Chapter 21 Blood Vessels and Hemodynamics

Basic components of the CV organ system The blood vessels of the body should not be thought of as mere “pipes” carrying blood – they are dynamic, interactive, essential components of the cardiovascular organ system. Vessel Structure and Function

Blood Vessel Types Arteries – carry blood away from the heart Large elastic arteries (>1 cm); medium muscular arteries (0.1 – 10 mm); arterioles (< 0.1 mm) Capillaries – site of nutrient and gas exchange Veins – carry blood towards the heart Venules are small veins (< 0.1 mm) Vessel Structure and Function

All blood and lymph vessels in the body share components of 3 basic layers or “tunics” which comprise the vessel wall: Tunica interna (intima) Tunica media Tunica externa Vessel Structure and Function

The tunica interna is the inner lining in direct contact with blood. – The epithelium of the intima is the same endothelium that makes up the endocardial lining of the heart. – It has an active role in vessel-related activities.  The tunica media is chiefly composed of smooth muscle that regulates the diameter of the vessel lumen.  The tunica externa helps anchor vessel to surrounding tissue through use of elastic and collagen fibers.

Vessel Structure and Function The largest arteries are the conducting arteries (elastic arteries), best exemplified by the garden hose-sized aorta. Their walls are thin compared to their overall size. Elastic arteries perform the important function of storing mechanical energy during ventricular systole and then transmitting that energy to keep blood moving after the aortic and pulmonary valves close.

Vessel Structure and Function

Medium sized muscular (distributing) arteries have more smooth muscle in their tunica media. Muscular arteries help maintain the proper vascular tone to ensure efficient blood flow to the distal tissue beds. Examples include the brachial artery in the arm and radial artery in the forearm. Vessel Structure and Function

An anastomosis is a union of vessels supplying blood to the same body tissue. Should a blood vessel become occluded, a vascular anastomosis provides collateral circulation (an alternative route) for blood to reach a tissue. The shaded area in this graphic shows overlapping blood supply to the ascending colon.

Vessel Structure and Function Arterioles deliver blood to capillaries and have the greatest collective influence on both local blood flow and on overall blood pressure. They are the primary "adjustable nozzles” across which the greatest drop in pressure occurs.

Capillaries are the only sites in the entire vasculature where gases, water and other nutrients are exchanged. Venules and veins have much thinner walls than corresponding arterioles and arteries of similar size. Vessel Structure and Function

The terminal end of an arteriole tapers toward the capillary junction to form a single metarteriole. At the metarteriole-capillary junction, the distal most muscle cell forms the precapillary sphincter which monitors and regulates blood flow into the capillary bed.

Capillaries are different from other vascular structures in that they are made of only a single endothelial cell sitting on a very thin basement membrane - there are no other tunics, layers or muscle. The minimalist nature of capillaries allows them to be freely permeable to many substances (gases, fluids, and small ionic molecules). Vessel Structure and Function

The body contains three types of capillaries: Continuous capillaries are the most common with endothelial cells forming a continuous tube, interrupted only by small intercellular clefts. Fenestrated capillaries (fenestra = windows), found in the kidneys, villi of small intestines, and endocrine glands are much more porous. Sinusoids form very porous channels through which blood can percolate, e.g., in the liver and spleen.

Vessel Structure and Function 3 Types of capillaries in the body

Vessel Structure and Function Veins have thinner walls, less muscle and elastic tissue, and are designed to operate at much lower pressures. Intravenous pressure in venules (16 mmHg) is less than half that of arterioles (35 mmHg), and drops to just 1-2 mmHg in some larger veins. Because intravenous pressure is so low, veins have valves to keep blood flowing in only 1 direction. When exposed to higher than normal pressures, veins can become incompetent (varicose veins).

Vessel Structure and Function

Fluid Exchange - Starling Forces As blood flows to the tissues of the body, hydrostatic and osmotic forces at the capillaries determine how much fluid leaves the arterial end of the capillary and how much is then reabsorbed at the venous end. These are called Starling Forces. Filtration is the movement of fluid through the walls of the capillary into the interstitial fluid. Reabsorption is the movement of fluid from the interstitial fluid back into the capillary.

Fluid Exchange - Starling Forces Two pressures promote filtration: Blood hydrostatic pressure (BHP) generated by the pumping action of the heart - decreases from 35 to 16 from the arterial to the venous end of the capillary Interstitial fluid osmotic pressure (IFOP), which is constant at about 1 mmHg

Fluid Exchange - Starling Forces Two pressures promote reabsorption: – Blood colloid osmotic pressure (BCOP) is due to the presence of plasma proteins too large to cross the capillary - averages 36 mmHg on both ends. – Interstitial fluid hydrostatic pressure (IFHP) is normally close to zero and becomes a significant factor only in states of edema.

