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What happens to fluid at the arterial end of capillaries?
Fluid is forced out of the clefts of capillaries at the arterial end
Where does most of the fluid go after being forced out at the arterial end of capillaries?
Most of the fluid returns to the blood at the venous end of the capillaries.
Why is fluid movement in capillaries important?
It is important in determining the relative fluid volumes in blood and interstitial space.
What two opposing forces determine the direction and amount of fluid flow in capillaries?
Hydrostatic pressures and colloid osmotic pressures.
What is hydrostatic pressure?
the force exerted by a fluid against the walls of its container (e.g., blood against capillary walls)
What is colloid osmotic pressure?
the pressure exerted by proteins, particularly albumin, that helps draw fluid back into the capillaries from the interstitial space
What is hydrostatic pressure (HP)?
is the force exerted by fluid pressing against the walls of its container, such as blood against the walls of a capillary
What is capillary hydrostatic pressure (HPc)?
the capillary blood pressure that tends to force fluids through capillary walls and out of the blood vessel
Where is capillary hydrostatic pressure greater: at the arterial end or venule end of the capillary bed?
greater at the arterial end (35 mm Hg) of the capillary bed than at the venule end (17 mm Hg)
What is interstitial fluid hydrostatic pressure (HPif)?
the pressure exerted by interstitial fluid, pushing fluid back into the blood vessel
What is the usual assumption about interstitial fluid hydrostatic pressure (HPif)?
usually assumed to be zero because lymphatic vessels drain interstitial fluid
What is capillary colloid osmotic pressure (OPc)?
the "sucking" pressure created by plasma proteins pulling water back into the capillary
What is the typical value of capillary colloid osmotic pressure (OPc)?
approximately 26 mm Hg
What is interstitial fluid colloid osmotic pressure (OPif)?
the pressure in the interstitial fluid due to proteins, but it is inconsequential because interstitial fluid has very low protein content
What is the typical value of interstitial fluid colloid osmotic pressure (OPif)?
around 1 mm Hg
What is net filtration pressure (NFP)?
the overall pressure that comprises all forces acting on the capillary bed, determining whether fluid will move out of or into the capillary
How is net filtration pressure (NFP) calculated?
NFP = (HPc + OPif) − (HPif + OPc)
What does the NFP formula represent?
the pressures driving fluid out (HPc + OPif) subtracted by the pressures driving fluid in (HPif + OPc)
What happens when the net filtration pressure (NFP) is positive?
indicates net fluid flow out at the arterial end (filtration), where fluid is forced out of the capillaries
What happens when the net filtration pressure (NFP) is negative?
indicates net fluid flow in at the venous end (reabsorption), where fluid is drawn back into the capillaries
Where does more fluid leave: at the arterial end or venous end of the capillary bed?
at the arterial end (filtration) than is returned at the venous end (reabsorption)
How is excess interstitial fluid returned to the blood?
via the lymphatic system
How does the velocity of blood flow change as blood travels through the systemic circulation?
s fastest in the aorta, slowest in capillaries, and then increases again in veins
What is the relationship between speed and total cross-sectional area in blood vessels?
Speed is inversely related to total cross-sectional area: as the area increases, speed decreases, and as the area decreases, speed increases
Why is the velocity of blood flow slowest in capillaries?
Capillaries have the largest total cross-sectional area, which leads to slower blood flow
Why is slow capillary flow important?
allows adequate time for exchange between blood and tissues, enabling nutrients, gases, and waste products to be exchanged
How do many molecules pass between blood and interstitial fluid in capillaries?
Many molecules
What are the four routes through which molecules can cross capillary walls?
Diffuse directly through endothelial membranes.
Pass through clefts.
Pass through fenestrations
Active transport vesicles or caveolae
What is tissue perfusion?
the blood flow through body tissues, involved in:
Delivery of O2 & nutrients to tissue cells (and waste removal)
Gas exchange in the lungs
Absorption of nutrients in the digestive tract
Urine formation in the kidneys
Why is the rate of blood flow important?
It’s precisely regulated to provide the right amount of blood to allow proper function of tissues or organs
What factors control the rate of blood flow?
controlled by both extrinsic and intrinsic factors
What is extrinsic control of blood flow?
involves the sympathetic nervous system and hormones that control blood flow throughout the whole body
How does extrinsic control affect blood flow to specific regions?
acts on arteriolar smooth muscle to reduce blood flow to regions that need it the least
What is intrinsic control of blood flow?
refers to autoregulation (local) control of blood flow, where blood flow is adjusted locally to meet a specific tissue’s requirements
How do local arterioles regulate blood flow to tissues?
