Summary
Highlights
Orel Enriquez introduces the discussion on blood vessels, the third part of the cardiovascular system. The circulatory system includes pulmonary blood vessels, which transport deoxygenated blood from the right ventricle to the lungs and oxygenated blood back to the left atrium, and systemic blood vessels, which transport oxygenated blood from the left ventricle to the rest of the body and deoxygenated blood back to the right atrium.
Blood vessels serve several key functions: carrying blood throughout the body, facilitating the exchange of nutrients, waste products, and gases within tissues, transporting substances (like hormones) within the blood, regulating blood pressure, and directing blood flow to specific tissues as needed.
Arteries carry blood away from the heart, have thick, elastic tissue, and are involved in both pulmonary and systemic circulation. Veins carry blood towards the heart. Capillaries are small blood vessels where the exchange of nutrients, waste, and gases occurs, typically formed by a single layer of cells for efficient diffusion. Blood flows from arteries to arterioles, then to capillaries, from venules to small veins, and finally back to the heart.
Both arteries and veins have three layers: the tunica intima (innermost, composed of simple squamous epithelium), the tunica media (middle, composed of smooth muscles with elastin and collagen fibers), and the tunica adventitia (outermost, made mostly of connective tissue). Arteries typically have a much thicker tunica media compared to veins.
Arteries are categorized into elastic arteries, which have a larger diameter and thickest walls (e.g., aorta and pulmonary trunk), and muscular arteries, which are medium to small in size and mostly composed of smooth muscle cells. Muscular arteries can control blood flow to different body regions by dilating or contracting.
Capillaries form branching networks and their blood flow is regulated by precapillary sphincters, which are smooth muscle cells found around the capillaries. Capillaries do not have a tunica media; instead, they are composed of a single layer of endothelium supported by loose connective tissue, allowing for easier gas exchange and nutrient/waste transfer.
All three tunics (intima, media, adventitia) are present in small veins, but their layers are thinner than in arteries. Medium veins collect blood from small veins, and large veins contain valves. These valves open and close to ensure unidirectional blood flow and prevent regurgitation.
Pulmonary circulation carries deoxygenated blood from the right ventricle, through the pulmonary trunk, to the lungs where it acquires oxygen. Pulmonary veins then return this oxygen-rich blood to the left atrium of the heart.
Systemic circulation carries oxygenated blood from the left ventricle, through the aorta, to all tissues of the body. The aorta branches out to distribute blood, and further branching ensures delivery to specific organs and regions.
The aorta has several parts: the ascending aorta (from the left ventricle), the aortic arch (containing three major arteries supplying the head and upper limbs), and the descending aorta, which extends through the thorax (thoracic portion) and abdomen (abdominal portion) to the pelvis.
Key arteries in this region include: the brachiocephalic artery (first branch of the aortic arch, supplying the right side of the head and neck), the left common carotid artery (second branch, supplying the left side of the head and neck), and the left subclavian artery (third branch, supplying the left upper limb). The right common carotid and right subclavian arteries branch off the brachiocephalic artery.
Important arteries of the upper limbs include the brachial arteries (where blood pressure measurements are taken), and the ulnar and radial arteries, both branches of the brachial artery. The radial artery is a common site for taking pulse.
The abdominal aorta branches into several arteries that supply abdominal organs. Examples include the celiac trunk (supplying stomach, pancreas, spleen, liver), superior mesenteric arteries (supplying small intestines and upper colon), inferior mesenteric arteries (supplying the majority of the large intestine), renal arteries (kidneys), hepatic arteries (liver), testicular/ovarian arteries (gonads), inferior phrenic arteries (diaphragm), and lumbar arteries (lumbar vertebra and back muscles).
Arteries of the lower limbs include the femoral arteries (thighs), popliteal arteries (knee), anterior and posterior tibial arteries (leg and foot), and fibular arteries (lateral leg and foot).
Veins return blood to the heart. In systemic circulation, veins carry deoxygenated blood to the heart, while in pulmonary circulation, pulmonary veins carry oxygenated blood from the lungs to the heart. The superior vena cava returns deoxygenated blood from the head, neck, and thorax to the right atrium, and the inferior vena cava returns deoxygenated blood from the abdomen, pelvis, and lower limbs to the right atrium.
Veins in this region include the external jugular vein (drains head and neck generally) and the internal jugular vein (drains blood specifically from the brain, face, and neck). Subclavian veins form the brachiocephalic veins, which then join to form the superior vena cava.
The median cubital vein, located near the elbow, is the first choice for venipuncture (blood collection) due to its stability. Other choices include the cephalic vein (outside of the arm) and the basilic vein (inside of the arm), though the basilic vein is less stable and near nerves.
The hepatic portal system is a vascular system that begins with capillaries in the viscera (digestive organs like stomach, spleen, intestines) and ends with capillaries in the liver. It utilizes the splenic vein and superior mesenteric vein to transport nutrient-rich blood from the digestive tract to the liver for processing.
Veins in the lower limbs include the femoral veins (drain blood from the thighs), the great saphenous vein (drains blood from the foot), and the popliteal vein (drains blood from the knee).
Blood pressure is the force exerted by blood against blood vessel walls, measured at the brachial artery. Systolic pressure is due to heart contraction, and diastolic pressure is due to heart relaxation. Normal blood pressure is 120/80 mmHg. Pulse pressure is the difference between systolic and diastolic pressure, typically measured at radial arteries or other pulse points.
Most material exchange (oxygen, carbon dioxide, nutrients) occurs across capillary walls by diffusion. Blood pressure, capillary permeability, and osmosis influence fluid movement. Local blood flow is controlled by the relaxation and contraction of precapillary sphincters, which respond to metabolic needs (e.g., increased blood flow when oxygen levels decrease).
The nervous system controls blood flow via the vasomotor center in the pons and medulla oblongata, which regulates blood vessel diameter. The vasomotor tone is the state of partial constriction of blood vessels. Hormonal control is regulated by the sympathetic division, releasing epinephrine and norepinephrine from the adrenal medulla. These hormones typically cause vasoconstriction, but can cause vasodilation in skeletal and cardiac muscles to increase blood flow during fight-or-flight responses.
Aging leads to arteriosclerosis, making arteries less elastic. Atherosclerosis, a type of arteriosclerosis, involves the deposit of materials (like cholesterol plaque) on artery walls. Factors contributing to atherosclerosis include lack of exercise, obesity, high-fat diet, and genetics.
A microscopic comparison shows arteries with a thicker tunica media than veins. Capillaries are composed of a single layer of endothelium. The space within blood vessels where blood flows is called the lumen. Nerves can be differentiated from blood vessels microscopically as nerves are solid structures, while blood vessels are hollow with a lumen.