Capillaries and Microcirculation Quiz

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Questions and Answers

What is the main characteristic of continuous capillaries?

They have large fenestrations for filtration.

They are wider with many clefts.

They consist of a tube of endothelial cells with intercellular clefts. (correct)

They allow free passage of plasma proteins.

Which method of capillary exchange involves substances moving down their concentration gradient?

Bulk flow

Diffusion (correct)

Active transport

Transcytosis

What type of capillary is characterized by having large fenestrations?

Continuous capillaries

Arterioles

Sinusoids

Fenestrated capillaries (correct)

What allows transcytosis to occur in capillaries?

Formation of vesicles around large molecules Signup and view all the answers

Which method of capillary exchange allows large, lipid-insoluble molecules to cross the endothelium?

Transcytosis Signup and view all the answers

What type of capillary allows smaller plasma proteins to pass but restricts most from crossing?

Sinusoids Signup and view all the answers

What is the main purpose of bulk flow in the body?

To regulate volumes of blood and interstitial fluid Signup and view all the answers

What pressure is primarily responsible for promoting reabsorption at the venous end of a capillary?

Blood colloid osmotic pressure (BCOP) Signup and view all the answers

At the arterial end of a capillary, what is the net filtration pressure (NFP)?

10 mmHg Signup and view all the answers

Which pressure is close to zero and affects filtration and reabsorption in capillaries?

Interstitial fluid hydrostatic pressure (IFHP) Signup and view all the answers

Which factor contributes minimally to interstitial fluid osmotic pressure (IFOP)?

Tiny amounts of proteins Signup and view all the answers

What defines the net filtration pressure (NFP) formula?

NFP = (BHP + IFOP) - (BCOP + IFHP) Signup and view all the answers

What is the typical range of blood hydrostatic pressure (BHP) across a capillary bed?

16 to 35 mmHg Signup and view all the answers

What is the significance of blood colloid osmotic pressure (BCOP)?

It draws water back into the capillaries from interstitial fluid. Signup and view all the answers

What happens to thoracic pressure during inhalation?

It decreases Signup and view all the answers

What is the formula for calculating cardiac output (CO)?

CO = heart rate x stroke volume Signup and view all the answers

Which factor does NOT affect blood pressure?

Body temperature Signup and view all the answers

What is the minimum mean arterial pressure (MAP) required to adequately perfuse vital organs?

60 mmHg Signup and view all the answers

Which of the following factors contributes to vascular resistance?

Size of blood vessel lumen Signup and view all the answers

Where is the velocity of blood flow slowest?

Capillaries Signup and view all the answers

What is the primary reason for the presence of venous valves?

To prevent backflow of blood Signup and view all the answers

What percentage of fluid filtered through tissues is typically reabsorbed?

85% Signup and view all the answers

What characteristic of venules allows for the exchange of elements?

Porosity of postcapillary venules Signup and view all the answers

Which statement best describes the structural adaptations of veins?

They have a large lumen and low resistance. Signup and view all the answers

What condition is most likely to result when filtration exceeds reabsorption?

Oedema Signup and view all the answers

Which type of vein is more numerous and located within the subcutaneous layer?

Superficial veins Signup and view all the answers

What is the primary role of the lymphatic system in relation to fluid balance?

To filter excess fluid and return it to the bloodstream Signup and view all the answers

What structural feature of veins contributes to their function as blood reservoirs?

Thin walls with large lumen Signup and view all the answers

What volume of blood can veins accommodate, representing their capacity as blood reservoirs?

65% Signup and view all the answers

As venules move away from capillaries, what structural change occurs?

They increase in diameter and smooth muscle content. Signup and view all the answers

