Source: Textbook Ch. 44, Pre-Session Assignment 11
Tags: blood vessels, arteries, veins, capillaries, capillary types, capillary exchange, Starling forces, hydrostatic pressure, colloidal pressure, oncotic pressure, blood pressure, vasoconstriction, vasodilation, baroreceptors, RAAS, antidiuretic hormone
Difficulty: Intermediate | Prerequisites: Parts 1 and 2 study notes (blood composition, heart structure)
Big Picture
This final section of Chapter 44 covers the vessels that carry blood and the physics of how fluid moves between the blood and the tissues. You need to understand the structural differences between arteries, veins, and capillaries, what drives fluid out of capillaries (hydrostatic pressure) and back in (colloidal/oncotic pressure), and how the body regulates blood pressure through nervous and hormonal mechanisms. This is where the plumbing meets the physics.
TL;DR
Arteries have thick elastic walls for high-pressure transport; veins have thinner walls with valves for low-pressure return; capillaries are one-cell-thick exchange sites. Fluid movement across capillary walls depends on the balance of hydrostatic and colloidal pressures. Blood pressure is regulated by cardiac output, blood volume, and peripheral resistance, all tuned by the autonomic nervous system and hormones (RAAS, ADH, ANP).
Tunica intima
The innermost layer of a blood vessel wall, consisting of endothelium and a thin layer of connective tissue. In simple terms, it is the smooth lining that blood touches.
Tunica media
The middle layer, composed of smooth muscle and elastic fibres. This is the layer that contracts or relaxes to change vessel diameter (vasoconstriction and vasodilation).
Tunica externa (adventitia)
The outermost layer of connective tissue that anchors the vessel to surrounding structures.
Arteriole
A small artery that leads into a capillary bed. Arterioles are the main site of resistance regulation in the circulatory system.
Venule
A small vein that drains a capillary bed and carries blood toward larger veins.
Precapillary sphincter
A ring of smooth muscle at the entrance to a capillary bed that opens or closes to regulate blood flow into that particular bed.
Vasoconstriction
Narrowing of a blood vessel due to contraction of smooth muscle in the tunica media. Increases resistance and blood pressure.
Vasodilation
Widening of a blood vessel due to relaxation of smooth muscle in the tunica media. Decreases resistance and blood pressure.
Blood pressure (BP)
The force exerted by circulating blood against the walls of blood vessels, typically measured in the brachial artery as systolic/diastolic (e.g. 120/80 mmHg).
Peripheral resistance
The resistance to blood flow caused by friction between blood and vessel walls. Determined mainly by vessel diameter, blood viscosity, and total vessel length.
Arteries
Carry blood away from the heart
Thick walls with a prominent tunica media (lots of smooth muscle and elastic tissue) to withstand high pressure
Elastic arteries (e.g. aorta, pulmonary trunk): large, stretch during systole and recoil during diastole to smooth out pressure pulsations
Muscular arteries (e.g. brachial, femoral): distribute blood to specific body regions; thicker smooth muscle layer for vasoconstriction/vasodilation
Arterioles: smallest arteries; the primary regulators of blood flow and systemic resistance. Their diameter is controlled by local metabolic signals, sympathetic nerves, and hormones.
Veins
Carry blood back to the heart
Thinner walls with less smooth muscle and elastic tissue than arteries; larger lumen relative to wall thickness
Contain one-way valves (especially in the limbs) to prevent backflow since venous pressure is low
Venous return is assisted by: skeletal muscle pump (contracting muscles squeeze veins), respiratory pump (pressure changes during breathing), and sympathetic venoconstriction
Veins are capacitance vessels: they hold roughly 60% of total blood volume at any given time and act as a blood reservoir
Capillaries
The smallest vessels, connecting arterioles to venules
Walls are one endothelial cell thick with a basement membrane, making them the only vessels where exchange of gases, nutrients, and waste occurs
Capillary beds are networks of capillaries within tissues; blood flow through each bed is regulated by precapillary sphincters
Types of capillaries
Continuous capillaries: endothelial cells are joined by tight junctions with small intercellular clefts. Found in most tissues (skin, muscle, lungs, brain). The blood-brain barrier uses continuous capillaries with especially tight junctions.
Fenestrated capillaries: endothelial cells have small pores (fenestrations) that increase permeability. Found in kidneys, intestinal villi, and endocrine glands where rapid exchange or filtration is needed.
Sinusoidal capillaries (sinusoids): large, irregularly shaped with wide gaps between endothelial cells and an incomplete basement membrane. Found in liver, spleen, and bone marrow. Allow passage of large molecules and even whole cells (e.g. new blood cells entering circulation from marrow).
Fluid movement across capillary walls is governed by the balance of two opposing pressures at any point along the capillary. This is described by the Starling equation.
