10.1 - Types of Circulatory System

Necessity of Transport Systems in Complex Multicellular Organisms

Every living cell requires oxygen and nutrients and expels cellular waste products such as carbon dioxide and nitrogenous wastes.
Unicellular organisms such as Amoeba sp. have a small body mass and a large total surface area to volume ratio (TSA/V).
Essential substances and wastes diffuse between the cell and its external environment; no specialised transport system is required.

Large complex multicellular organisms have a small TSA/V.
Cells are too far from the external environment for direct substance exchange by diffusion.
An internal transportation system transports substances; in vertebrates, this is the blood circulatory system.

Haemolymph flows directly into the body cavity (haemocoel) and bathes the cells.
Haemolymph is a blood-like nutritious liquid found in most invertebrates, including insects and molluscs.

Blood is always contained in continuous, closed blood vessels and is distributed throughout the body.
Oxygen and nutrients are exchanged across blood capillary walls.

One or more hearts pump haemolymph through haemolymph vessels into the haemocoel.
Heart contracts → haemolymph flows into the haemocoel → substances diffuse between haemolymph and body cells.
Heart relaxes → haemolymph returns through tiny openings called ostia (singular: ostium).
A separate tracheal system transports oxygen.

Heart: two chambers—one atrium and one ventricle.
Ventricle → gill capillaries for gaseous exchange → systemic capillaries → atrium through veins.
In systemic capillaries, oxygen diffuses into tissues and carbon dioxide diffuses into the capillaries.
Blood passes through the heart once in one complete circulation: single circulatory system.

Heart: three chambers—two atria and one ventricle.
Deoxygenated blood enters the right atrium; oxygenated blood from the lungs and skin enters the left atrium through the pulmonary vein.
Blood from both atria enters one ventricle; some oxygenated and deoxygenated blood mixes.
Pulmocutaneous circulation transports blood to the lungs and skin; systemic circulation transports oxygenated blood to body tissues and returns deoxygenated blood to the right atrium.
Blood passes through the heart twice in one complete circulation: incomplete double circulatory system.

Heart: four chambers—two atria and two completely separated ventricles.
Pulmonary circulation: deoxygenated blood travels through the pulmonary artery to the lungs; oxygenated blood returns to the left atrium and flows into the left ventricle.
Systemic circulation: blood travels through the aorta to all body tissues; deoxygenated blood returns through the vena cava to the right atrium.
Blood passes through the heart twice in one complete circulation; oxygenated and deoxygenated blood do not mix: complete double circulatory system.