ZOO502 Final Term Merged PPT Handouts Topics 133-265

Topic-133 · Viscosity of Blood · Viscosity of Blood

• Plasma has a viscosity of about 1.8 relative to water.
• Viscosity increases with the addition of blood cells.
• Relative viscosity of mammalian and bird blood at 37°C is between 3 and 4. Thus, blood is three or four times more viscous than water.

Effects of Blood Viscosity on Flow

• According to Poiseuille's law, the flow of blood is inversely related to its viscosity.
• Due to high viscosity, larger pressure gradients are required to maintain flow of blood through vascular system.

Blood Viscosity and Flow in Arterioles

Blood flowing through small vessels (arterioles) behaves as if its relative viscosity is much reduced.

In vessels less than 0.3 mm in diameter, the apparent relative viscosity of blood approaches the viscosity of plasma i.e. 1.8 only.

It happens due to plasma skimming i.e.

Plasma Skimming

RBCs accumulate in the center of blood vessel during flow and the walls remain relatively free of cells. This causes difference in viscosity between the center and the walls and results in increase in velocity of blood flow.

Fahraeus-Lindqvist Effect

This phenomenon is called the Fahraeus-Lindqvist effect.

Phenomenon of plasma skimming and resultant increased blood flow velocity is called FahraeusLindqvist effect

It reduces the energy required to drive blood through microcirculation.

Blood Viscosity and Flow in Capillaries

In very small vessels (diameters 5 to 7 µm) Fahraeus-Lindqvist effect is inverted i.e. apparent viscosity of blood increases. In these vessels, red blood cells move in a queue, pressing against the walls. So blood flow is slowest in capillaries.

Topic-134 · Compliance in Circulatory System · Compliance

• Blood vessels contain elastic fibers that enable them to distend. As the pressure in a vessel increases, its walls are stretched and the volume of the vessel increases.
• The ratio of change in volume to change in pressure is termed as compliance of the system.
• The compliance of a system is related to its size and elasticity of its walls.
• The greater the initial volume and elasticity of the walls, the greater is the compliance of a system.

Compliance in Venous System

• Venous system is very compliant because a small change in pressure produces large change in volume. Similarly large changes in volume have little effect on venous pressure.

Effect of Compliance in Venous System

• Due to greater compliance, venous system acts as a volume reservoir without causing a significant change in blood pressure.

Compliance in Arterial System

• Arterial system is less compliant than the venous system. It acts as a pressure reservoir in order to maintain capillary blood flow.
• Some portions of arterial system are more elastic e.g. near the heart. They help to reduce the oscillations in pressure generated by contractions of the heart and to maintain flow in distal arteries during diastole.

Topic-135 · Peripheral Circulation · Vascular Pathways

Cardiovascular system includes two pathways of blood circulation:

• Pulmonary circuit that circulates blood through lungs
• Systemic or peripheral circuit

Peripheral Circulation

The pathway followed by blood pumped from the left ventricle of the mammalian heart till its return to the right atrium is known as peripheral or systemic circulation.

Pathway of Peripheral Circulation

• The oxygenated blood is pumped by the left ventricle into the aorta which is the largest artery in the peripheral circulation.
• Branches from aorta form the arterial system and supply blood to the organs and major body regions.
• Arteries branch to form capillary beds in the tissues where oxygen is exchanged for carbon dioxide.
• The deoxygenated blood returns through the venous system to the superior and inferior vena cavae which open into the right atrium.

Blood Vessels

Three types of blood vessels carry blood during circulation:

• Arteries which carry blood away from the heart to organs and tissues.
• Veins which return blood to the heart.
• Capillaries which have thin walls formed of only a single layer of endothelial cells and permit exchange of materials between blood and tissues.

Topic-136

Blood Pressure

Contraction of the ventricle of heart generates blood pressure. This pressure causes the blood to flow in the arteries away from the heart from higher pressure to lower pressure areas.

Changes in Blood Pressure during Circulation

The pressure is highest in the aorta and decreases during circulation through tiny arterioles and capillaries which offer substantial resistance to flow due to narrow diameters. Blood pressure is almost nonexistent in the venous system.

Changes in Blood Pressure With Cardiac Cycle

Arterial blood pressure is highest during ventricular systole. The pressure at this time is called systolic pressure.

This pressure causes the arteries to stretch.

During diastole, when the ventricles are relaxed, pressure drops. However, a lower pressure (diastolic pressure) is maintained in the arteries due to contraction of elastic walls of arteries.

Regulation of Blood Pressure

Blood pressure fluctuates with vasoconstriction and vasodilation due to contraction or relaxation of smooth muscles in arteriole walls.

Vasoconstriction narrows the arterioles and increases the blood pressure while vasodilation causes an increase in diameter of arterioles that causes blood pressure to fall.

Contraction of smooth muscles of arteriole walls is regulated through chemical, nervous and hormonal controls. These controls are also affected by physical or emotional states e.g. exercise and stress

Chemical Control of Blood Pressure

• Hormones that affect body state during stress also act to cause vasodilation or vasoconstriction and, thereby, regulate blood pressure. These hormones include cortisol, epinephrine and norepinephrine.
• The renin‐angiotensin‐aldosterone system of the kidneys that regulates blood volume also regulates blood pressure.
• Nitric oxide (NO) serves as a major inducer of vasodilation in the cardiovascular system.
• A peptide, endothelin is a potent inducer of vasoconstriction.

Topic-137 · Effect of Gravity on Pressure and Flow · Effect of Gravity on Arterial blood Pressure

Gravity affects the column of blood in larger arteries. So pressure in limbs and head region varies in different positions of the body.

In lying down position, the heart is at the same level as the limbs and head with respect to gravity. So, blood pressure in the arteries of head and limbs is equal.

In a standing position, the relationship between the head, heart, and limbs changes with respect to gravity.

In this position heart is below the head and about a meter above the lower limbs. So, the arterial pressure in lower limbs increases while in the head region, it decreases.

Effect of Gravity on Venous Flow

Because the vascular system is elastic, an increase in absolute pressure expands blood vessels, particularly the veins which are more compliant.

Thus, pooling of blood tends to occur in veins, in different regions of the body.

Pooling of Blood

Pooling of blood causes difficulty in maintaining capillary flow in those regions that may have severe physiological consequences e.g. change in kidney filtration rates.