The Cardiovascular System: Blood Vessels
Vessel structure, blood pressure, capillary exchange and the major circulatory routes
By the end of this unit you can…
- ✓Compare the structure of arteries, capillaries and veins
- ✓Relate blood flow, pressure and resistance and describe blood pressure across the system
- ✓Explain short- and long-term regulation of blood pressure
- ✓Describe capillary exchange and the forces of bulk flow; explain edema
- ✓Trace the major arteries and veins, the hepatic portal system and fetal circulation basics
Key terms
Practice →1Vessel structure
| Tunic | Made of | Role |
|---|---|---|
| Tunica intima (interna) | Endothelium (simple squamous) + subendothelial layer | Smooth lining; minimizes friction |
| Tunica media | Smooth muscle + elastic fibers; sympathetic control | Vasoconstriction / vasodilation — controls diameter, flow and BP. Thickest in arteries |
| Tunica externa (adventitia) | Loose collagen fibers; vasa vasorum in large vessels | Protects and anchors; thickest layer in veins |
| Vessel | Features |
|---|---|
| Elastic (conducting) arteries | Aorta and major branches; lots of elastin → act as pressure reservoirs, expanding and recoiling to smooth flow |
| Muscular (distributing) arteries | Deliver blood to organs; thickest tunica media → active vasoconstriction |
| Arterioles | Smallest arteries; control flow into capillary beds — the main site of resistance |
| Capillaries | Only a tunica intima (endothelium + basement membrane) → ideal for exchange. Types: continuous (most common; tight junctions in brain), fenestrated (pores — kidneys, small intestine, endocrine glands), sinusoid (large gaps — liver, bone marrow, spleen) |
| Venules → veins | Thin walls, large lumens; capacitance vessels holding ~65% of blood; low pressure; valves in limb veins prevent backflow |
Capillary beds: a vascular shunt (metarteriole → thoroughfare channel) connects arteriole to venule, and true capillaries branch from it. Precapillary sphincters regulate flow into the true capillaries (e.g. after a meal, digestive capillaries open).
2Blood flow, pressure & resistance
- Blood flow (F)
- Volume of blood flowing per minute. For the whole body = cardiac output.
- Blood pressure (BP)
- Force per unit area exerted on a vessel wall (mmHg). Pressure gradients keep blood moving.
- Resistance (R)
- Opposition to flow — mostly in the systemic (peripheral) vessels. Depends on blood viscosity, total vessel length and, most importantly, vessel diameter (resistance varies inversely with the fourth power of the radius: halving the radius raises resistance 16×).
F = ΔP / R — flow is directly proportional to the pressure difference and inversely proportional to resistance.
| Measure | Normal / meaning |
|---|---|
| Systolic pressure | Peak arterial pressure during ventricular ejection (~120 mmHg) |
| Diastolic pressure | Lowest arterial pressure as aortic valve is closed (~80 mmHg) |
| Pulse pressure | Systolic − diastolic (120 − 80 = 40 mmHg) — felt as the pulse |
| Mean arterial pressure (MAP) | Diastolic + ⅓ pulse pressure (80 + 13 ≈ 93 mmHg) — the pressure that propels blood to tissues |
| Capillary pressure | ~35 mmHg at arterial end → ~17 mmHg at venous end (low to protect fragile capillaries) |
| Venous pressure | Low (~15 mmHg → ~0 at right atrium); helped by skeletal muscle pump, respiratory pump and venoconstriction |
3Regulating blood pressure
Blood pressure depends on cardiac output, peripheral resistance and blood volume (MAP = CO × R).
- Baroreceptor reflex (short-term, neural)
- Baroreceptors in the carotid sinuses and aortic arch detect stretch. High BP → signals to the cardiovascular center in the medulla → ↓ sympathetic, ↑ parasympathetic → HR and contractility fall, arterioles dilate → BP falls. Low BP → the reverse.
- Chemoreceptor reflex
- Carotid and aortic bodies detect falling O₂ or rising CO₂/H⁺ → increase sympathetic output → ↑ BP to speed blood to the lungs.
