Capillary and Lymphatic System

Capillary dynamics

- exchange of gases and nutrients through diffusion

- as diffusion occurs, bulk fluid flows also occur

hydrostatic pressure (HP) – force exerted by a fluid pressing against a wall.

In capillaries, HPC = capillary Bp = filtration pressure

HPC greater at the arterial end (35mmHg) then at the venous end (16 mmHg)

HPC is opposed by the hydrostatic pressure of the interstitial fluid; however,

HPIF = 0mmHg (because of excess fluid being reclaimed by the lymphatic vessels)

Osmotic pressure (OP) – created by the presence of large, non-diffusible molecules (plasma proteins) in a fluid. These molecules draw water towards them.

OPC = 26mmHg; OPIF = 5mmHg

SEE Figure (PowerPoint) with calculations

[Recall that:

At the arterial end of the capillary, fluid leaves the capillary because Net HP > Net OP

(greater than)

In our example, pressure differential of 14mmHg

At the venous end of the capillary, some fluid is reclaimed by capillary because

Net OP > Net HP

(greater than)

In our example, pressure differential of 5mmHg]

Net fluid loss of 1.5mL/min

The excess fluid (now interstitial fluid) and any leaked proteins will be picked up by the lymphatic system.

The lymphatic system will return fluid and proteins to the vascular system (blood)

- if not, the body would be depleted of plasma in a day!

The Lymphatic System

I. Components and functions

Lymphatic vessels – transport excess fluid from interstitial spaces and return the fluid to the venous system

Lymphoid tissues and organs- phagocytic cells (lymphocytes) “cleanse” lymph – body defense

II. Lymphatic pathways

- fluid leaves the arterial end of a capillary, most is reclaimed at the venule end of capillary

- excess fluid remaining in the interstitial space causes a temporary increase in the HPIF

- this increase in HPIF coupled with the design of lymphatic capillaries allows fluid to enter the lymphatic capillaries.

Lymphatic capillaries – blind-ended (open at one end) – fluid moves in one direction, backflow is prevented by the presence of valves

Lymph capillaries occur almost everywhere blood capillaries occur except bones, teeth, bone marrow, and central nervous system.

Once fluid enters lymphatic capillaries it is called lymph.

Lacteals: specialized lymphatic capillaries located in the villi (fingerlike projections) of the intestinal mucosa. This fatty lymph is called chyle (whitish in coloration)

Pathway: lymphatic capillaries – afferent lymphatic vessels – lymphatic organs (nodes) – efferent lymphatic vessels – lymphatic trunks – two lymphatic ducts (right lymphatic duct and thoracic duct)

The thoracic duct begins as the enlarged sac called the cisterna chyli – which collects lymph from the lumbar trunks (legs) and intestinal trunk (digestive organs)

The thoracic duct collects lymph from the left side of head, neck, and thorax (including left arm) and the abdomen and legs. The thoracic duct empties the lymph into the venous system at the juncture of the left subclavian vein and left internal jugular vein.

The right lymphatic duct collects lymph from the right side of head, neck, and thorax (including right arm). The right lymphatic duct empties the lymph into the venous system at the juncture of the right subclavian vein and right internal jugular vein.

III. Lymph Transport

Pumpless – low pressure conditions, very (3L/Day)

Movement controlled by muscle contractions

1. skeletal muscle contraction

2. actions of breathing

3. contraction of smooth muscle in lymph vessel wall

(4.) valves – prevent back flow

-when physical activity increases, lymph flow increases, this is why is it important to immobilize an infected area.

IV. Cells, tissues, and organs

A. cells: Lymphocytes: T cells and B cells

Activated T cells - manage the immune response and some directly attack antigens.

Foreign Antigens - non-self antigens (body perceives as foreign: bacteria- toxins,

viruses, cancer cells, mismatched RBCs)

B cells – produce plasma cells, which produce antibodies. Antibodies help to immobilize antigens until they can be destroyed.

B. lymphoid tissue - reticular connective tissue. Macrophages live on fibers, lymphocytes in clusters & spaces (sinuses)

C. lymphoid organs

1. lymph nodes - clustered along lymph vessels

- Basic function: filter lymph; have a role in monitoring the presence of antigens

and activating the immune system-

Note: more afferent lymphatic vessels entering lymph node than efferent lymphatic

vessels exiting lymph node. Why?

Where are clusters of lymph nodes located in the body? Why?

Tonsils: partially encapsulated lymph nodules located in the pharynx

Peyer’s patches: aggregations of nodules scattered throughout the ileum of the small

Intestine

Appendix

(Note MALT Powerpoint slide)

2. Thymus

- located in thorax, behind the sternum. Large during infancy, grows through childhood

and after adolescence becomes smaller with age (replaced by connective tissue)

- subdivided into lobules

- contains numerous lymphocytes

- secretes thymosin – hormone that stimulates T cells after leaving thymus

- differentiation of lymphocytes in T cells

3. Spleen

- largest lymphatic organ, filters BLOOD

- located on left side, abdominal cavity

- also RBC graveyard and site of RBC production in a fetus

- white pulp: splenic nodules, large number of lymphocytes

- red pulp: spaces of lobules, contains many RBCs as well as lymphocytes & macrophages