Blood Circulation and Artery Function

• Blood transports gases, hormones, and cells throughout the body via the circulatory system.

Blood Circulation Vessels

• Arteries carry blood away from the heart, usually oxygenated, except for the pulmonary artery.
• Blood's red color results from oxygen binding to the heme (iron) on red blood cells; other substances can cause different colors, like carbon monoxide turning it black.
• Arterioles constitute smaller versions of arteries.
• Capillaries are thin vessels connecting arterioles and venules, facilitating nutrient, oxygen, and waste exchange.
• Veins carry blood to the heart, usually deoxygenated, except for pulmonary veins.
• Blood circulation relies on pressure, with high pressure in arteries leaving the heart and low pressure in veins returning to the heart.

Arteries: Structure and Function

• Arteries are thick vessels that carry oxygenated blood away from the heart to tissues, with at least one artery supplying each organ.
• Arteries' thick walls withstand high pressure and consist of three layers:
  • Tunica Externa: A tough, outer layer made of connective tissue.
  • Tunica Media: The thickest layer, composed of smooth muscle and elastic fibers (elastin).
  • Tunica Interna/Intima: A smooth endothelium lining the vessel's inside.
• A basement membrane separates the artery's cells from connective tissue.
• As the heart pumps, the artery stretches, causing a change in diameter that can be felt as a pulse.
• Blood flow in major arteries is pulsatile, coinciding with the heart's pumping action.
• Systolic pressure is the peak pressure in an artery during ventricular contraction or depolarization.
• When blood is pumped, the lumen and arterial wall expand, storing potential energy in the Tunica Media's elastic fibers.
• During ventricular relaxation (repolarization), the elastic fibers return to their original position, releasing stored potential energy and driving blood through the muscle, which measures as diastolic pressure.
• Artery walls maintain steady pressure for continuous blood flow.
• Smooth muscle in the Tunica Media can contract (vasoconstriction) or relax (vasodilation) to adjust artery diameter, regulating blood flow.

Arterioles: Regulating Blood Flow

• Arterioles are smaller arteries that blood flows into from the arteries.
• The arterial walls resemble those of the artery.
• The autonomic nervous system (ANS) controls arteriole diameter.
• Vasoconstriction decreases blood flow to tissues, while vasodilation increases it.
• Vasodilation causes blushing as more blood is directed to surface capillaries to release excess heat.
• Vasoconstriction causes paleness by diverting blood to areas needing more oxygen and glucose during stress.
• Vasodilation occurs to increase blood flow during inflammation, leading to increased interstitial space and secretion of fluids and antimicrobial chemicals.
• Arterioles primarily vasodilate to allow blood flow into capillaries only when needed.

Capillaries: Exchange Vessels

• Capillaries connect arterioles to venules, with walls made of a single layer of endothelium and a basement membrane.
• At around 10 micrometers wide, they are the narrowest vessels, allowing only one red blood cell to pass at a time.
• Capillaries facilitate the diffusion of oxygen and substances but are susceptible to high-pressure aneurysms.
• Minerals and vitamins can be exchanged between tissues and capillaries.
• The endothelium is coated in a gel-like membrane that acts as a filter increasing permeability and allowing for nutrient and oxygen-rich plasma to pass through.
• Plasma picks up waste and transports it away from tissues as it re-enters the capillaries.

Venules and Veins: Returning Blood to the Heart

• Blood flows from capillaries into venules and then into veins.
• Veins and venules have greatly reduced pressure.
• Valves in veins prevent backflow of blood.
• Veins have thinner layers of smooth muscle compared to arteries due to lower pressure.
• Each body part has at least one vein to return blood to the heart, except for the hepatic portal vein that brings blood from the stomach and intestines to the liver for filtration.
• Valves open for blood flow toward the heart and close to prevent backflow, ensuring unidirectional movement.
• Skeletal muscles squeeze veins to open valves and promote blood flow toward the heart.
• Veins act as blood reservoirs, holding up to 65% of the body’s blood, or 85% in sedentary individuals.
• Nerve impulses can cause smooth muscle in the Tunica Media to vasoconstrict, increasing fluid pressure and driving more blood to the heart during stress.

