(Reading Assignment: Chapter 16 of Berne and Levy)

A. Major function of the circulatory system

The circulatory system maintains the flow of oxygen, water, electrolytes, nutrients, hormones and immune cells to all tissues of the body. Waste products such as carbon dioxide and unwanted metabolites are carried away from the tissues.

A normal 70 kg adult has a blood volume of about 5 liters.

B. Circuitry

Pulmonary and systemic circulation

Circulation is divided into systemic (also called greater circulation or peripheral circulation) and pulmonary circulation. The systemic circulation supplies blood to all organs and tissues of the body and consists of aorta, arteries, arterioles, capillaries, venules, small veins and vena cavae. The pulmonary circulation consists of pulmonary veins and arteries, and is a low pressure, low resistance system for exchange of oxygen and carbon dioxide.

B. Physical properties of blood vessels

Distribution system: The arteries and arterioles serve as a distribution system. These are relatively thick-walled, high-pressure conduits that transport blood to small arteries and arterioles. The aorta and large arteries serve as a pressure-storing function, and small arteries and arterioles serve as a resistance function. The small arteries and arterioles are sometimes referred to as resistance vessels as they provide the highest resistance to blood flow. Abundant elastic tissue in the arteries allow the vessel to stretch during systole (contraction of ventricle) and recoil. This is important in the maintenance of blood pressure and in reducing the pulsatile nature of blood flow. Abundant smooth muscle of arterioles allows flow through a particular organ to be decreased or increased easily.

Exchange system:The capillaries serve as an exchange system. These are numerous, highly branched thin-walled tubes that allows diffusion of gases, and transport of water, nutrients and electrolytes through their walls. Capillaries are made up of only one layer of endothelial cells.

Collecting system:Veins and venules serve as the collecting systems that transport the blood back to the heart. Veins are large capacity, thin-walled vessels (capacitance vessels). The pressure is low in these vessels. As the amount of smooth muscle and elastic tissue is low in the venules, they can be easily distended. The venous system normally holds 2/3 of the blood volume.

C. Pressure and volume distributions

The pressure in the left ventricle fluctuates between zero and 120 mmHg. During contraction, the pressure reaches 120 mmHg, and during early in diastole, the pressure is near zero mmHg. The high pressure produced by the pumping of the blood out of the heart stretches the aorta and temporarily stores it as the pressure energy on the walls of the aorta. Due to the elastic ability of aorta and large arteries, they recoil and propel the blood toward the small arteries and arterioles in the tissues. The pressure in the aorta and large arteries fluctuate between ~120-125 mmHg and ~80 mmHg. The slightly higher systolic pressure in the large arteries is believed to be due to the slightly less compliant quality of the arteries than the aorta, and to the rebound effect of pulsatile blood flow.

As the blood travels through small arteries and arterioles, the pressure falls markedly and the pulsatility of the flow is also reduced. These vessels therefore serve as the “stopcock” of the circulation and are called the resistance vessels. The number and the total cross sectional area of small arteries and arterioles exceed that of large arteries. Nevertheless, the total resistance of arterioles is higher than the total resistance of large arteries, due to the smaller radius of the arterioles (see Poiseuille’s Law).

The pressure in the capillaries are low (10-25 mmHg). The large cross-sectional area of capillaries and their short length results in low resistance to blood flow within the capillary bed. The pressure in the veins is low (0-10 mmHg). Due to their ability to distend, they can store 2/3 of the blood volume.

D. Compliance and Distensibility of blood vessels

Distensibility (percentage of volume increase per unit pressure change) in a normal large artery and vein. Note the decreased slope of the arterial curve at higher pressure (B). Also note the steep slope and much higher distensibility of the veins (C) at their physiological pressures (0-10 mmHg) compared to the slope and distensibility of arteries (A) at their usual pressures (75-150 mmHg).

(Reprinted from Circulatory Physiology-the essentials 2nd ed.,(1984)by J.J.smith & J.P. Kampine, Figure 1.6 Page 11, Lippincott, Williams & Wilkins)

Vascular distensibility = Increase in vol. / (Increase in pressure x original vol.)

Vascular compliance (or capacitance) = Distensibility x volume.

Distensibility is the % volume change per unit pressure. Compliance relates to the total quantity of blood that can be stored in a given portion of a vascular bed for each mmHg rise in pressure. Compliance of a vein is ~24 times that of an artery because it is ~8 times as distensible and it has ~3 times the volume.

E. Summary of systemic circulation: pressure, velocity of flow, cross sectional area, blood volume for different parts of the systemic circulation.

(Reprinted from Principles of Physiology 3rd ed., (1993) by R.M.Berne & M.N.Levy, page 362 Fig 22.2 with permission from Elsevier)

F. Pulmonary Circulation

Contraction of the right heart propels the blood to the pulmonary circulatory system. The pressure within the pulmonary system ranges from 10 to 24 mmHg, and therefore is a low-pressure system. The main purpose of this system is for reoxygenation of blood by the lung. Pulmonary capillaries form a rich network within the alveolar walls for efficient exchange of gases.

G. Blood

Blood is a suspension of different types of cells (red, white and platelets) in a solution that contains various salts, proteins and dissolved gases (plasma). A 70-kg adult typically has a blood volume of 5000 ml with about 55% as plasma volume and 45% as red cell mass.

Total blood volume is a linear function of body weight (-normally ~75ml/kg).

When lying down (recumbent), ~25-30% of total boold volume is “thoracic” or central blood volume. When standing up, about a third of the central blood volume moves to the lower parts of the body. Standing up quickly from a recumbent position sometimes produces “postural hypotension” due to sudden lowering of the thoracic blood volume and associated decrease in cardiac output (discussed in later lectures).