries.

• Pressure filters some plasma fluid out of capillaries; cells and most plasma proteins remain in blood.
• Oxygen and nutrients move towards cells; carbon dioxide and other wastes move towards blood.
• Tissue fluid returns to circulation directly or through lymph vessels.

Keep in mind: Red blood cells normally stay inside capillaries. Oxygen leaves haemoglobin and diffuses through tissue fluid to the cells.

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.

A donor red-cell antigen must not meet its matching antibody in the recipient plasma.

• A cells have A antigen; B cells have B; AB cells have both; O cells have neither.
• A plasma has anti-B; B has anti-A; O has both; AB has neither.
• ABO red-cell donations: O to all four groups; A to A/AB; B to B/AB; AB to AB.

Keep in mind: Group O has no A or B antigen on its red cells, but its plasma contains both anti-A and anti-B. In the ABO model it receives O red cells only.

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.