Capillaries

I. Capillaries are the site of exchange of nutrients and waste products between blood and tissues.

II. Blood flows through the capillaries more slowly than through any other part of the vascular system because of the huge cross-sectional area of the capillaries.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

III. Capillary blood flow is determined by the resistance of the arterioles supplying the capillaries and by the number of open precapillary sphincters.

IV. Diffusion is the mechanism by which nutrients and metabolic end products are exchanged between capillary plasma and interstitial fluid.

a. Lipid-soluble substances move across the entire endothelial wall, whereas ions and polar molecules move through water-filled intercellular clefts or fused-vesicle channels.

b. Plasma proteins move across most capillaries only very slowly, either by diffusion through water-filled channels or by vesicle transport.

c. The diffusion gradient for a substance across capillaries arises as a result of cell utilization or production of the substance. Increased metabolism increases the diffusion gradient and increases the rate of diffusion.

V. Bulk flow of protein-free plasma or interstitial fluid across capillaries determines the distribution of extracellular fluid between these two fluid compartments.

a. Filtration from plasma to interstitial fluid is favored by the hydrostatic-pressure difference between the capillary and the interstitial fluid. Absorption from interstitial fluid to plasma is favored by the protein concentration difference between the plasma and the interstitial fluid.

b. Filtration and absorption do not change the concentrations of crystalloids in the plasma and interstitial fluid because these substances move together with water.

c. There is normally a small excess of filtration over absorption.

Veins

I. Veins serve as low-resistance conduits for venous return.

II. Veins are very compliant and contain most of the blood in the vascular system.

a. Their diameters are reflexly altered by sympathetically mediated vasoconstriction so as to maintain venous pressure and venous return.

b. The skeletal-muscle pump and respiratory pump increase venous pressure locally and enhance venous return. Venous valves permit the pressure to produce only flow toward the heart.

The Lymphatic System

I. The lymphatic system provides a one-way route for movement of interstitial fluid to the cardiovascular system.

II. Lymph returns the excess fluid filtered from the blood-vessel capillaries, as well as the protein that leaks out of the blood-vessel capillaries.

III. Lymph flow is driven mainly by contraction of smooth muscle in the lymphatic vessels, but also by the skeletal-muscle pump and the respiratory pump.

SECTION D KEY TERMS

compliance

prostacyclin (PGI2)

systolic pressure (SP)

endothelin-1 (ET-1)

diastolic pressure (DP)

shear stress

pulse pressure

flow-induced arterial

mean arterial pressure

vasodilation

(MAP)

angiogenesis

vasodilation

angiogenic factors

vasoconstriction

intercellular cleft

intrinsic tone

fused-vesicle channel

local controls

metarteriole

hyperemia

precapillary sphincter

active hyperemia

crystalloids

bradykinin

colloids

kininogen

absorption

kallikrein

net filtration pressure (NFP)

flow autoregulation

Starling forces

myogenic response

capillary filtration coefficient

reactive hyperemia

peripheral vein

nitric oxide

skeletal-muscle pump

angiotensin II

respiratory pump

vasopressin

lymphatic system

atrial natriuretic factor

lymphatic vessel

endothelium-derived

lymph

relaxing factor (EDRF)

lymphatic capillary

SECTION D REVIEW QUESTIONS

Draw the pressure changes along the systemic and pulmonary vascular systems during the cardiac cycle.

What are the two functions of the arteries? What are normal values for systolic, diastolic, and mean arterial pressures? How is mean arterial pressure estimated?

What are two major factors that determine pulse pressure?

What denotes systolic and diastolic pressure in the measurement of arterial pressure with a sphygmomanometer?

What are the major sites of resistance in the systemic vascular system?

What are two functions of arterioles? Write the formula relating flow through an organ to mean arterial pressure and the resistance to flow offered by that organ.

List the chemical factors thought to mediate active hyperemia.

Name the mechanism other than chemical factors that contributes to flow autoregulation. What is the only autonomic innervation of most arterioles? What are the major adrenergic receptors influenced by these nerves? How can control of sympathetic nerves to arterioles achieve vasodilation?

Name four hormones that cause vasodilation or vasoconstriction of arterioles and specify their effects.

Vander et al.: Human I III. Coordinated Body I 14. Circulation I I © The McGraw-Hill

Physiology: The Functions Companies, 2001 Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

13. Describe the role of endothelial paracrine agents in mediating arteriolar vasoconstriction and vasodilation, and give three examples.

14. Draw a flow diagram summarizing the factors affecting arteriolar radius.

15. What are the relative velocities of flow through the various segments of the vascular system?

16. Contrast diffusion and bulk flow. Which is the mechanism of exchange of nutrients, oxygen, and metabolic end products across the capillary wall?

17. What is the only solute to have significant concentration differences across the capillary wall? How does this difference influence water concentration?

18. What four variables determine the net filtration pressure across the capillary wall? Give representative values for each of them in the systemic capillaries.

19. How do changes in local arteriolar resistance influence local capillary pressure?

20. What is the relationship between cardiac output and venous return in the steady state? What is the force driving venous return?

21. Contrast the compliances and blood volumes of the veins and arteries.

22. What three factors influence venous pressure?

23. Approximately how much fluid is returned to the blood by the lymphatics each day?

24. Describe the forces that cause lymph flow.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • marianna trevisani
    What is the function of arterioles in the body?
    8 years ago
  • jonas
    What is the function of capillaries in the body?
    8 years ago
  • polo
    What are the relative velocities of flow through the various segments of the vascular system?
    8 years ago
  • juhana
    How do changes in arteriole resistance influence corresponding capillary pressure?
    8 years ago
  • linda
    How do different concentration of crystalloids affect absorption?
    8 years ago
  • Troy Bell
    What are the four things that promote venous return and lymph transport?
    8 years ago
  • alexander
    Are very compliant and contain most of the blood in the vascular system.?
    8 years ago
  • aziza
    What two factors determine bulk flow in capillaries?
    8 years ago
  • Willow
    What three factors influence capillary exchange?
    8 years ago
  • ADIAM
    Does arterioles or capillaries control vasoconstriction or vasodilatin in the human body?
    8 years ago
  • tytti
    What four variables determine the net filtration pressure across the capillary wall?
    8 years ago
  • seija
    What are the major sites of resistance in the systtemic vascular system?
    8 years ago
  • Eden
    What are the major sites of resistance in the systemic vascular system?
    7 years ago
  • hope
    What is polo capillaritis?
    7 years ago
  • eden
    What is the role of endothelial paracrine agens in mediating arteriolar constriction?
    7 years ago
  • Stephanie Pitts
    How do changes in local arteriolar resistance influence local capillary pressure?
    7 years ago
  • JUNIOR WALKER
    Which factor serves to push proteinfree plasma out of the capillaries?
    6 years ago
  • juhana
    What is the only solute to have significant concentration difference across the capillary wall?
    8 months ago

Post a comment