Varicose Veins

• Varicose veins are enlarged, bulging veins caused by blood pooling in one-way valves.
• They occur due to one-way valve damage causing blood backflow that reduces blood flow to the heart.
• They often occur in surface veins due to daily pressure, compression, prolonged standing, and restricted movement.

Atherosclerosis: Arterial Disease

• Atherosclerosis, a form of coronary artery disease, involves cholesterol deposits on the arterial wall, narrowing the lumen and inhibiting blood flow.
• Mineral and calcium deposits form plaque.
• If plaque breaks off and damages the arterial wall, it can trigger blood clotting by platelets, potentially blocking the artery and causing a heart attack.
• LDLs containing fats and cholesterols form atheroma (fatty tissue) on the endothelium layer of the arterial wall.
• Endothelium cells signal phagocytes to engulf LDLs, causing the phagocytes to become foamy and initiate a pro-inflammatory response.
• Smooth cells from the Media Media migrate to the top of the plaque, forming a hard layer.
• Anoxia, the lack of blood (and thus oxygen and nutrients) in the heart due to blockage in a coronary artery, can cause the heart to beat faster.
• Increased pressure can rupture the plaque and cause a blood clot, leading to critical heart failure.
• Angina, or pain is felt at first.
• Atheroma buildup results from high LDL blood concentration, chronic high glucose levels (obesity, diabetes), high blood pressure (stress, smoking), consumption of trans fats, and arterial wall infection.

Aneurysm

• An aneurysm is a bulge that forms due to a weakened arterial wall, often caused by atherosclerosis.
• Wall rupture can lead to less oxygen, internal bleeding, and cell death.
• Types include saccular and fusiform aneurysms.
• Aneurysms in the brain can cause a stroke.

The Heart: Structure and Function

• A muscular organ pumps blood throughout the body with two parallel pumps separated by the septum.
• The right side receives deoxygenated blood, and the left side receives oxygenated blood.
• Two main circuits:
  • Pulmonary: The right side pumps deoxygenated blood to the lungs for gas exchange, with blood returning to the left side via pulmonary veins.
  • Systemic: The left side pumps oxygenated blood into the aorta and throughout the body.
• The left ventricle’s muscles are larger than the right's to pump blood with greater force.
• The aorta ascends and then splits into five branches:
  • Aortic root: Supplying blood to coronary arteries, opens during ventricular relaxation.
  • Descending aorta: Supplying blood to the lower body.
  • Rightmost branch: Supplying blood to the right arm and right brain.
  • Middle-most branch: Supplying blood to the left brain.
  • Leftmost branch: Supplying blood to the left arm.
• The heart has four chambers: two thin-walled atria (upper) and two thick-walled ventricles (lower), separated by the septum.

Blood flow Through the Heart

• Deoxygenated blood from the body enters the superior and inferior Vena Cava.
• They join together to dump into the right atrium, where it's then pumped through the tricuspid valve into the right ventricle.
• Blood is pumped through the pulmonary valve into the pulmonary trunk, where it deposits carbon dioxide and picks up oxygen in the lungs.
• Oxygenated blood returns to the left atrium via the left and right pulmonary veins.
• Blood is pumped into the left ventricle through the bicuspid valve.
• Blood goes from the left ventricle, is pumped through the aortic valve into the aorta, and then to the body.

Heart Rhythm: Intrinsic Control

• Cardiac muscle can generate its own action potential and contract without external influence (myogenic).
• The SA and AV nodes, along with the bundle of HIS and Purkinje fibers, send, transmit, and receive electrical signals telling the cardiac muscle when to contract.
• The SA node, the heart's pacemaker, typically operates at around 70 bpm.
• The SA node’s bpm can be affected by coronary heart disease.
• After the SA node sends an impulse to the AV node, a slight delay prevents simultaneous contraction of all four chambers.
• The SA node sends electrical signals through the bundle of HIS to the Purkinje fibers, causing ventricular depolarization.
• Depolarization of heart cells spreads rapidly due to the branched nature of cardiac muscle.
• The SA node has the highest rate of spontaneous contraction or depolarization, with extensive membranes that aid in the spread of depolarization.
• If the SA node fails, the AV node can maintain heart rhythm at around 60 bpm; if the AV node also fails, the bundle of HIS cannot generate a sufficient heart rate.
• Electric pacemakers can be placed under the skin with electrodes on the heart to initiate contraction and set the pace of the heart.

