Physiological Basis of Tissue Fluid Formation and Oedema

Flashcards covering fluid compartments, HP and OP dynamics, and the physiological mechanisms of oedema.

What is the total body water volume and its percentage of body weight?

$40\,L$ and $60\%$ of body weight.

How is the Extracellular fluid (ECF) volume distributed between interstitial fluid and plasma?

Interstitial fluid accounts for $12\,L$ ($80\%$ of ECF) and plasma accounts for $3\,L$ ($20\%$ of ECF).

What are the two major components of blood?

Formed elements (RBCs, WBCs, and platelets) and plasma, which is $90\%$ water.

What creates osmotic pressure (OP) in the blood and where are these substances produced?

Osmotic pressure is created by plasma proteins that cannot move through the capillary membrane; these proteins are produced in the liver.

What is the average osmotic pressure generated by plasma proteins?

Between $25\,mmHg$ and $28\,mmHg$.

Define hydrostatic pressure (HP) in the context of blood vessels.

The pushing force exerted by a fluid, such as blood, against the wall of a blood vessel.

What are the specific hydrostatic pressures at the arterial and venous ends of a capillary?

Arterial pressure is $35\,mmHg$ and venous pressure is $17\,mmHg$.

Why does fluid move out of the capillary at the arterial end?

Because the hydrostatic pressure ($35\,mmHg$) is greater than the osmotic pressure ($25\,mmHg$).

Why is fluid reabsorbed at the venous end of the capillary?

Because the osmotic pressure ($25\,mmHg$) is greater than the hydrostatic pressure ($17\,mmHg$).

What is the Net Filtration Pressure (NFP) at the arterial and venous ends respectively?

At the arterial end, NFP is $10\,mmHg$; at the venous end, NFP (for reabsorption) is $8\,mmHg$.

Define oedema.

An atypical accumulation of fluid in the interstitial spaces, leading to palpable swelling.

At what point does interstitial fluid accumulation typically become obvious as oedema?

When the interstitial volume has increased by $2-3\,L$.

What are the four primary factors that cause the formation of oedema?

1. Decreased capillary osmotic pressure; 2. Increased capillary permeability; 3. Increased capillary hydrostatic pressure; 4. Obstruction of the lymphatic system.

What are the common causes of decreased capillary osmotic pressure?

Inadequate production of plasma proteins (e.g., liver disease/failure, malnutrition) or abnormal loss of proteins like albumin (e.g., glomerulonephritis).

Why is oedema from decreased osmotic pressure not gravity dependent?

Because plasma proteins are evenly distributed throughout the blood, allowing oedema to occur in areas like the face or abdomen.

What occurs during increased capillary permeability to cause oedema?

Capillary pores enlarge or the wall is damaged, allowing plasma proteins to leak out, which increases tissue osmotic pressure.

What are common causes of increased capillary permeability?

Burns, immune responses (e.g., allergic reactions), and inflammation.

Which factors can lead to increased capillary hydrostatic pressure?

Increased arterial pressure (hypertension), capillary distension from increased vascular volume (cardiac failure, pregnancy, salt and water retention), or increased venous pressure (liver disease, thrombosis).

Where is oedema typically located when caused by increased capillary hydrostatic pressure?

It is often gravity dependent, appearing in the legs and feet (peripheral oedema).

Which parasitic worms are associated with lymphatic obstruction leading to elephantiasis?

Wuchereria bancrofti and Brugia malayi.

List five clinical or environmental causes of lymphatic obstruction.

Injury, radiation therapy, skin infections (such as cellulitis), surgery, and tumours.