Hydrostatic pressure (HP)
The pressure of the blood pushing fluid out of the capillary into the interstitial space. Higher at the arteriolar end of a capillary (roughly 35 mmHg) and lower at the venular end (roughly 15 mmHg) because pressure drops as blood moves through the capillary.
Colloidal (oncotic) pressure (COP)
The osmotic "pull" exerted by plasma proteins (mainly albumin) that draws fluid back into the capillary from the interstitial space. Remains relatively constant along the capillary at roughly 25 mmHg.
Net fluid movement
At the arteriolar end: HP (~35 mmHg) exceeds COP (~25 mmHg), so the net pressure pushes fluid out of the capillary into the tissues (filtration).
At the venular end: HP (~15 mmHg) is less than COP (~25 mmHg), so the net pressure pulls fluid back into the capillary (reabsorption).
Roughly 85% of filtered fluid is reabsorbed. The remaining 15% is collected by the lymphatic system and returned to the blood, preventing oedema.
Control of blood flow through capillary beds
Precapillary sphincters open or close in response to local chemical signals (low O2, high CO2, histamine, lactic acid)
When a tissue is metabolically active, local metabolites cause sphincters to relax, increasing blood flow to that area (active hyperaemia)
When a tissue is at rest, many sphincters are closed, and blood bypasses the capillary bed through a metarteriole-thoroughfare channel (shunt)
Real-world application
Oedema (tissue swelling) results when the balance of Starling forces is disrupted. Causes include: elevated capillary hydrostatic pressure (e.g. heart failure, venous obstruction), reduced plasma protein concentration (e.g. liver failure, malnutrition, nephrotic syndrome), increased capillary permeability (e.g. inflammation, burns), or blocked lymphatic drainage.
Definition
Blood pressure is the force exerted by blood against the walls of the blood vessels. It is highest in the aorta and large arteries, drops progressively through arterioles and capillaries, and is lowest in the venae cavae.
Systolic pressure is the peak pressure during ventricular contraction. Diastolic pressure is the minimum pressure during ventricular relaxation. A normal reading is roughly 120/80 mmHg.
The three main determinants of blood pressure
Cardiac output (CO): higher CO raises BP. CO = SV x HR.
Blood volume: more blood in the system increases pressure. Dehydration or haemorrhage lowers it.
Peripheral resistance: primarily determined by arteriolar diameter. Vasoconstriction increases resistance and raises BP; vasodilation decreases resistance and lowers BP.
Blood pressure across the circulatory system
Aorta and large arteries: highest pressure (pulsatile, ~120/80 mmHg)
Arterioles: significant pressure drop due to high resistance
Capillaries: low, steady pressure (~35 mmHg at arteriolar end, ~15 mmHg at venular end)
Veins: very low pressure (~5 to 15 mmHg); rely on valves and muscle/respiratory pumps
Venae cavae: near 0 mmHg as blood returns to the right atrium
Nervous regulation (short-term)
Baroreceptors in the carotid sinus and aortic arch detect changes in arterial pressure and relay signals to the cardiovascular centre in the medulla oblongata
If BP rises: the medulla increases parasympathetic output (slows HR) and decreases sympathetic output (vasodilation, lower HR and contractility), bringing BP down
If BP drops: the medulla increases sympathetic output (vasoconstriction, faster HR, stronger contraction) and decreases parasympathetic output, raising BP
Hormonal regulation (longer-term)
Renin-angiotensin-aldosterone system (RAAS): when BP drops, the kidneys release renin, which triggers formation of angiotensin II (a potent vasoconstrictor). Angiotensin II also stimulates aldosterone release from the adrenal cortex, promoting sodium and water reabsorption in the kidneys, increasing blood volume and BP.
Antidiuretic hormone (ADH / vasopressin): released by the posterior pituitary in response to low blood volume or high blood osmolarity. Promotes water reabsorption in the kidneys and causes vasoconstriction, raising BP.
Atrial natriuretic peptide (ANP): released by atrial cells when stretched by high blood volume. Promotes sodium and water excretion, and inhibits renin and aldosterone, lowering blood volume and BP.
Adrenaline and noradrenaline: from the adrenal medulla; increase HR, contractility, and cause vasoconstriction in most vascular beds (but vasodilation in skeletal muscle arterioles during exercise).
Students often think blood pressure is constant throughout the circulatory system. It is not. Pressure is highest in the aorta and drops continuously as blood moves through arterioles, capillaries, and veins.
Veins are sometimes dismissed as passive tubes. They play an active role as capacitance vessels (holding ~60% of blood volume) and rely on valves and external pumps (skeletal muscle, respiratory) to return blood.
