A&P 2 Lecture Notes - Blood Vessels, Hemostasis, and Coronary Circulation
Tissue Layers of the Blood Vessels: The Tunicas
The blood vessels, encompassing arteries, veins, and capillaries, are lined by specialized tissue layers known as tunicas. There are three primary tissue layers that line arteries and veins, while vessels in a capillary bed are only lined by a single layer. These layers are critical for the structure and function of the circulatory system. The heart itself is located in a central area of the chest called the mediastinum, and its vessels follow these specific histological patterns.
The innermost layer is the tunica interna, also known as the tunica intima. This layer directly faces the lumen, or the internal space of the blood vessel where blood flows. It is composed of simple squamous epithelial tissue, which is referred to specifically as endothelial tissue or endothelium. On the apical side of these endothelial cells, there are no microvilli or cilia. Instead, the tissue consists of flat, tile-like cells that provide a smooth surface to ensure that blood can flow easily without getting caught or damaged. This layer also includes a basement membrane and an internal elastic lamina.
The middle layer, known as the tunica media, is typically the thickest of the three layers. It is primarily made up of smooth muscle and is under the involuntary or autonomic control of the nervous system. When the smooth muscle in this layer contracts, the process is called vasoconstriction. This contraction is usually localized to a specific segment rather than occurring throughout the entire length of the vessel simultaneously. Conversely, when these muscles relax, the vessel undergoes vasodilation, resulting in an increased lumen diameter. The tunica media also contains an external elastic lamina.
The most superficial layer is the tunica externa. This outer layer is primarily composed of connective tissue containing a significant amount of collagen fibers and elastic fibers within its extracellular matrix. It also houses numerous nerves and lymphatic vessels. The primary function of the tunica externa is to anchor the blood vessel to the surrounding tissues, ensuring it stays in place. During surgical procedures such as a coronary artery bypass, a vascular surgeon must meticulously snip away this connective tissue to remove a vessel for use without puncturing the vessel itself. Within these layers, elastic fibers are essential as they allow the vessel to stretch and recoil, while collagen fibers provide necessary structural strength. It is worth noting that smoking is known to damage these elastin fibers.
Arterial Structure and Function in Systemic Circulation
Arteries are defined as blood vessels that transport blood away from the heart. Large arteries possess three layers in their walls that enable them to stretch significantly when the heart pumps blood into them and subsequently recoil. This recoil action is vital because it lessens the amount of pumping energy lost and decreases the fluctuation or change in blood pressure throughout each cardiac cycle. This helps keep blood moving forward even when the heart is in a relaxed state, which saves cardiac energy. The radial pulse, felt on the wrist just below the thumb, is the tactile representation of this alternating expansion and recoil of the tunicas in a superficial artery. Arteries generally follow specific paths through the body, and most are located deep within the tissues. Deep arteries often travel alongside veins that share the same name. Only a few large arteries are located superficially.
To monitor the internal environment, specialized sensory neurons called arterial baroreceptors, or stretch receptors, are located within the walls of certain arteries. These sensors detect how much the arterial wall is stretching or relaxing due to changes in blood pressure. Two critical reflexes involving these sensors are the aortic reflex and the carotid sinus reflex. These receptors generate or diminish action potentials based on pressure changes. Specifically, baroreceptors respond very quickly to short-term increases in blood pressure by decreasing the heart rate. Conversely, if there is a short-term decrease in blood pressure, the heart rate increases to compensate and maintain homeostasis.
Pathological Conditions: Aneurysms and Hypertension
The continuous force exerted by the beating heart puts the endothelium of large arteries at risk for rips and tears. If a tear occurs, blood can seep between the tissue layers in a localized area, causing the artery wall to balloon or stretch outward, a condition known as an aneurysm. If an aneurysm ruptures, it can lead to massive internal bleeding and potential death. Aneurysms are named according to their anatomical location. An abdominal aortic aneurysm occurs in the abdomen, a thoracic aortic aneurysm is located in the chest, and a cerebral aneurysm occurs in the brain. The thoracic aortic aneurysm is considered particularly dangerous and is often not associated with lifestyle factors.
The aorta is the largest artery in the body, carrying oxygenated blood from the heart to the systemic circulation. It typically measures approximately , which is equivalent to , in diameter. Three large arteries branch off the arch of the aorta to supply oxygen-rich blood to the shoulders, arms, and most importantly, the brain. These include the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery. The brachiocephalic trunk eventually branches into the right subclavian artery and the right common carotid artery. In anatomical naming, terms like "common," "trunk," or "base" signify that the region has the widest diameter, with the vessel narrowing further along its path. For example, the pulmonary trunk narrows into the pulmonary artery. If a vessel is labeled "left" or "right," it implies the existence of a corresponding vessel on the opposite side; if no such designation exists, there is only one such vessel.
