Cardiovascular: Lymphatics and Capillaries L5

Question 1

What is the main function of the capillaries?

Answer 1

To be the site of exchange of materials such as nutrients, waste, gases, fluids, between the blood and the tissues.

Question 2

What are the three characteristics of the capillaries to allow them to function?

Answer 2

1. Very thin walls.
2. Large cross-sectional area of capillary bed.
3. Slow and smooth blood flow.

Question 3

How are the capillaries arranged and how does this contribute to blood flow?

Answer 3

Arranged into capillary beds, a network with a large cross-sectional area (compared to arterioles) that leads to a blood flow that is smooth and slow, also enabling exchange. (Can be likened as a river flowing into a lake.)

Question 4

How is blood flow regulated at the capillaries, and how is blood able to bypass a capillary bed?

Answer 4

Blood flow is regulated by autonomic control of smooth muscle sphincters called precapillary sphincters. If these sphincters are completely closed, then blood can bypass the capillary bed, straight to a venule via a connection called an anastamosis.

Question 5

Does structure of capillaries vary? Why is this?

Answer 5

Structure of the capillaries vary depending on how rapid the rate of exchange between the blood and tissues must be, and how controlled this exchange must be.

Question 6

What are the three types of capillaries in order from least leaking to most leaking?

Answer 6

Continuous capillary, Fenestrated capillary, Sinusoidal capillary.

Question 7

Describe structural features of the continuous capillary.

Answer 7

These are the most common capillary and are found in skeletal and cardiac muscle, the lungs and connective tissue.
Diameter is 5-10µm, similar to that of a single red blood cell, so RBCs must travel in a single-file through capillaries.
Have a continuous layer of endothelial cells and therefore the least permeable of the 3 types.
The endothelial cells produce a thin layer of protein called the basement membrane which acts like a filter. Capillaries are bound together by tight junctions.

Question 8

Describe how exchange can occur in continuous capillaries.

Answer 8

1. Some substances can travel through the endothelial cells by passive diffusion.
2. Others (e.g proteins and lipids) can be transported through the endothelial cells in vesicles.
3. White blood cells of the immune system (and some molecules) are able to travel through the intercellular spaces between endothelial cells.

Question 9

Where are the fenestrated capillaries and how do they compare to continuous capillaries?

Answer 9

Fenestrated capillaries are found where there needs to be a lot of exchange of molecules between tissues and blood, for example in the intestines, endocrine glands, and the glomeruli of the kidney. These capillaries are much more permeable than continuous capillaries, the endothelial cells have small pores or ‘fenestrations’. These provide an additional pathway for molecules to be exchanged between the blood and tissues. Similar to continuous capillaries, the endothelial cells of fenestrated capillaries are supported by a thin basement membrane - this basement membrane also extends across fenestrations. They also have a similar diameter.

Question 10

Compare the structure of the sinusoidal capillaries to the other capillary types.

Answer 10

The sinusoidal capillaries are the most permeable of the 3 types. They have a larger diameter of 20-30µm. They are found in places like the bone marrow, liver, lymph nodes and spleen, where the greatest amount of exchange (involving substances such as red blood cells) must occur. The endothelial cells of sinusoidal capillaries have the larger fenestrations than fenestrated capillaries. The basement membrane is not continuous and does not extend across fenestrations, permeability increased.

Question 11

What is lymph?

Answer 11

90% of the cardiac output returns back to the heart via veins, 10% returns back via the lymphatics. The fluid that is not reabsorbed into the capillary and remains in the extracellular space is known as ‘lymph’.

Question 12

Name of the duct that most lymph drains into? What duct and what area is the minority to this?

Answer 12

Most lymph drains into the thoracic duct. Lymph from the right arm, right side of the chest, neck and head and lower left lobe of the lung, is drained by the right lymphatic duct.

Question 13

Name 3 roles of the lymph vascular system.

Answer 13

1. Drainage/recovery of excess fluid.
2. Transports immune cells and foreign material to lymph nodes.
3. Transports fats from intestine to the blood.

Question 14

Name 3 major components of the lymphatic system.

Answer 14

1. Lymphatic capillaries.
2. Lymphatic vessels.
3. Lymph nodes.

Question 15

What are the lymphatic capillaries?

Answer 15

Vessels that are closed at one end which drain lymph into lymph vessels.

Question 16

What are the lymphatic vessels?

Answer 16

Larger vessels designed for transporting lymph around the body and back into the blood vascular system. E.g Cisterna Chyli is the main lymph vessel that transports lymph and chyle (lymph and fats) from the abdomen. Valves are present.

Question 17

Name 3 collections of lymph nodes.

Answer 17

Cervical Nodes (around the neck). Axillary Nodes (around armpits). Inguinal Nodes (around groin).

Question 18

What are the walls of lymphatic capillaries made of?

Answer 18

Overlapping endothelial cells.

Question 19

Are lymphatic capillaries more permeable than blood capillaries?

Answer 19

Yes, the cytoplasm of the endothelial cells is thin and often do not have a basement membrane (if they do its a basic one).

Question 20

Where do lymphatic capillaries lead to?

Answer 20

Lymphatic capillaries form interconnected networks that drain lymph into lymphatic vessels.

Question 21

Lymphatic capillaries are ___-ending as they form the beginning of the lymph vascular system which drains lymph back into the blood rather than around the body in a closed circuit.

Answer 21

blind

Question 22

How does lymph move unidirectionally?

Answer 22

As lymphatic vessels get further away from the capillaries, they get larger and also have valves to prevent backflow of lymph and smooth muscle cells in their walls to help pump lymph onwards.