Fluid Exchange - Starling Forces

Normally there is nearly as much fluid reabsorbed as there is filtered. At the arterial end, net pressure is outward at 10 mmHg and fluid leaves the capillary (filtration). At the venous end, net pressure is inward at –9 mmHg (reabsorption). On average, about 85% of fluid filtered is reabsorbed.

Fluid Exchange - Starling Forces Fluid that is not reabsorbed (about 3L/ day for the entire body) enters the lymphatic vessels to be eventually returned to the blood.

Gas And Nutrient Exchange In contrast to the bulk flow of fluids at the capillaries, the exchange of gases and small particles (like certain nutrients and wastes) is a purely passive diffusion process. Gases and these other substances simply move into or out of the capillary down their concentration gradient.

Venous Reserve Because systemic veins and venules contain a large percentage of the blood volume (about 64% at rest), they function as blood reservoirs from which blood can be diverted quickly if needed. To counteract a drop in BP, stimulation of the sympathetic NS will cause venoconstriction, allowing a greater volume of blood to flow to skeletal muscles.

The volume of blood returning through the veins to the right atrium must be the same amount of blood pumped into the arteries from the left ventricle – this is called the venous return. Besides pressure, venous return is aided by the presence of venous valves, a skeletal muscle pump, and the action of breathing. Venous Return

The skeletal muscle pump uses the action of muscles to milk blood in 1 direction (due to valves). The respiratory pump uses the negative pressures in the thoracic and abdominal cavities generated during inspiration to pull venous blood towards the heart. Venous Return

Proximal valve Distal valve 1 Proximal valve Distal valve 12 Proximal valve Distal valve 123

Although the venous circulation flows under much lower pressures than the arterial side, usually the small pressure differences (venule 16 mmHg to right atrium 0 mmHg), plus the aid of muscle and respiratory pumps is sufficient. Venous Return

Pressure, Flow, And Resistance Blood pressure is a measure of the force (measured in mmHg) exerted in the lumen of the blood vessels. Blood flow is the amount of blood which is actually reaching the end organs (tissues of the body). Resistance is the sum of many factors which oppose the flow of blood.

Pressure, Flow, And Resistance Cardiovascular homeostasis is mainly dependent on blood flow… but blood flow is hard to measure. Clinically, we check blood pressure because it is easier to measure, and it is related to blood flow. The relationship between blood flow, blood pressure, and peripheral resistance follows a simple formula called Ohms Law. BP = Flow x Resistance

Pressure, Flow, And Resistance In an effort to meet physiological demands, we can increase blood flow by: Increasing BP Decreasing systemic vascular resistance in the blood vessels Usually our body will do both – when we exercise, for example. figure adapted from

Pressure, Flow, And Resistance As we have already seen, peripheral resistance is itself dependent on other factors like the viscosity of blood, the length of all the blood vessels in the body (body size), and the diameter of a vessel. The first two of these factors (viscosity and the length of blood vessels) are unchangeable from moment to moment. The diameter, however, is readily adjusted if the body needs to change blood flow to a certain capillary bed.

Pressure, Flow, And Resistance

Pressure, Flow, And Resistance Example: If the diameter of a blood vessel decreases by one-half, its resistance to blood flow increases 16 times! “Hardening of the arteries” (loss of elasticity) seriously hampers the body’s ability to increase blood flow to meet metabolic demands.

Pressure, Flow, And Resistance (Interactions Animation) Vascular Regulation You must be connected to the internet to run this animation

Autoregulation Homeostasis in the body tissues requires the cardiovascular system to adjust pressure and resistance to maintain adequate blood flow to vital organs at all times – a process called autoregulation. Autoregulation is controlled through negative feedback loops.

Autoregulation of blood pressure and blood flow is a complex interplay between: The vascular system The nervous system The endocrine hormones and organs like the adrenal gland and the kidney The heart Autoregulation

The vascular system senses alterations of BP and blood flow and signals the cardiovascular centers in the brain. – The heart then appropriately modifies its rate and force of contraction. – Arterioles and the precapillary sphincters of the metarterioles adjust resistance at specific tissue beds.

Autoregulation For example, during emergencies, the autonomic nervous system will vasodilate the precapillary sphincters of metarterioles in the skeletal muscles, lungs, and brain, while constricting the precapillary sphincters found in tissues such as the skin, GI tract, and kidneys. This sends the majority of the cardiac output (blood flow) to those organs important in a fight or flight response, while temporarily depriving (through vasoconstriction) the nonessential organs.