Study Notes

### Capillaries

Smallest blood vessels
Diameter between 5–10µm
U-shaped vessels connect arterial and venous blood flow
Thin walls with no tunica media or externa
Extensive network of branched vessels throughout the body ### Microcirculation
Blood flow through metarterioles, capillaries, and venules
Single metarteriole supplies a capillary bed
Thoroughfare channel: route from arteriole to venule ### Types of Capillaries
Continuous Capillaries: Tube of endothelial cells with intercellular clefts
Fenestrated Capillaries: Contain pores known as fenestrations
Sinusoids: Wider capillaries with large fenestrations and clefts ### Capillary Exchange
Movement of substances between blood and interstitial fluid
Diffusion: Most important exchange method, substances move down their concentration gradient. Water-soluble substances move through intercellular clefts or fenestrations. Lipid-soluble substances move through endothelial cells.
Transcytosis: Small amount of material from blood plasma become enclosed in vesicles and transported across the cell. Mainly large, lipid-insoluble molecules.
Bulk Flow: Based on pressure gradients, promotes movements of large volumes of fluid. Important for regulating blood and interstitial fluid volumes.
- Filtration: Movement from capillaries into interstitial fluid.
- Reabsorption: Movement from interstitial fluid into capillaries. ### Pressures Regulating Filtration and Reabsorption
Blood Hydrostatic Pressure (BHP): Pressure of water in the plasma against vessel walls; ranges from 35 to 16 mmHg.
Interstitial Fluid Hydrostatic Pressure (IFHP): Close to 0 mmHg.
Interstitial Fluid Osmotic Pressure (IFOP): 0.1-5mmHg, small amount of proteins in interstitial fluid.
Blood Colloid Osmotic Pressure (BCOP): Due to the presence of plasma proteins which are too big to cross the capillary walls. Averages 26 mmHg. ### Net Filtration Pressure (NFP)
The balance of pressures promoting filtration and reabsorption.
Calculated using the formula: (BHP + IFOP) – (BCOP + IFHP).
- Arterial End of Capillary: NFP = 10 mmHg, promoting filtration.
- Venous End of Capillary: NFP = -9mmHg, promoting reabsorption. ### Starling’s Law
Nearly as much fluid is reabsorbed as filtered.
About 85% of filtered fluid is reabsorbed.
20 liters of fluid are filtered daily, 17 liters are reabsorbed, 3 liters enter the lymphatic system and return to the blood. ### Edema
Filtration exceeds reabsorption.
Increase in interstitial fluid volume. ### Venules
Thin walls that collect capillary blood.
Postcapillary venules: (10–50 µm) porous and allow for exchange.
As venules become thicker with more smooth muscle (50–200 µm), exchange becomes less likely.
Very elastic walls make them a good reservoir for blood. ### Veins
Diameter from 0.5 mm to 3 cm.
Thinner walls than arteries, larger lumen, less muscle.
Thick tunica externa with collagen, elastic fibers, and lack of internal and external elastic laminae.
Larger lumen, can distend well but cannot withstand high pressure.
Superficial veins: Subcutaneous layer.
Deep veins: Between skeletal muscles.
Anastomotic veins: Connections between veins, allows blood from capillaries to flow to the heart even if the primary vein is obstructed. ### Veins and Valves
Can accommodate large volumes of blood.
Hold up to 65% of blood volume.
Low blood pressure due to larger lumen.
Adaptations to ensure blood returns to the heart:
- Large lumen: Low resistance
- Venus valves: Prevents backflow. Formed from tunica intima, resembles semi-lunar valves of the heart. ### Respiratory Pump
Pressure changes in the thoracic and abdominal cavities aid blood return.
- Inhalation: Diaphragm moves down, decreasing thoracic pressure and increasing abdominal pressure.
- Blood flows from abdominal veins to thoracic veins.
- Exhalation: Valves prevent backflow. ### Hemodynamics: Blood Flow
Volume of blood through a given tissue in a given time period, measured in mL/min.
Total blood flow is equivalent to cardiac output (CO).
CO = heart rate (HR) x stroke volume (SV)
Distribution of CO depends on:
- Pressure difference, blood flows from high to low pressure areas.
- Resistance, opposition to blood flow. ### Blood Pressure
Generated by contraction of the ventricles.
Pressure decreases progressively with distance from the left ventricle.
- Systolic BP: Highest pressure during systole, ventrical contraction
- Diastolic BP: Lowest arterial pressure, relaxation of ventricles. ### Mean Arterial Pressure (MAP)
Average blood pressure in arteries.
Calculated as: MAP = SBP + 2 (DBP) / 3.
Must be above 60 mmHg to perfuse vital organs.
Depends on:
- Total volume of blood.
- Cardiac output
- Vascular resistance. ### Vascular Resistance
Opposition to blood flow due to friction between blood and vessel walls.
Depends on:
- Size of Lumen: Smaller lumen, greater the resistance.
- Blood Viscosity: Thicker the blood, greater the resistance.
- Total Blood Vessel Length: Longer the length, greater the resistance. ### Velocity of Blood Flow
Speed of blood flow is inversely related to cross-sectional area, slower where the area is greatest.
Slowest in capillaries
Faster in venules when they unite
The total circulation time is 1 minute. ### Chemoreceptors
Monitor chemical composition of blood.
Located near baroreceptors in carotid sinuses and aortic arch.
Detect:
- Hypoxia: Low oxygen levels
- Acidosis: High acidity
- Hypercapnia: High carbon dioxide levels.
Transmit impulses to the cardiovascular center (CVC).
Stimulate sympathetic nervous system to cause vasoconstriction. ### Hormonal Regulation of BP
Renin-Angiotensin-Aldosterone (RAA) System:
- Reduced blood flow to kidneys: Stimulates renin release.
- Renin and ACE enzyme stimulate angiotensin II: Vasoconstrictor and increases aldosterone secretion.
- Aldosterone: Increases Na+ and water reabsorption, increasing blood volume.
Epinephrine and Norepinephrine:
- Adrenaline hormones released by the adrenal medulla.
- Increase HR and force of contraction, leading to higher CO.
- Vasoconstrict arterioles.
Antidiuretic Hormone (ADH):
- Produced by hypothalamus in response to low blood volume.
- Causes vasoconstriction.
- Promotes water movement from kidney tubules to bloodstream.
Atrial Natriuretic Peptide (ANP):
- Released by atria of heart in response to high blood volume.
- Promotes vasodilation and loss of salt and water.
- Reduces CO.

### Autoregulation of BP

Tissue's ability to automatically adjust blood flow.
Causes vasodilation and vasoconstriction.
Important for distributing blood to active areas.
Types of changes in blood flow:
- Physical changes: Warming (vasodilate), cooling (constrict), and myogenic response (smooth muscle reacts to changes in blood flow).
- Chemicals: Cells release chemicals for vasodilation or vasoconstriction.

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Description

Test your knowledge on capillaries and microcirculation, including the types of capillaries and their roles in blood flow and exchange. This quiz covers the structure, function, and significance of these smallest blood vessels within the circulatory system.