Question

Each student must pick two of the questions below.

Answers should be 5-7 sentences or 2 paragraphs

1. Name or describe a condition that might increase the viscosity of blood and how this would affect blood pressure, blood flow and resistance.
2. List 2 conditions that would cause and increase in total vessel length increase and how this would affect blood pressure, lood flow and resistance ______________
3. Distinguish between continuous capillaries, sinusoid capillaries and fenestrated capillaries. Where is each found
4. Why do medium sized veins have valves where arteries of the same size lack valves?

Answer 1

1. Name or describe a condition that might increase the viscosity of blood and how this would affect blood pressure, blood flow and resistance.

Factors which can increase the blood viscosity are:

• Increase in red cell mass
• Increased red cell deformity
• Increased level of fibrinogen and coagulation factors in plasma
• Dehydration
• Decreased shear rate with increased vessel diameter
• Increased red cell aggregability
• Flow condition

Increased viscosity impose more work to the heart by increasing the resistance to blood flow. At constant systolic BP, increased viscosity causes increase in total peripheral resistance TPR which would result in reduced blood flow.

2. List 2 conditions that would cause and increase in total vessel length increase and how this would affect blood pressure, blood flow and resistance

The length of the blood vessels increases throughout childhood but remains constant in adults under normal physiological condition. Anyways, not all tissues have same length of vessel. Adipose tissues have half the length of blood vessel as compared to skeletal muscles. There is a linear relationship between length of the vessel and resistance to the blood flow. Longer the vessel, greater the resistance. Length of the vessel indirectly proportional to blood flow; this is due to the fact that lengthy vessel impart greater resistance to the flow which decreases the blood flow. This can be explained by the Poiseuille’s equation:

Blood flow = π ΔP r⁴ / 8ηλ

Where, ΔP - Pressure difference

r= radius of vessel

η = Viscosity of blood

λ = length of blood vessel.

3.Distinguish between continuous capillaries, sinusoid capillaries and fenestrated capillaries. Where is each found

The most abundant form of blood vessels are capillaries, are smallest which can penetrate tissues and enable exchange of gases, nutrients and waste products by connecting arterioles to venules. The process involved are passive diffusion and pinocytosis. They usually ranges from 3 to 4 µm and can be upto 40 µm at times. They are composed of tunica interna (epithelial cells) encircled by pericytes provided with precapillary sphincters which aids in regulating blood flow to capillaries. Capillaries are of three types:

• Continuous
• Fenestrated and
• Sinusoidal.

Continuous

It is composed of continuous endothelial lining with tight junctions between their endothelial cells. They are provided with intercellular clefts which enable the passage of small molecules, like ions. They are predominant in nervous system, fat and muscle tissues. The continuous endothelial cells serves the nervous tissue by forming BBB blood –brain-barrier. They serves brain protection from large molecules by restricting the movement of cells and larger molecules between blood and interstitial fluid in the brain.

Fenestrated

Fenestrated capillaries are predominantly found in kidney, intestine and endocrine system where most of the molecular exchange occurs. More particularly in the glomeruli of the kidneys, as they are involved in filtration of the blood during the formation of urine. They are facilitated with minute opening of 80 to 100 nm in diameter called fenestra provided with diaphragm like structure. It has permeable membrane and spanned with fibrils which enable the quick movement of macromolecules in and out of the capillary.

Sinusoidal

They are also called as discontinuous capillaries or sinusoids, have endothelial linings. They have enormous openings and of around 30 to 40 nm in diameter. They are either a discontinuous or non-existent basal lamina which allows passage of blood cells and serum proteins. They are abundant in the liver, the sinusoids of the spleen, lymph nodes, bone marrow and few glands of the endocrine system.

4. Why do medium sized veins have valves where arteries of the same size lack valves?

Most veins are equipped with valves to ensure blood flow in one direction; to prevent reverse flow of blood. Arteries does not have valves because of the higher pressure from heart cause to blood to flow in one direction. Veins have much lower blood pressure than arteries as it carries blood from various parts and returning to heart has a risk of back flow. To prevent this back flow, veins have valves.