PSIO 202, Lecture 9

PSIO 202, Lecture 9 - PSIO 202 Human Anatomy and Physiology...

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Unformatted text preview: PSIO 202 Human Anatomy and Physiology Lecture 9 Hemodynamics; Capillary Exchange Objectives and Reading Assignment • Reading: Tortora, pages 769-772 • Objectives – Be able to describe and draw the general features of a capillary network. – Compare and contrast the anatomy and important physiological features of each type of capillary – Explain how fluid and solutes pass through the capillary wall – Describe the forces that result in movement fluid into and out of capillaries Capillaries-Overview • Oxygen and nutrients move out of the blood by crossing the capillary wall and entering the interstitial fluid (ISF) • Carbon dioxide and metabolic waste products move in the opposite direction • Estimates indicate that if all body capillaries were placed end-to-end, they would reach 60,000 miles, and the surface area would be 5,000 cm2 Capillaries-Overview • The average capillary is about 1 mm long, and the lumen diameter is about 8 micrometers • Blood flow velocity in a capillary is about 3 cm/sec • The concentration, or density, of capillaries in a given tissue is proportional to the tissue’s metabolic activity The “Capillary Bed” • The term “capillary bed” refers to a network of capillaries where exchange of materials with the tissue cells can take place • Every tissue cell is usually within 2-3 cell diameters from a capillary • This is important, because most substances move by diffusion, which is efficient only when diffusion distances are short. Capillary Exchange of Respiratory Gases and Nutrients Figure 18.14.1 continuous -- Found in lungs, skeletal muscle, connective tissue fenestrated --Fenestration means “little window (70-100 nm). Found in kidneys, endocrine glands, small intestine, etc. sinusoidal -- Found in liver, spleen, bone marrow, and anterior pituitary gland Routes for Capillary Exchange • intercellular clefts, or pores—water and most small substances, cross the capillary wall by diffusion through these pores • endothelial cell membranes--some small molecules and gases can diffuse or be transported across the endothelial cell layer • fenestrations--large molecules can pass easily through the fenestrations Mechanisms of Fluid Exchange • Materials pass through the capillary walls by one of two major mechanisms – Diffusion--movement of molecules or ions from a region of higher concentration to one of lower concentration until equilibrium is reached. This is a passive process – Bulk flow --bulk flow refers to the movement of a fluid from a region of higher pressure to one of lower pressure; also a passive process – Transcytosis--substances enter pinocyotic vesicles, move to endothelial cells via endocytosis, and exit on opposite side via exocytosis; this is an active process Bulk Flow • Fluid exchange between ISF and capillaries is based primarily on bulk flow • Two major forces tend to push fluid OUT of capillaries and into ISF – Blood hydrostatic pressure (BHP)--pushes fluid out through the capillary pores – interstitial fluid osmotic pressure (IFOP)—’pulls” fluid out via osmosis; this pressure is very small compared to the BHP. Bulk Flow, Continued • There are also two forces that pull fluid INTO the capillaries: – The blood colloid osmotic pressure (BCOP)-The BCOP is the result of differences in protein concentration between plasma and ISF, which tends to pull water from the ISF and into the capillaries – The Interstitial fluid hydrostatic pressure (IFHP)-- The IFHP is due to the pressure exerted by interstitial fluid, but it is normally very small The “Net Filtration Pressure” • The net movement of fluid is driven by the difference between the inward and outward pressures, and is called the “net filtration pressure” NFP = (BHP + IFOP) - (BCOP + IFHP) Push (Filtration) - Pull (Reabsorption) • The BHP and the BCOP are the major factors that determine fluid movement across the capillary wall NFP; SUMMARY FILTRATION results in a constant flow of fluid that washes over the tissue cells at the arterial end of the capillary, carrying nutrients and oxygen with it REABSORPTION results in a return of fluid to the capillary at the venous end, thereby depositing wastes into the venous system NOTE: Under normal conditions, slightly more fluid leaves than enters the capillaries. The lymphatic vessels absorb this excess fluid and return it to the circulatory system. ...
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