Venous System and Blood Pressure Dynamics
Veins are responsible for returning blood to the heart under significantly lower pressure than that found in arteries. Consequently, vein walls are thinner and have less strength. However, veins have the unique ability to distend or stretch outward to hold large volumes of blood without strong recoil. This allows veins to function as the body's blood reservoir, storing a large portion of the total blood volume. This reservoir function ensures that even during dehydration or moderate blood loss, the heart still has enough blood to pump and maintain constant pressure. Superficial veins are also the routine site for venipuncture, used to administer medications or withdraw blood because they are accessible and under lower pressure than systemic arteries.
Venous blood return to the heart, often working against gravity, is assisted by three main mechanisms. First, the respiratory pump utilizes pressure changes in the chest and abdominal cavities during breathing to move blood. Second, one-way valves found in many veins prevent the backflow of blood. Third, the skeletal muscle pump involves the "milking action" of skeletal muscles squeezing nearby veins to push blood toward the heart. Each heartbeat constitutes one cardiac cycle consisting of two phases: diastole and systole. Normal blood pressure is recorded as at the brachial artery. The bottom number, , represents diastole, when the atria are filling and the heart is relaxing. During this phase, the tricuspid and mitral valves (also known as the bicuspid or AV valves) slam shut, producing the quieter "lub" sound. The top number, , represents systole, when both ventricles contract simultaneously. This causes the pulmonary and aortic valves (semilunar valves) to shut, creating the louder "dub" sound. Hypertension is a disease state where blood pressure remains elevated above . Known as the "silent killer," it can lead to strokes or aneurysms.
The Process of Hemostasis
Hemostasis is the normal, localized physiological process used to repair injured blood vessels and prevent excessive blood loss through five sequential steps. The first step is vasoconstriction, where the smooth muscle of the tunica media contracts on either side of the rupture to decrease the vessel diameter and reduce blood loss. The second step is the formation of a platelet plug. When the collagen fibers in the tunica externa are exposed by a break in the vessel lining, passing platelets bind to the collagen and become activated. These activated platelets release substances to initiate the clotting process. This can be compared to breaking off pieces of a soft taco to mimic the accumulation of platelets.
The third step is coagulation, a complex, multi-stepped process involving the activation of different clotting factors. These factors work to form a fibrin mesh using sticky, thread-like fibrin proteins. This mesh traps healthy red blood cells as they pass by, forming a temporary clot over the injury. The fourth step is clot retraction, during which serum (the fluid remaining after clotting) is squeezed out as the red blood cells within the clot die. This causes the temporary clot to shrink, tighten, and dry up. The fifth and final step is fibrinolysis. As new tunica layers move in to seal the rupture, the body produces enzymes that digest the fibrin mesh and dead red blood cells, dissolving the clot components.
Proper hemostasis requires that calcium ion concentrations, denoted as , remain within the optimum range of of blood. Abnormalities in hemostasis includes Deep Vein Thrombosis (DVT), where blood clots fail to break down and may travel to the heart and then the lungs. If a clot becomes stuck in a lung artery, it is called a pulmonary embolism. Hemophilia A is another disorder, often inherited, caused by a non-functioning clotting factor VIII, which prevents the body from forming a strong fibrin clot. To manage clotting risks, doctors may prescribe anticoagulants or blood thinners, such as Heparin, Coumadin, or Aspirin.
Coronary Circulation and Heart Autonomy
Despite being constantly filled with blood, the heart muscle or myocardium cannot be nourished by the blood within its chambers because the muscle is too thick for adequate diffusion. Furthermore, the heart muscles never rest and require a continuous supply of oxygen and nutrients. This demand is met by the coronary circulation. The right and left coronary arteries are the only arteries in the body that branch directly from the base of the aorta, ensuring the heart receives the freshest, most recently oxygenated blood. In approximately of people, there are only these two main coronary arteries. These vessels run along the surface of the heart and branch into smaller arteries, then into arterioles, and finally into coronary capillaries where perfusion occurs and wastes are collected.
After passing through the capillary beds, the blood enters coronary veins. These veins converge into a single large vessel on the posterior surface of the heart called the coronary sinus. The coronary sinus then drains the deoxygenated blood into the right atrium to complete the circuit. To ensure the heart survives potential blockages, there are backup branches called collateral vessels or anastomoses that provide alternate pathways for blood flow. The heart also possesses intrinsic control over its rhythm. About of cardiac cells differentiate during embryonic development into auto-rhythmic fibers. These cells can spontaneously generate electrical signals called action potentials to stimulate contraction without needing instructions from the brain. This electrical signal follows a specific path through four nodes: the sinoatrial (SA) node (the natural pacemaker), the atrioventricular (AV) node, the Bundle of His, and the Purkinje fibers. While the heart can beat on its own, extrinsic controls like hormones (such as adrenaline), neurotransmitters, and the autonomic nervous system can modify the heart rate and contraction strength. For instance, the vagus nerve (cranial nerve X) acts to slow down the SA node and reduce the heart rate.