Local arterioles that feed capillaries can undergo modification of their diameters to regulate blood flow
How do organs regulate their own blood flow?
by varying the resistance of their own arterioles
How much of the body's total blood does skeletal muscle receive at rest?
about 20% of the total blood in the body
How much blood does skeletal muscle receive during exercise?
over 70% of the blood
How do intrinsic controls affect blood flow to skeletal muscles during exercise?
cause skeletal muscle arterioles to dilate, increasing blood flow to the muscles during exercise
How do extrinsic controls affect blood flow during exercise?
decrease blood flow to other organs, such as the kidneys and digestive organs, during exercise
How does blood flow in the skin change as temperature rises?
As temperature rises (e.g., from heat exposure, fever, or vigorous exercise), hypothalamic signals reduce vasomotor stimulation of skin vessels, causing dilation, which allows warm blood to flush into capillary beds
What happens when blood flow increases to the skin due to temperature rise?
Warm blood flushes into capillary beds, and heat radiates from the skin
How does blood flow in the skin change as temperature decreases?
blood is shunted to deeper, more vital organs, and superficial skin vessels constrict strongly, trapping blood in the vessels
Why do people get rosy cheeks in cold weather?
The constriction of superficial skin vessels in cold weather can trap blood
When do transient elevations in blood pressure (BP) occur?
during changes in posture, physical exertion, emotional upset, or fever
What factors can cause blood pressure to vary?
Age, sex, weight, race, mood, and posture
What is considered hypertension in terms of blood pressure?
sustained elevated arterial pressure of 140/90 mm Hg or higher
What are the risks of prolonged hypertension?
major cause of heart failure, vascular disease, renal failure, and stroke. It causes the heart to work harder, leading to myocardial enlargement, weakness, and flabbiness
What is considered low blood pressure (hypotension)?
typically below 90/60 mm Hg
Is low blood pressure usually a concern?
usually not a concern unless it causes inadequate blood flow to tissues. It is often associated with long life and a lack of cardiovascular illness
What is circulatory shock?
condition where blood vessels inadequately fill and cannot circulate blood normally, leading to inadequate blood flow that cannot meet tissue needs
What is hypovolemic shock?
results from large-scale blood loss, causing a significant decrease in blood volume and circulation
What is cardiogenic shock?
occurs when an inefficient heart cannot sustain adequate circulation, leading to insufficient blood flow to tissues
Vascular system consists of two main circulations
Pulmonary circulation and Systemic circulation
What is pulmonary circulation?
a short loop that runs from the heart to the lungs and back to the heart
What is systemic circulation?
a long loop that carries blood to all parts of the body and then back to the heart
How do the locations of arteries and veins differ?
Arteries run deep, while veins can be both deep and superficial
How do deep veins and arteries compare in terms of naming?
Deep veins share the same name as the corresponding artery
Do superficial veins correspond to arteries?
Superficial veins do not correspond to the names of any arteries
How do venous pathways differ from arterial pathways?
Venous pathways are more interconnected than arterial pathways.
Why can venous pathways be harder to follow?
Venous pathways can be harder to follow because veins can have more than one name.
What makes the venous drainage of the brain unique?
The brain contains dural venous sinuses, which are specialized channels for venous drainage
How does the venous system of the digestive system drain?
drains into the hepatic portal system, which passes through the liver before returning to the heart
Why is the hepatic portal system important?
because blood passing through it isn’t directly for use by the liver. Instead, substances absorbed in the small intestine travel first to the liver for processing
How do substances absorbed in the small intestine travel to the liver?
travel through the hepatic portal system to the liver, where they are processed before going to the heart
Why do some medicines need to be taken in specific ways related to the hepatic portal system?
because if they pass through the liver (via the hepatic portal system), the liver may deactivate the medication before it can have its intended effect
What type of blood flows through the liver?
receives both oxygenated and deoxygenated blood.
What determines whether a blood vessel becomes an artery or a vein?
Vascular endothelial growth factor (VEGF)
What is the purpose of fetal shunts like the foramen ovale and ductus arteriosus?
The foramen ovale and ductus arteriosus are fetal shunts that bypass the nonfunctional lungs, allowing blood to flow directly to the heart and body
What is the function of the ductus venosus in fetal circulation?
bypasses the liver in fetal circulation, allowing blood to flow directly to the heart.
What is the role of the umbilical vein and arteries?
circulate blood to and from the placenta during fetal development
Why does vessel formation occur in the body?
occurs to support body growth, wound healing, or to rebuild vessels lost during menstrual cycles
How does aging affect the vascular system?
may result in conditions such as varicose veins, atherosclerosis, and increased blood pressure
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