10.2 - Circulatory System of Humans

Components of the Human Circulatory System

Blood: Connective tissue made up of blood plasma, blood cells and platelets; acts as a transportation medium.
Heart: Muscular pump that circulates blood throughout the body.
Blood vessels: Arteries, capillaries and veins connected to the heart; transport blood to body tissues.

The heart lies between the lungs in the thoracic cavity.
Four chambers: left atrium, right atrium, left ventricle and right ventricle.
Atria receive blood returning to the heart; ventricles pump blood out.
Ventricular walls are thicker and contract more strongly than atrial walls.
The left ventricle wall is thicker than the right because it generates greater pressure to pump blood through the aorta to the whole body; the right ventricle pumps only to the lungs.
The septum separates the left and right sides and prevents oxygenated blood from mixing with deoxygenated blood.

Tricuspid valve: Between the right atrium and right ventricle; three leaflets; prevents backflow into the right atrium.
Bicuspid valve: Between the left atrium and left ventricle; two leaflets; prevents backflow into the left atrium.
Semilunar valves: At the bases of the aorta and pulmonary artery; prevent backflow into the ventricles when they relax.

Aorta: Main artery; transports oxygenated blood to the whole body.
Vena cava: Main vein; transports deoxygenated blood back to the heart.
Pulmonary artery: Transports deoxygenated blood from the heart to the lungs.
Pulmonary veins: Transport oxygenated blood from the lungs to the heart.
Coronary arteries: Transport oxygenated blood to heart tissues.
Coronary veins: Transport deoxygenated blood from heart tissues.

Arteries:
- Transport blood away from the heart rapidly and at high pressure.
- Thick, muscular, elastic wall; small lumen.
- No valves except semilunar valves at the bases of the aorta and pulmonary artery.
- Branch into arterioles, which end in capillary networks.
Blood capillaries:
- Wall is one cell thick, non-muscular and non-elastic; lumen is very small.
- Allow gases, nutrients, excretory substances and hormones to diffuse between blood and cells.
- Join to form venules; flow is from arteries to veins at low pressure.
Veins:
- Transport blood from the body back to the heart at very low pressure.
- Thin, less muscular, less elastic wall; large lumen.
- Valves maintain one-way blood flow.

Water (90% of plasma): Transportation medium and solvent for respiratory gases, ions, digestive products and excretory substances.
Fibrinogen: Blood clotting.
Albumin: Controls blood osmotic pressure.
Globulin: Antibody involved in body defence.
Solutes: Nutrients such as glucose, excretory substances such as urea, and respiratory gases.
Hormones: Control physiological activities.
Enzymes: Involved in cellular metabolic processes.

Erythrocytes (red blood cells):
- Elastic plasma membrane; biconcave disc shape provides a large TSA/V for efficient gaseous exchange.
- No nucleus when mature, allowing more haemoglobin to be loaded.
- Produced in bone marrow such as the sternum and ribs; live up to 120 days; destroyed in the liver or spleen by phagocytosis.
- Haemoglobin contains an iron atom that binds oxygen; it forms oxyhaemoglobin at high oxygen partial pressure and releases oxygen at low oxygen partial pressure.
Platelets:
- Fragments of cell cytoplasm originating from bone marrow; lifespan of about one week.
- Involved in blood clotting.
Leucocytes (white blood cells):
- Irregular shape; nucleus present; no haemoglobin; produced in bone marrow; lifespan under five days.
- Diffuse through capillary pores to fight pathogens in tissue fluid.
- Granulocytes: neutrophils engulf bacteria and dead cells by phagocytosis; eosinophils release enzymes that fight inflammation and allergic reactions; basophils contain heparin that prevents blood clotting.
- Agranulocytes: lymphocytes produce antibodies and antitoxins; monocytes engulf bacteria and dead cells by phagocytosis.