Lecture 7

The Lymphatic System

• There is more filtration than absorption along the length of a capillary.
• Excess fluid is returned to the circulatory system by the lymphatic system.

Components of the Lymphatic System

• Lymphatic system consists of:
  • Lymph nodes (small organs)
  • Lymphatic vessels (tubes through which lymph flows)

Lymphatic Capillaries

• Distinct from blood capillaries.
• Made of a single layer of endothelial cells on a basement membrane.
• Have large water-filled channels permeable to all interstitial fluid components, including proteins.
• Closed-ended (no tubes flow into them).
• Interstitial fluid enters via bulk flow.
• Lymphatic capillaries empty into lymph vessels.

Lymph Vessels

• Contain one-way valves to ensure lymph flows in one direction only: into the right atrium.
• Lymph passes through lymph nodes, which are part of the immune response.
• Extend into the interstitial space surrounding tissue cells.
• Thin walls to allow tissue fluid, or interstitial fluid, to enter the lymphatic capillaries.
• Once the tissue fluid enters the lymphatic capillaries, it is called lymph.

Mechanism of Blood Flow

• The lymph system returns interstitial fluid to the circulatory system, along with small amounts of plasma proteins that have escaped the blood vessel capillaries.
• Mechanisms contributing to lymph flow:
  • Lymph vessels beyond the lymphatic capillaries have smooth muscle:
    • Generates rhythmic contractions.
    • Responds to stretch.
    • Innervated by the sympathetic nervous system.
  • One-way valves.
  • Skeletal muscle contractions.
  • Respiratory pump.

Arterial Blood Pressure

Blood Pressure

• Blood pressure is determined by the volume of blood in the vessels and the compliance of a vessel.

Compliance

• Ability of a vessel to distend and increase volume with increasing transmural pressure (pressure inside the vessel minus the pressure outside the vessel)
• \text{Compliance} = \frac{\Delta \text{Volume}}{\Delta \text{Pressure}}
• The greater the compliance of a vessel, the more easily it can be stretched.

Systole

• During systole, the ventricle pumps blood into the adjacent artery.
• Approximately 1/3rd of the volume of blood ejected by the ventricle leaves the artery; the remainder of the stroke volume remains in the arteries during systole, distending the arterial walls and increasing the arterial pressure.
• When ventricular contraction ends, the stretched arterial walls recoil passively, and blood continues to be driven into the arterioles during diastole, even though no new blood is entering the artery from the ventricle.
• This passive recoil maintains perfusion through the capillaries while the ventricles are in diastole.
• As blood leaves the arteries, the arterial volume and pressure slowly decrease.
• The next ventricular contraction occurs while the artery walls are still stretched by the remaining blood, and this means that the arterial pressure does not decrease to zero.
• Large arteries (aorta) act as pressure reservoirs due to their elastic recoil and maintain blood flow while the ventricles relax.
• Arteries are compliant (but not as compliant as veins).

Arterial Blood Pressure

• Systolic pressure: the maximum arterial pressure reached during peak ventricular ejection.
• Diastolic pressure: the minimum arterial pressure reached just before ventricular ejection begins.
• Arterial pressure is generally recorded as systolic pressure divided by diastolic pressure.
• Pulse pressure: systolic pressure minus the diastolic pressure (\text{PP} = \text{SP} - \text{DP}).
• 120/80 mmHg is normal blood pressure.
• Hypertension: chronically increased arterial blood pressure (high blood pressure).
• Hypotension: abnormally low arterial blood pressure.

Mean Arterial Pressure (MAP)

• Blood pressure changes during the cardiac cycle:
  • Maximal during systole.
  • Minimal during diastole (pulsatile).
• \text{MAP} = \text{diastolic pressure} + \frac{\text{pulse pressure}}{3}
• MAP is the pressure driving blood into the tissues averaged over the cardiac cycle.
• At pressures below 60 mmHg, the tissue cells in the body will not get sufficient blood and the oxygen and nutrients they need.
• The blood pressure is pulsatile as the blood leaves the heart, increasing and decreasing during systole and diastole.
• The aorta is very compliant and dampens (or reduces) the pulsatile output of the left ventricle, thereby reducing the pulse pressure.
• As the distance from the heart increases, the pulse pressure decreases due to the cumulative effects of elastic rebound along the arterial system.
• At the level of the aorta, we see very large waves, but at the level of the arterioles, the waves become smaller and smaller until they disappear.
• The pressure surge from the ventricles is absorbed as it moves along the arterial system and eventually disappears at the level of the arterioles.
• No pressure oscillations are seen in the capillaries.
• MAP decreases as the distance from the heart increases.
• The largest drop in pressure occurs at the level of the arterioles.
• The large pressure drop is due to the high resistance of the arterioles.
• Arterioles are small diameter arteries and provide resistance to blood flow and can alter state of constriction of smooth muscle.

