Physiology
Lec: 6 د. زيـد الاطرقجيCardiovascular system
Circulation
The blood is pumped by the left side of the heart into the systemic circulation which is distributed in various proportions to the systemic organs through a parallel arrangement of vessels that braches from the aorta. This arrangement ensures that all organs receive blood of the same composition. That is any organ does not receive leftover blood that has passed through another organ.
Because of this parallel arrangement blood flow through each systemic organ can be independently adjusted as needed.
Blood is constantly reconditioned so that its composition remains relatively constant despite an ongoing drain of supplies to support metabolic activities and despite the continual addition of wastes from the tissues.
Organ that recondition the blood normally receive much more blood than is necessary to meet their basic metabolic needs so that can adjust the extra blood to achieve homeostasis, for example large percentage of the cardiac output are distributed to:
1. The digestive tract to pick up nutrient supplies.
2. To the kidneys for elimination of metabolic waste and adjust water and electrolyte composition.
3. To the skin to control body heat.
Blood flow to the other organs e.g heart, Skeletal muscles, brain and so on is solely for filling these organs metabolic needs and can be adjusted according to their level of activity. For example during exercise additional blood is delivered to the active muscles to meet their increased metabolic needs.
Because reconditioning organs [digestive organ, kidneys and skin] receive blood flow in excess of their own needs. They can withstand temporary reduction in blood much better than can other organs that do not have this extra margin of blood supply. The brain in particular suffers irreparable damage when transiently deprived of blood supply.
After only four minutes without O2 permanent brain damage occurs, this high priority in the overall operation of the circulatory system is the constant delivery of adequate blood to the brain which can least tolerate disrupted blood supply.
In contrast the reconditioning organs can tolerate significant reductions in blood flow for quite a long time and often do for example
During exercise some of the blood that normally flows through the digestive organs and kidneys is diverted to the skeletal muscles. Like wise to conserve body heat blood flow through the skin is markedly restricted during exposure to cold.
Blood flow
Flow rate of blood through a vessel = the volume of blood passing through it per minute.
Flow rate is directly proportional to the pressure gradient and inversely proportional to vascular resistance.
F=P\R
P is the pressure gradient which is the difference in pressure between the beginning and end of a vessel. Blood flow from area of higher pressure to an area of lower pressure down a pressure gradient.
Contraction of the heart imparts pressure to the blood which is the main driving force for flow through a vessel. Contraction of the ventricle produce pressure which drive blood to the tissue through the circulation. The greater the pressure gradient forcing blood through a vessel the greater is the flow rate through that vessel. It is the difference in pressure between the two ends of a vessel not the absolute pressure within the vessel determines flow rate.
Resistance: R
Is the measurement of the opposition to blood flow through a vessel caused by friction between the moving fluid and the stationary vascular walls. As resistance to flow increases it is more difficult for blood to pass through the vessel so flow rate decreases [as long as the pressure gradient remains unchanged]. When resistance increases the pressure gradient must increase correspondingly to maintain the same flow rate, accordingly when the vessels offer more resistance to flow, the heart must work harder to maintain adequate circulation.
Resistance to blood flow depends on three factors:
1. Viscosity of the blood.
2. Vessel length.
3. Vessel radius.
Viscosity refers to the friction developed between the molecules of fluid as they slide over each other during flow of the fluid. The greater the viscosity the greater is the resistance to flow. In general the thicker a liquid the more viscous it is. Blood viscosity is determined primarily by the number of circulating red blood cells when excessive red blood cells are present blood flow are more sluggish than normal. Because blood rubs against the lining of the vessels as it flows.
The greater the vessel surface area in contact with the blood the greater the resistance to flow.
Surface area is determined by both the length [L] and radius of the vessel.
The major determinant of resistance to flow is the vessel is radius. Fluid passes more readily through a large vessel than through a smaller vessel. The reason is that a given volume of blood come in contact with much more of the surface area of a small-radius vessel than of a large radius vessel resulting in greater resistance.
Resistance is inversely proportional to the fourth power of the radius [multiplying the radius by itself four times]. Thus doubling the radius reduces the resistance to 1\16 its original value and therefore increases flow through the vessel 16 fold [at the same pressure gradient]. The converse is also true only 1\16th as much blood flows through a vessel at the same driving pressure when its radius is halved.
Importantly the radius of arterioles can be regulated and is the most important factor in controlling resistance to blood flow throughout the vascular circuit.
The entire purpose of the circulatory system is for capillary exchange. All other activities of the system are directed toward ensuring an adequate distribution of replenished blood to capillaries for exchange with all cells
The arterioles, capillaries, and venules are collectively referred to as the micro-circulation because they are visible through a microscope [the micro-circulatory vessels are all located within the organs].