Heart Rate Regulation

• The sympathetic nervous system increases heart rate during stress; the parasympathetic nervous system returns it to normal.
• Both are part of the autonomic nervous system.
• Epinephrine, secreted by the adrenal gland, mediates stress responses.
• Epinephrine is also used to deal with anaphylactic shock caused by allergies.

Heart Medications

• Foxglove, containing digitalis, strengthens heart contractions and treats heart congestion caused by a weakened heart.
• Nitroglycerin prevents heart attacks by reducing blood pressure and vasodilating arteries, reducing the chance of plaque bursting.
• Beta Blockers prevent cells from receiving epinephrine.
  • Beta 1 Blockers: Block receptors on cardiac muscle, preventing increased heart rate and high blood pressure.
  • Beta 2 Blockers: Block receptors in blood vessels and bronchioles.

Blood Flow Calculations

• Heart rate (bpm): The number of heartbeats per minute.
• Stroke volume (L/beat): The amount of blood pumped per heartbeat.
• Cardiac output (L/min): The amount of blood pumped from the heart per minute; calculated as Heart Rate x Stroke Volume.
• Athletes typically have lower heart rates with higher stroke volumes due to stronger cardiovascular systems.

Blood Pressure

• Measured with a sphygmomanometer.
• It depends on cardiac output and arterial resistance (elastic fibers).

ECG/EKG (Electrocardiogram)

• Tracks the heart's electrical activity, giving an accurate representation of heart function non-invasively.
• Components:
  • P wave: Represents atrial depolarization (systole) and contraction; AV valves are open, and semi-lunar valves are closed.
  • PR interval: Delay of the electrical impulse caused by the AV node while ventricles fill.
  • QRS complex:
    • Atrial repolarization (diastole) occurs during ventricular depolarization; blood dumps into the atria.
    • Q dip: Represents the depolarization of the septum as electrical signals pass through the Bundle of HIS to reach the Purkinje fibers.
    • First half of the R wave: Electrical signals pass the Bundle of HIS; AV valves close, and semi-lunar valves open.
    • Second part of the R wave: Represents the depolarization of the ventricles as the electrical signal reaches both left and right Purkinje fibers; AV valves are closed, and semi-lunar valves are open.
    • AV valves closing makes “lub” is the initial and louder sound of the heartbeat.
    • S dip: Represents final and complete ventricular depolarization.
  • ST segment: Represents ventricular contraction (systole); no electrical signal, ventricles are already completely depolarized.
  • T wave: Represents ventricular repolarization (diastole).
• The Semi-lunar valves close and make the “dub” sound, is the second and weaker sound of the heartbeat.

Regulation of Blood Pressure

• Receptors in the walls of the aorta (sensing high pressure) and carotid artery (sensing low pressure) regulate blood pressure.
• High pressure stimulates the medulla oblongata in the brain to engage the parasympathetic nervous system.
  • Vasodilation.
  • Reduced heart rate and stroke volume.
• Low pressure engages the sympathetic system until receptors in the aorta are activated.
• Blood pressure is measured in systolic (100-130 mmHg) and diastolic (60-80 mmHg) pressure.
• Normal blood pressure is 120/80.

Hypertension

• Sustained high blood pressure weakens blood vessels.
• The body adds connective tissue to the arterial wall to combat this, reducing elasticity and hardening the arterial walls.
• Could be caused by Factors such as: diet, obesity, smoking, alcohol, stress, age, and genetics.

Thermoregulation

• Body temperature is kept at 37 degrees Celsius.
• High temperatures or exercise cause the hypothalamus to engage sweat glands and dilate surface blood vessels.
• Sweat evaporates off the skin to cool it.
• Cold temperatures cause the hypothalamus to constrict surface blood vessels to conserve heat.
• Hair follicles also stand up to maintain heat.
• Shivering warms the body.

Explore the roles of blood and blood vessels in the circulatory system, focusing on arteries and blood pressure. Understand the structural adaptations of arteries for maintaining blood flow and withstanding pressure. Also learn how artery elasticity affects blood flow.