Factors Controlling Mean Arterial Pressure

Mean Arterial Pressure

• Maintaining sufficient mean arterial pressure is a prerequisite for ensuring adequate perfusion or blood flow to all our organs and tissues.
• MAP is maintained within a specific range
  • Too low a blood pressure would mean that tissues of the body would not receive sufficient oxygen and nutrients, and waste products would accumulate.
  • Chronically high blood pressure can cause damage to the arteries, the heart and kidneys, along with other organs in the body.
• Mean systemic arterial pressure (MAP) is the arithmetic product of two factors:
  • \text{MAP} = \text{CO} \times \text{TPR}
    • CO = cardiac output
    • TPR = total peripheral resistance
  • CO and TPR are the main factors used to calculate the mean arterial pressure.
• TPR is the combined resistance to flow of all the systemic blood vessels.
  • Note: Systemic blood vessels and not pulmonary vessels as the pulmonary circulation provides little resistance to flow.
  • Friction between the blood and the walls of the blood vessels produces resistance, which impedes blood flow.
  • Major site of resistance in the systemic circuit → arterioles.
  • Changes in TPR are due primarily to changes in the resistance of the arterioles.
  • TPR is determined primarily by total arteriolar resistance.
  • Blood flowing through arterioles experiences resistance as it contacts the arteriolar wall, as they have a small diameter, and the pressure drop through the arterioles will be great.

Cardiovascular Regulatory Mechanisms

Regulation of Mean Arterial Pressure

• Short-term regulation: seconds to hours
  • Baroreceptors reflexes modify the activity of autonomic nerves supplying the heart and blood vessels as well as changes in the secretion of hormones.
  • Adjusts cardiac output (CO) and total peripheral (TPR) resistance by ANS.
• Long-term regulation:
  • Adjust blood volume.
  • Restore normal salt and water balance through mechanisms that regulate urine output and thirst.

Arterial Baroreceptors

• Arterial baroreceptors are mechanoreceptors that detect changes in your blood pressure.
  • Carotid sinus and the aortic arch baroreceptors.
  • Respond to changes in MAP as well as changes in pulse pressure.
  • Respond to changes in pressure when the walls of the vessel stretch and relax.
  • Pulse pressure is the difference between the systolic and the diastolic pressures.
  • Afferent neurons travel from the baroreceptors to the brainstem and provide input to the neurons of the cardiovascular control center.

Baroreceptor Action Potential Frequency

• The rate of discharge of the carotid sinus baroreceptor is directly proportional to the MAP.
• The baroreceptors continuously generate action potentials in response to ongoing pressure in the arteries.
• An increase in MAP will increase the frequency of action potentials generated by the baroreceptors.
• A decrease in MAP will decrease the frequency of firing of the baroreceptors.
• An increase in pulse pressure can occur with the calculated MAP still being normal.
• An increase in pulse pressure will cause an increase in the overall action potential frequency.

The Medullary Cardiovascular Center

• Located in the medulla oblongata in the brainstem.
• The neurons in this center receive input from the baroreceptors which determines the frequency of action potentials sent from the medullary cardiovascular center to alter vagal stimulation (parasympathetic) to heart and sympathetic innervation to heart, arterioles and veins.
• Increase in arterial pressure increases the rate of firing of the arterial baroreceptors signals medullary cardiovascular center to decrease sympathetic activity to the heart, arterioles and veins, and increase parasympathetic neuron activity to the heart to decrease the arterial pressure.
• Baroreceptors adapt to sustained changes in arterial pressure.
• Only used for short term regulation of blood pressure.

Other Reflexes

• Chemoreceptors primarily function to regulate respiratory activity.
• Aortic and carotid bodies