Students confuse hydrostatic pressure and colloidal pressure. Hydrostatic pushes fluid out; colloidal pulls fluid back in. At the arteriolar end of a capillary, hydrostatic wins (filtration). At the venular end, colloidal wins (reabsorption).
"Vasoconstriction raises blood pressure" is true, but students sometimes assume this only happens via sympathetic nerves. Angiotensin II, ADH, and endothelin are also powerful vasoconstrictors.
⚠️ Know the three vessel wall layers (tunica intima, media, externa) and how they differ between arteries, veins, and capillaries.
⚠️ Be able to name and distinguish the three capillary types (continuous, fenestrated, sinusoidal) and give a tissue example for each.
⚠️ Understand Starling forces: at which end of the capillary does filtration occur (arteriolar) and reabsorption occur (venular), and what happens to the ~15% of fluid that is not reabsorbed (lymphatic system).
⚠️ The RAAS pathway is commonly tested. Be able to trace: low BP, renin, angiotensinogen, angiotensin I, ACE, angiotensin II, aldosterone, sodium/water reabsorption, BP rises.
⚠️ Know the baroreceptor reflex: what happens when BP is too high vs too low.
True or false: Arteries have thicker walls than veins.
True. Arteries have a thicker tunica media to withstand higher pressure.
Fill in the blank: The three types of capillaries are ________, ________, and ________.
Continuous, fenestrated, and sinusoidal.
True or false: At the venular end of a capillary, fluid is filtered out into the tissues.
False. At the venular end, colloidal (oncotic) pressure exceeds hydrostatic pressure, so fluid is reabsorbed back into the capillary.
Fill in the blank: The enzyme ACE converts ________ into ________.
Angiotensin I into angiotensin II.
True or false: Veins contain valves to prevent backflow.
True.
Q: Compare arteries and veins in terms of wall thickness, lumen size, and the presence of valves.
A: Arteries have thicker walls (especially the tunica media), a smaller lumen relative to wall thickness, and no valves (except semilunar valves at the aorta and pulmonary trunk). Veins have thinner walls, a larger lumen relative to wall thickness, and contain one-way valves to prevent backflow.
Q: A patient has low plasma albumin levels due to liver disease. Predict what happens to capillary fluid exchange and explain why.
A: Low albumin reduces colloidal (oncotic) pressure in the capillaries. With less inward "pull," more fluid stays in the interstitial space. The result is oedema (tissue swelling), because filtration outpaces reabsorption.
Q: Explain the baroreceptor reflex in response to a sudden drop in blood pressure (e.g. standing up quickly).
A: Baroreceptors in the carotid sinus and aortic arch detect the drop and send fewer signals to the cardiovascular centre in the medulla. The medulla increases sympathetic output (vasoconstriction, increased HR and contractility) and decreases parasympathetic output. This raises peripheral resistance and cardiac output, restoring BP.
Q: What is the role of renin in blood pressure regulation?
A: Renin is an enzyme released by the kidneys when blood pressure or blood flow to the kidneys drops. It cleaves angiotensinogen (from the liver) into angiotensin I, which is then converted to angiotensin II by ACE in the lungs. Angiotensin II causes vasoconstriction and stimulates aldosterone secretion, both of which raise blood pressure.
Q: Where in the circulatory system is blood pressure highest and where is it lowest?
A: Pressure is highest in the aorta and large arteries (roughly 120/80 mmHg) and lowest in the venae cavae (approaching 0 mmHg) as blood returns to the right atrium.
Capillary exchange ties directly to the lymphatic system, which collects the ~15% of filtered fluid that is not reabsorbed and returns it to the blood. Blood pressure regulation connects to kidney physiology (RAAS, ADH, sodium/water balance) and endocrinology (adrenal hormones, ANP). The autonomic nervous system's role here reinforces concepts from the nervous system chapter (sympathetic vs parasympathetic, neurotransmitters).
Related Terms / Search Tags
blood vessels, arteries, veins, capillaries, arterioles, venules, tunica intima, tunica media, tunica externa, elastic arteries, muscular arteries, continuous capillaries, fenestrated capillaries, sinusoidal capillaries, sinusoids, blood-brain barrier, precapillary sphincter, capillary bed, capillary exchange, Starling forces, Starling equation, hydrostatic pressure, colloidal pressure, oncotic pressure, filtration, reabsorption, oedema, edema, lymphatic system, blood pressure, systolic pressure, diastolic pressure, peripheral resistance, vasoconstriction, vasodilation, baroreceptor, baroreceptor reflex, carotid sinus, aortic arch, cardiovascular centre, medulla oblongata, RAAS, renin, angiotensin, angiotensinogen, ACE, angiotensin II, aldosterone, ADH, vasopressin, ANP, atrial natriuretic peptide, skeletal muscle pump, respiratory pump, venous return, capacitance vessels