- Hormonal (short-term)
- Epinephrine/NE (↑ CO, vasoconstriction), angiotensin II (powerful vasoconstrictor), ADH (vasoconstriction at high levels), ANP (vasodilation, ↓ BP).
- Renal (long-term)
- Direct: high BP → kidneys filter more → more urine → blood volume and BP fall. Indirect: low BP → kidneys release renin → renin–angiotensin–aldosterone system.
- 1Low arterial BP → juxtaglomerular cells of the kidney release renin.
- 2Renin converts angiotensinogen (from the liver) → angiotensin I.
- 3ACE (angiotensin-converting enzyme, mostly in lung capillaries) converts angiotensin I → angiotensin II.
- 4Angiotensin II: vasoconstriction, stimulates aldosterone (Na⁺ & water retention), ADH release and thirst → BP rises.
Circulatory shock: blood vessels are inadequately filled. Hypovolemic (blood/fluid loss — rapid weak pulse, falling BP), vascular (extreme vasodilation — anaphylaxis, septic shock), cardiogenic (pump failure).
4Capillary exchange
Most exchange of O₂, CO₂, nutrients and wastes happens by diffusion through the endothelium (lipid-soluble molecules through cells; small water-soluble ones through clefts and fenestrations). Bulk flow of fluid is driven by two opposing pressures:
- Hydrostatic pressure (HP)
- Blood pressure pushing fluid out of the capillary (filtration). High at the arterial end (~35 mmHg), low at the venous end (~17 mmHg).
- Colloid osmotic pressure (OP)
- Created by plasma proteins (mainly albumin) that can't leave; pulls fluid into the capillary (~26 mmHg), roughly constant along its length.
- Net filtration pressure (NFP)
- NFP = (HP_c − HP_if) − (OP_c − OP_if). Positive at the arterial end (fluid out), negative at the venous end (fluid back in).
More fluid leaves than returns (~1.5 mL/min); the lymphatic system picks up the excess and returns it to the blood.
5Major circulatory routes
Pulmonary circuit: pulmonary trunk → right and left pulmonary arteries (carry O₂-poor blood) → lungs → pulmonary veins (O₂-rich) → left atrium.
| Aorta region | Main branches |
|---|---|
| Ascending aorta | Right and left coronary arteries |
| Aortic arch | Brachiocephalic trunk (→ right common carotid + right subclavian), left common carotid, left subclavian |
| Thoracic (descending) aorta | Intercostal, bronchial and esophageal arteries |
| Abdominal aorta | Celiac trunk (liver, stomach, spleen), superior mesenteric (small intestine, proximal colon), renal, gonadal, inferior mesenteric (distal colon, rectum); ends by splitting into the common iliac arteries at L4 |
Head & neck: common carotids → internal carotid (brain, eye) and external carotid (face, scalp). Vertebral arteries (from the subclavians) join to form the basilar artery. The cerebral arterial circle (circle of Willis) connects anterior and posterior brain circulations, providing alternative routes if one artery is blocked. Upper limb: subclavian → axillary → brachial (BP is measured here) → radial and ulnar → palmar arches. Lower limb: external iliac → femoral → popliteal → anterior tibial (continues as the dorsalis pedis) and posterior tibial (gives off the fibular artery).
Major veins: the superior vena cava drains everything above the diaphragm (formed by the brachiocephalic veins, which receive the internal jugular and subclavian veins); the inferior vena cava drains below the diaphragm (renal, gonadal, hepatic veins, common iliacs). Superficial limb veins: cephalic and basilic (joined at the elbow by the median cubital — used for drawing blood); in the leg the great saphenous — the longest vein, used for coronary bypass grafts. The azygos system drains the thoracic wall.
Hepatic portal circulation: veins from the stomach, intestines, spleen and pancreas (superior mesenteric, splenic, inferior mesenteric) drain into the hepatic portal vein, which carries nutrient-rich blood to the liver for processing before it enters the general circulation. Liver blood then exits via the hepatic veins → IVC.
Pulse points — where an artery lies close to the surface over firm tissue: temporal, facial, common carotid, brachial, radial (most used), femoral, popliteal, posterior tibial and dorsalis pedis.