6.2 The Blood System

Galen

Humans have 4 humours, produced in the liver, sickness is caused by humours being out of balance

William Harvey

English physician, proposed blood system in 1600s, blood is pumped in a circle by the heart, valves in the veins

Arteries

• Thick walls with muscle and elastin fibres

• High blood pressure

• Small lumen

• Blood away from heart

• Mostly oxygenated

Capillaries

• Very thin walls, 1 cell thick

• Blood pressure gradient

• Lumen 1 blood cell wide

• Distribute materials and collect waste

• Continuous: endothelial cells held together by tight junctions to limit permeability of large molecules

• Fenestrated: endothelial layer contains pores to emphasise absorption, e.g. in intestines

• Sinusoidal: open spaces between cells, permeable to large molecules and cells, e.g. in liver

Veins

• Thin walls

• Low blood pressure, maintained by skeletal muscle groups

• Large lumen

• Blood back to heart

• Mostly deoxygenated

Arterial Walls

• Muscle Fibres → rigid arterial wall, contract to narrow lumen and maintain pressure

• Elastic Fibres → let arterial walls stretch and expand, recoil and maintain pressure

Vasoconstriction

When cold, blood vessels close to the skin constrict to keep blood away from the surface and maintain heat

Vasodilation

When hot, blood vessels close to the skin dilate to increase surface contact and lose heat

Double Circulatory System

Pulmonary to lungs, systemic to body

Valves in Heart

• Tricuspid(AV) from right atrium to right ventricle

• Semilunar from right ventricle to pulmonary artery, left ventricle to aorta

• Bicuspid(AV) from left atrium to left ventricle

Heartbeat

• Myogenic → controlled by impulses from the heart itself

• Sinoatrial Node → ‘pacemaker’ in the right atrium, atria contract

• Atrioventricular Node → transmits impulse to Bundle of His in septum

• Bundle of His → transmits impulse to Purkinje Fibres in ventricle walls, ventricles contract

Cardiac Cycle

• Blood flows into atria and ventricles

• Once atrium is 70% full, atrial systole starts and pushes remaining blood into ventricles

• Ventricular systole starts, increase in pressure forces AV valves closed as ventricular pressure exceeds atrial pressure

• Once ventricular pressure exceeds arterial pressure semilunar valves open and blood is ejected

• Diastole starts, once ventricular pressure drops below arterial pressure semilunar valves close

• Once ventricular pressure drops below atrial pressure AV valves reopen

Heart Rate

• Epinephrine → hormone produced by adrenal glands, increase heart rate during activity

• Nerve Signals → rapid changes from medulla oblongata, vagus nerve decreases heart rate, sympathetic cardiac nerves increase heart rate and force of contraction

Atherosclerosis

Hardening and narrowing of arteries due to cholesterol deposition, genetics, age, smoking, obesity, diet, exercise, hypertension all risk factors for Coronary Heart Disease