Arteries serve as:
1. Rapid transit passage ways to the organ.
2. pressure- reservoir.
Arteries are specialized to serve as rapid transit passageways for blood from the heart to the organs because of their large radius. Arteries offer little resistance to blood flow and act as pressure reservoir to provide the driving force for blood when the heart is relaxing.
All vessels are lined with a thin layer of smooth flat endothelial cells that are continuous with the endothelial lining of the heart [endocardium].
A thick wall made up of smooth muscle and connective tissue surrounds the arteries endothelial lining.
Arterial connective contains two types of connective tissue fibers:
1. Collagen fibers: which provide tensile strength against the high driving pressure of blood ejected from the heart.
2. Elastin fibers which give the arterial walls elasticity so that they behave much like a balloon.
As the heart pumps blood into arteries during ventricular systole a greater volume of blood enters the arteries from the heart than leaves them to flow into smaller vessels which have a greater resistance to flow. The arteries elasticity enable them to expand to temporarily hold this excess volume of ejected blood , storing some of the pressure energy imparted by cardiac contraction in their stretched walls just as a balloon expands to accommodate the extra volume of air you blow into .
When the ventricle relaxes and ceases pumping blood into the arteries, the stretched arterial walls passively recoil like an inflated balloon that is released. This recoil pushes the excess blood contained in the arteries into the vessels downstream ensuring continued blood flow to the organs when the heart is relaxing and not pumping blood into the system. The heart alternately contract to pump blood into the arteries and then relaxes to refill from the veins. When the heart is relaxing and refilling no blood is pumped out. However capillary flow does not fluctuate between cardiac systole and diastole that is blood flow is continuous through the capillaries supplying the organs.
The driving force for the continued flow of blood to the organs during cardiac relaxation is provided by the elastic properties of the arterial walls.
Arterioles:
Arterioles are the major resistance vessels in the vascular tree because their radius is small enough to offer considerable resistance to flow. In contrast to the low resistance of the arteries. The high degree of arteriolar resistance causes a marked drop in mean pressure as blood flows through these small vessels. On average the pressure falls from 93mmHg [the mean arterial blood pressure entering the arterioles] to 37mmHg the pressure of blood leaving the arterioles and entering the capillaries. This decline in pressure helps establish the pressure differential that encourages the flow of blood from the heart to the various organs downstream.
The radius and accordingly the resistance of arterioles supplying individual organ can be adjusted independently to accomplish two facts:
1. To variably distribute the cardiac output among the systemic organs depending on the body's momentary needs.
2. To help regulate arterial blood pressure.
Arteriolar walls contain very little elastic connective tissue. However they do have a thicker layer of smooth muscle that is richly innervated by sympathetic nerve fibers , the smooth muscle is also sensitive to many local chemical changes and to a few circulating hormones .
The driving force [pressure] for flow is identical for each organ. Therefore differences in flow to various organs are completely determined by differences in the vascularization and by differences in resistance offered by the arterioles supplying each organ. On a moment to moment basis the distribution of cardiac output can be varied by differentially adjusting arteriolar resistance in the vascular beds.
Active- hyperemia: it is an abnormal need for blood. When muscle cells are more active metabolically they need more blood to bring in 02 and nutrients and to remove metabolic wastes. The increased blood flow meets these increased local needs. Conversely when muscle is in relaxed state the muscle cells need less blood and therefore there will local arteriolar vasoconstriction and a subsequent reduction in blood flow to the area.
Local metabolic changes can thus adjust blood flow as needed without involving nerves or hormones.
Local chemical factors which produce relaxation of arterioles:
1. Decrease local 02 concentration.
2. Increase local CO2 concentration.
3. Increase local H+ concentration.
4. Increase local K+ concentration.
5. Increase osmolarity.
6. Increased adenosine release.
7. Increased prostaglandin release.
Function of endothelial cells:
1. Line the blood vessels and heart chambers; serve as a physical barrier between the blood and the remainder of the vessel wall.
2. Secrete vaso-active substances in response to local chemical and physical changes, these substances cause relaxation [vasodilatation]or contraction[vasoconstriction]of the underlying muscle.
3. Secrete substances that stimulate new vessel growth and proliferation of smooth muscle cells in vessel walls.
4. Participate in the exchange of materials between the blood and surrounding tissue cells across capillaries through vesicular transport.
5. Influence formation of platelet plugs, clotting and clot dissolution.
6. Participate in the determination of capillary permeability by contracting to vary the size of the pores between adjacent endothelial cells.