Chapter 20: The circulatory System: Blood Vessels and circulation

Blood vessels

• There are three main caregpries of blood vessels:

  1. Arteries carry blood away from the heart.

  2. Veins carry blood toward the heart.

  3. Capillaries are microscopic, thin-walled vessels that connect the smallest arteries to the smallest veins. They are the site of the transfer of materials into and out of tissues.

Capillaries

• Exchange of materials (gases, nutrients, wastes, hormones, ect.) with the tissue occurs only in the capillaries and in some of the smallest veins.

• No cell in the body is more than about five cell width always a capillary (except in ligament, tendons, cartilage, cornea, and the lens).

• Capillary exchange (passage of materials across the walls of capillaries) occurs via three routes:

  1. through the endothelial cells.

  2. Through spaces between the endothelial cells.

  3. Through the filtration pores of fenestrated capillaries.

• Capillaries are arranged in web-like networks called capillary beds.

• Blood flow to particular capillaries is regulated by constriction or dilation of upstream arterioles or by precapillary sphincters.

Blood Flow

• Flow is the amount of blood flowing through an organ, tissue, or blood vessel in a given amount of time (mL/min).

• In a resting individual, the flow is equal to cardiac output. Flow through individual organs, however, is consistently variable as blood is redirected to different parts of the body.

• Flow = Difference in pressure/Resistance

• F = P/R

• Blood pressure is the force that blood exerts against the wall of a vessel.

• Can be measure directly by inserting a catheter or needle connected to an external manometer.

• Clinically, we are most interested in the BP in the systemic arteries close to the heart. Because it is at the same level as the heart, pressure in the brachial artery is used as an approximation. it is measured with a sphygmomanometer.

• Systolic pressure is the peak pressure recorded during ventricular systole.

• Diastolic pressure is the minimum arterial pressure, measured during ventricular diastole.

• In a healthy adult, these values are about 120 mm Hg and 75 mm Hg, respectively.

• The differnce between the systolic pressure and the diastolic pressure is the pule pressure.

  (120 - 75 = 45 mm Hg)

• Another measure of pressure is the mean arterial pressure (MAP)

• MAP = diastolic pressure + 1/3 pule pressure. For the typical adult, this would be about 90 mm Hg. (at least for the vessels at the level of the heart.)

• Gravity affects MAP (and all measurements of blood pressure). In a standing person, MAP is about 62 mm Hg in arteries of the head and about 180 mm Hg in arteries of the ankle.

• High blood pressure can lead to health complications:

  • Heart attack — due to atherosclerosis

  • Stroke — due to atherosclerosis

  • Heart Falure

  • Aneurysm

  • Metabolic Syndrome — high “bad” cholesterol, low “good” cholesterol, high triglycerides, high insulin levels, extra body weigh. Can lead to heart disease, stroke, and diabetes mellitus.

• The ability of arteries to stretch and recoil is one of the primary ways that the effects of blood pressure are limited.

• Blood pressure can increase with age as the arteries become less distensible (arteriosclerosis).

• Atherosclerosis is the growth of lipid deposits in the walls of the arteries. These deposits can become calcifies, thus making the arteries more rigid. Associated with high blood pressure.

• Hypertension is a chronic resting blood pressure higher than 140/90.

• Hypotension is a chronic low blood pressure.

• Blood pressure is determined by three main variables: cardiac output, blood volume, and resistance to flow.

• We’ve talked about cardiac output. Regulation of blood volume will be discussed in later chapters. Lets focus on resistance to flow.

Peripheral Resistance

• Depends on:

  1. Blood Viscosity

  2. Vessel Length

  3. Vessel Radius- this can be altered via vasoconstriction and vasodilation

Regulation of blood pressure and flow

• Neural control (Baroreflexes)

• Baroreceptors can be found in the aortic arch and in the carotid sinuses (at base of internal carotid artery).

• If BF increases,, this causes decreases in heart rate/ cardiac output and vasodilation in general. This is regulated though the autonomic nervous system.

• Neural Control (Chemoreflexes)

• Chemoreceptors for oxygen, carbon dioxide, and pH are found in the aortic arch and in the carotid bodies (at base of external carotid artery).

• Low levels of oxygen, high levels of carbon dioxide, and acidosis all cause widespread vasoconstriction. This increases overall BP, increasing blood flow to the lungs to promote gas exchange. Respiration rate is also increased.

• Hormonal Control —Several hormones will cause increases in blood pressure:

  a) Aldosterone promotes sodium and water retention in the kidneys.

  b) Angiotensin II causes vasoconstriction. (Synthesis of angiotensin II requires agiotensin-converting enzyme, known as ACE. ACE inhibitors are often prescribed for hypertension. )

• c) Antidiuretic hormone promotes water retention, but it will also cause vasoconstriction.

Venous Return

• The flow of blood back to the heart is called venous return. it is achieved via five mechanisms:

  1. Pressure Gradient — Pressure falls as we move from venues to the venue cavae. this promotes flow towards the heart.

  2. Gravity — At least from the head and neck.

  3. The Skeletal Muscle Pump – Contractions of muscles and presence of valves pushes blood in one direction.

  4. The Thoracic Pump – When you inhale, the thoracic cavity expands and thoracic pressure drops, while at the same time the diaphragm moves downward to increase abdominal pressure. This moves blood toward the thoracic cavity.

  5. Cardiac Suction – Suction from the empty atria draws blood in.