Blood Vessels Closed circuit carrying blood from heart

Blood Vessels Closed circuit carrying blood from heart to cells and back again

Arteries • Carry blood AWAY from heart under high pressure • Become progressively smaller and thinner: arterioles • See figure 13. 18 for blood vessel walls • Vasoconstriction: Vasoconstriction smooth muscles reduce diameter of vessel • Vasodilation: Vasodilation smooth muscles increase diameter of vessel

Capillaries • Smallest diameter blood vessels • Connect arterioles to venules • Thin enough for materials to leave and enter • Smooth muscles encircle entrances to capillaries called precapillary sphincters – Controlled by O 2 concentrations

Exchange • Gases, nutrients, and metabolic by-products are exchanged between capillaries and the tissue fluid surrounding cells • Plasma proteins will affect pressure differences • Substances move from higher to lower pressure – Materials move out faster at arteriolar end where pressure is higher – On average more material leaves than enters • Materials also move by diffusion

Veins • Vessels carry blood back to atria under low pressure • Venules: microscopic vessels than form from merging capillaries • Venous valves project in from lining to prevent back flow of blood

Blood Pressure • The force blood exerts against the inner walls of blood vessels • Systolic pressure: pressure during ventricular contraction • Diastolic pressure: pressure lowest pressure right before next ventricular contraction • Pulse: alternate expanding and recoiling of arterial walls

Factors Influencing Blood Pressure • Heart Action – Stroke volume: amount of blood released by left ventricle in a contraction – Cardiac output: stroke volume X heart rate • Blood Volume • Peripheral Resistance – Friction between walls of vessels and the blood • Blood Viscosity – Ease of flow – Blood cells and plasma proteins increase viscosity

Control of Blood Pressure • Regulation of cardiac output – The more blood that enters the heart, the stronger the contraction – (Starling’s law of the heart) – Medulla oblongata cardiac center – Parasympathetic/sympathetic nerves • Regulation of peripheral resistance – Vasomotor center of medulla oblongata will change diameter of blood vessels – Control: concentrations of chemicals

Paths of circulation • • • Pulmonary circuit: circuit Lungs Systemic circuit: circuit Body Aorta branches into major arteries: – Brachiocephalic – Common carotid – Subclavian • Major veins: – Jugular: drain blood from head – Hepatic portal system