The Functions Of The Heart And Blood Vessels

Heart

The heart sometimes is thought of as an organ that pumps blood through the organs of the body. While this is true, it is more accurate to view the heart as a pump that receives blood from venous blood vessels at a low pressure, imparts energy to the blood (raises it to a higher pressure) by contracting around the blood within the cardiac chambers, and then ejects the blood into the arterial blood vessels.

It is important to understand that organ blood flow is not driven by the output of the heart per se, but rather by the pressure generated within the arterial system as the heart pumps blood into the vasculature, which serves as a resistance network. Organ blood flow is determined by the arterial pressure minus the venous pressure, divided by the vascular resistance of the organ (see Chapters 5 and 7). Pressures in the cardiovascular system are expressed in millimeters of mercury (mm Hg) above atmospheric pressure. One millimeter of mercury is the pressure exerted by a 1-mm vertical column of mercury (1 mm Hg is the equivalent of 1.36 cm H2O hydrostatic pressure). Vascular resistance is determined by the size of blood vessels, the arrangement of the vascular network, and the viscosity of the blood flowing within the vasculature.

The right atrium receives systemic venous blood (venous return) at very low pressures (near 0 mm Hg) (Fig. 1-3). This venous return then passes through the right atrium and fills the right ventricle; atrial contraction also contributes to the ventricular filling. Right ventricular contraction ejects blood from the right ventricle into the pulmonary artery. This generates a maximal pressure (systolic pressure) that ranges from 20 to 30 mm Hg within the pulmonary artery. As the blood passes through the pulmonary circulation, the blood pressure falls to about 10 mm Hg. The left atrium receives the pulmonary venous blood, which then flows passively into the left ventricle; atrial contraction provides a small amount of additional filling of the left ventricle. As the left ventricle contracts and ejects blood into the systemic arterial system, a relatively high pressure is generated (100-140 mm Hg maximal or systolic pressure). Therefore, the left ventricle is a high-pressure pump, in contrast to the right ventricle, which is a low-pressure pump. Details of the pumping action of the heart are found in Chapter 4.

FIGURE 1-3 Blood flow within the heart. Venous blood returns to the right atrium (RA) via the superior (SVC) and inferior vena cava (VC). Blood passes from the RA into the right ventricle (RV), which ejects the blood into the pulmonary artery (PA). After passing through the lungs, the blood flows into the left atrium (LA) and then fills the left ventricle (LV), which ejects the blood into the aorta (A) for distribution to the different organs of the body.

FIGURE 1-3 Blood flow within the heart. Venous blood returns to the right atrium (RA) via the superior (SVC) and inferior vena cava (VC). Blood passes from the RA into the right ventricle (RV), which ejects the blood into the pulmonary artery (PA). After passing through the lungs, the blood flows into the left atrium (LA) and then fills the left ventricle (LV), which ejects the blood into the aorta (A) for distribution to the different organs of the body.

The pumping activity of the heart is usually expressed in terms of its cardiac output, which is the amount of blood ejected with each contraction (i.e., stroke volume) multiplied by the heart rate. Any factor that alters heart rate or stroke volume will alter the cardiac output. The heart rate is determined by groups of cells within the heart that act as electrical pacemakers, and their activity is increased or decreased by autonomic nerves and hormones (see Chapter 2). The action potentials generated by these pacemaker cells are conducted throughout the heart and trigger contraction of cardiac myocytes (see Chapter 3). This results in ventricular contraction and ejection of blood. The force of ventricular contraction, and therefore stroke volume, is regulated by mechanisms intrinsic to the heart, by auto-nomic nerves and hormones (see Chapters 3, 4, and 6).

In recent years, we have learned that the heart has other important functions besides pumping blood. The heart synthesizes several hormones. One of these hormones, atrial na-triuretic peptide, plays an important role in the regulation of blood volume and blood pressure (see Chapter 6). Receptors associated with the heart also play a role in regulating the release of antidiuretic hormone from the posterior pituitary, which regulates water loss by the kidneys.

Vascular System

Blood vessels constrict and dilate to regulate arterial blood pressure, alter blood flow within organs, regulate capillary blood pressure, and distribute blood volume within the body. Changes in vascular diameters are brought about by activation of vascular smooth muscle within the vascular wall by au-tonomic nerves, metabolic and biochemical signals from outside of the blood vessel, and vasoactive substances released by cells that line the blood vessels (i.e., the vascular endothelium; see Chapters 3, 5, and 6).

Blood vessels have another function besides distribution of blood flow and exchange. The endothelial lining of blood vessels produces several substances (e.g., nitric oxide [NO], endothelin-1 [ET-1], and prostacyclin [PGI2]) that modulate cardiac and vascular function, hemostasis (blood clotting), and inflammatory responses (see Chapter 3).

Interdependence of Circulatory and Organ Function

Cardiovascular function is closely linked to the function of other organs. For example, the brain not only receives blood flow to support its metabolism, but it also acts as a control center for regulating cardiovascular function. A second example of the interdependence between organ function and the circulation is the kidney. The kidneys excrete varying amounts of sodium, water, and other molecules to maintain fluid and electrolyte homeo-stasis. Blood passing through the kidneys is filtered and the kidneys then modify the composition of the filtrate to form urine. Reduced blood flow to the kidneys can have detrimental effects on kidney function and therefore on fluid and electrolyte balance in the body. Furthermore, renal dysfunction can lead to large increases in blood volume, which can precipitate cardiovascular changes that sometimes lead to hypertension or exacerbate heart failure. In summary, organ function is dependent on the circulation of blood, and cardiovascular function is dependent on the function of organs.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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