Cardiovascular System Part 3 PDF

‫ بسمـ هللا‬2024 - Cardiovascular System Part 3 Functions of Veins: Blood reservoir (capacity vessels) veins have a large capacity for blood, significant volumes of blood can be added to or removed from veins with only little changes in...

‫ بسمـ هللا‬2024 - Cardiovascular System Part 3 Functions of Veins: Blood reservoir (capacity vessels) veins have a large capacity for blood, significant volumes of blood can be added to or removed from veins with only little changes in venous pressure. The venous system have large compliance. Transport vessels veins are transport vessels that offer little resistance to blood flow. Venous pressures - Central Venous Pressure (CVP) “Right Atrial Pressure - Mean systemic filling pressure (MSFP).MSFP is the pressure that drives the venous return back to the heart. Central Venous Pressure (CVP) “Right Atrial Pressure” Normally equals 0-2 mm Hg. It is important, since it is: - The main force for ventricular filling. - Considered an index of blood volume: CVP is decreased in hemorrhage. This is due to decreased blood volume. Venous return decreases and CVP is decreased. CVP is increased in right-sided heart failure. The failing right ventricle is unable to pump the volume of blood returning to the right atrium. Blood will accumulate in the right atrium and the CVP will increase. Central Venous Pressure is determined by two factors: - Venous blood volume. - Compliance of veins: determined mainly by venous tone. Sympathetic stimulation increases venous tone and decreases compliance of vein.  Vascular Compliance the ratio between changes in blood volume per change in pressure in a blood vessel. Compliance = ∆ volume/∆ pressure. Veins have a compliance that is about 24 times the compliance of arteries. This means that veins can accommodate much more blood than arteries per 1 mmHg change in pressure. This is determined mainly by venous tone. Sympathetic stimulation increases venous tone and decreases compliance of vein. Regulation of Venous Return Venous return is the volume of blood that flows back to the heart per minute. Under normal steady conditions, venous return is equal to cardiac output (i.e. about 5 l/min). Venous return depends on the difference between MSFP and RAP. This difference is considered the “pressure gradient for venous return”  Factors affecting venous return Mean systemic filling pressure (MSFP): is the pressure that drives the venous return back to the heart. The value of MSFP ranges between 6-8 mmHg. MSFP reflects the relationship between two factors: A- Blood volume. Decreased blood volume (e.g. by hemorrhage) or increased capacity (e.g. by decreased sympathetic activity) will decrease VR. Conversely, increased blood volume (e.g. in renal failure) or decreased capacity of the circulation (e.g. by sympathetic stimulation) will increase VR. B- Capacity of the veins. Factors affecting venous return Right atrial pressure (RAP) Venous blood (pushed by MSFP) returns to the right atrium. If the right atrial pressure is high, it will be more difficult for the MSFP to drive venous blood back to the right atrium. Therefore, increased right atrial pressure opposes venous return. Resistance to venous This is the vascular resistance to venous blood as it flows back to the return (RVR): heart. Venous Return Curve Effect of changes in RAP on venous return curve - Increasing RAP causes a decrease in VR. When the RAP reaches 7 mmHg, VR decreases to zero. (VR stops) - Conversely, decreasing RAP causes VR to increase. Effect of changes in MSFP on venous return curve - Increasing MSFP (e.g. by increasing blood volume or decreasing capacity of the circulation) causes increase in VR. - Conversely, decreasing MSFP causes decrease in VR. Effect of changes in RVR on venous return curve - Decreasing RVR, the VR is increased. - Increasing RVR, the VR is decreased. - whatever the value of RVR, venous return will be zero when RAP equals MSFP  Interaction between Cardiac Factors and Peripheral Vascular Factors in the Control of Cardiac Output Under steady states, the heart and the peripheral circulation must operate together. This means that the following two conditions must be fulfilled: - Venous return must equal cardiac output. - The right atrial pressure is the same for the heart to pump its output, and for the circulation to bring venous return back to the heart. The only point that fulfills these two criteria is point A. At this point: o Venous return equals cardiac output, both equal 5 l/min. o Right atrial pressure is maintained at 0 mmHg. Effects of exercise on the interaction between CO and VR curves This shift the cardiac function curve up and to the left through the following effects: o Increasing heart rate o Increasing contractility. o Decreasing afterload by decreasing total peripheral resistance. o This alone can only increase CO a little (from point A to point B)  Cardiac stimulation in a normal heart has only a little effect on cardiac output if acting alone. However, if venous return is additionally increased, the cardiac output is able to increase to much higher levels. Effects of exercise on the interaction between CO and VR curves The venous return curve also shift up and to the right (meaning increased VR) and this allows the CO to increase to much higher level (point C) There are several reasons for the shift of venous return curve up and to the right and the increase in venous return during exercise: o Increased MSFP. o Increased muscle pumping action and thoracic and cardiac sucti mechanisms to venous return against gravity o Arteriolar dilation in contracting muscles allows rapid flow of blood from arterial to venous side, increasing VR. o Decreased RVR The effects of exercise on muscle blood flow Muscle blood flow can increase a maximum of about 25-fold during the most strenuous exercise. Almost one-half this increase in flow results from intramuscular vasodilation caused by the direct effects of increased muscle metabolism. The remaining increase results from the moderate increase in arterial blood pressure that occurs in exercise.